The Pennsylvania State University

The Graduate School

Department of Mechanical Engineering

FLAMEHOLDING STUDIES FOR LEAN PREMIXED FUEL

INJECTORS FOR APPLICATION IN GAS TURBINE ENGINES

A Thesis in

Mechanical Engineering

by

Steven Marzelli

© 2010 Steven Marzelli

Submitted in Partial Fulfillment

of the Requirements

for the Degree of

Master of Science

May 2010

ii

The thesis of Steven Marzelli was reviewed and approved* by the following:

Robert J. Santoro

George L. Guillet, Professor of Mechanical Engineering

Director of the Propulsion Engineering Research Center

Thesis Advisor

Domenic A. Santavicca

Professor of Mechanical Engineering

Karen A. Thole

Professor of Mechanical Engineering

Head of the Department of Mechanical and Nuclear Engineering

*Signatures are on file in the Graduate School.

iii

ABSTRACT

Due to the ever-increasing demand for energy, it is likely that stationary gas turbine

engines will require the use of fuels with a diverse range of chemical compositions in the near

future. Utilizing fuels, such as syngas, bio-derived fuels, hydrogen and liquefied natural gas,

present serious challenges in lean premixed gas turbine engines. In particular, combustion

phenomena, such as flashback and flameholding, are known to be sensitive to inlet flow

conditions and fuel composition. Changes in composition can lead to an increase in the flame

speed, resulting in upstream flame propagation. This has the potential to cause catastrophic

damage to gas turbine hardware if the flame anchors at an undesired upstream location.

The current research effort investigated flameholding characteristics of a fuel injector

nozzle used in lean premixed gas turbine engine operation. A single vane of a swirled, cross-flow

fuel injector was examined. A test rig with optical access to the fuel injector nozzle vane was

designed and constructed, such that combustion phenomena within the nozzle could be viewed

and recorded through the use of a high-speed camera. It should be noted that the likely location

for flameholding was presumed to be near the location of the fuel injection jets within the nozzle

vane.

For hot-fire testing, flow conditions in the combustor apparatus were closely matched to

those experienced during the full-scale operation of a stationary gas turbine engine. The test

apparatus was outfitted with both high- and low- frequency pressure and temperature transducers

in order to determine inlet fuel and air flow conditions as well as monitor combustor

performance. Once full operating conditions were reached for a given test, a hydrogen torch

ignition system was fired in order to simulate a severe combustion event upstream of the fuel

injector nozzle. This practice is commonly used in examining fuel injector nozzle performance.

iv

A series of tests were conducted at a wide range of flow conditions with a variety of fuel

mixtures in order to develop a threshold for flameholding in terms of reference air velocity and

fuel composition. A range of inlet air velocities, from 190 ft/s to 55 ft/s, and several fuel

compositions were examined. Fuels studied included natural gas (~95% methane) and various

ethane-natural gas mixtures to determine the nozzle‟s susceptibility to flashback and

flameholding with natural gas and higher hydrocarbon blends.

For all cases tested, there was no evidence of flameholding within the fuel injector nozzle

after hydrogen torch firing. However, during tests at lower inlet velocities with ethane-natural

gas mixtures, a flame did appear to anchor at a location downstream of the fuel injector nozzle as

observed via high-speed pressure and temperature measurements. Because the flame held outside

the field of view of the high-speed camera, the exact location of the hold is unknown.

v

TABLE OF CONTENTS

List of Figures ............................................................................................................................. . viii

List of Tables .................................................................................................................................. x

Nomenclature ............................................................................................................. ................... xi

Acknowledgements ..................................................................................................................... xiv

Chapter 1 – Introduction ............................................................................................................. 1

1.1 – Review of Relevant Literature .............................................................................................. 1

1.1.1 –Lean Premixed Combustion .................................................................................... 1

1.1.2 – Variations in Natural Gas Composition ................................................................. 6

1.2 – Motivation for the Current Work .......................................................................................... 9

1.3 – Approach ............................................................................................................................... 9

Chapter 2 – Experimental Apparatus ....................................................................................... 11

2.1 – Experimental Facilities ....................................................................................................... 11

2.2 – Air Supply Systems ............................................................................................................. 11

2.3 – Gaseous Fuel Supply Systems ............................................................................................. 13

2.4 – Combustor Apparatus .......................................................................................................... 15

2.4.1 – Test Chamber ........................................................................................................ 16

2.4.1.1 – Test Chamber Setup ............................................................................... 16

2.4.1.2 – Acetone-PLIF Experiments .................................................................. 19

2.4.2 –Afterburner Chamber ............................................................................................. 20

2.4.2.1 – Afterburner Chamber Hardware ............................................................ 20

2.4.2.2 – Afterburner Ignition Check ................................................................... 23

2.4.2.3 – Afterburner Flame Stabilization Test Rig ............................................. 24

vi

2.4.3 – Hardware Modifications Made During High Velocity Flameholding Tests......... 27

2.5 – Diagnostics and Data Acquisition ....................................................................................... 29

2.6 – Abort Systems ..................................................................................................................... 31

Chapter 3 – Afterburner Flame Stabilization .......................................................................... 33

3.1 – Introduction ............................................................................................. ............................ 33

3.2 – Operating Conditions and Test Matrix ................................................................................ 35

3.3 – Results and Discussion ..................................................................... ................................... 36

Chapter 4 – Flameholding Experiments ................................................................................... 44

4.1 – High Velocity Flameholding Tests ...................................................................................... 44

4.1.1 – Operating Conditions and Test Plan ..................................................................... 44

4.1.2 – Results and Discussion ......................................................................................... 45

4.1.3 – High-Speed Video Summary ............................................................................... 48

4.2 – Low Velocity Flameholding Tests .......................................................................... ............ 51

4.2.1 – Operating Conditions ........................................................................................... 51

4.2.2 – Results and Discussion ........................................................................................ 53

4.2.3 – High-Speed Video Summary ............................................................................... 66

Chapter 5 – Conclusions and Future Work ............................................................................. 71

5.1 – Conclusions for Current Study ............................................................................................ 71

5.2 – Suggestions for Future Work .............................................................................................. 72

Bibliography ............................................................................................................................... 74

Appendix A – Orifices Used ...................................................................................................... 76

A.1 – Orifices for Afterburner Flame Stabilization ..................................................................... 76

A.2 – Orifices for High Velocity Flameholding Tests ................................................................. 77

vii

A.3 – Orifices for Low Velocity Flameholding Tests ............................... ................................... 78

Appendix B – Properties and Chemical Compositions of Fuels ............................................. 79

Appendix C – Afterburner Part Drawings .............................................................................. 80

Appendix D – Detailed Results for Flameholding Experiments ............................................ 88

D.1 – Results from High Velocity Flameholding Experiments ................................................... 89

D.2 – Results from Low Velocity Flameholding Experiments .................................................... 91

viii

LIST OF FIGURES

Figure 2-1 Air Supply System at Gas Turbine Combustion Laboratory ...................................... 12

Figure 2-2 Air Supply System at Cryogenic Combustion Laboratory ......................................... 13

Figure 2-3 Schematic of the Combustor Apparatus ..................................................................... 16

Figure 2-4 Section View of the Test Chamber ............................................................................. 17

Figure 2-5 Section View of the Afterburner Chamber ................................................................. 21

Figure 2-6 Afterburner Flame Stabilization Test Rig .................................................................. 25

Figure 2-7 Section View of the Hydrogen Injection System ....................................................... 26

Figure 2-8 LabVIEW VI Used for Flameholding Tests ............................................................... 30

Figure 2-9 Test Chamber with Photron Camera Mounted at the Side Window ......................... 31

Figure 3-1 Percent Pressure Oscillations in the Chamber for Tests at 55 ft/s ............................. 39

Figure 3-2 Percent Pressure Oscillations in the Chamber for Tests at 70 ft/s ............................. 40

Figure 3-3 Percent Pressure Oscillations in the Chamber for Tests at 85 ft/s ............................. 41

Figure 3-4 Percent Pressure Oscillations in the Chamber for Tests at 100 ft/s ........................... 42

Figure 4-1 Hydrogen Torch Flow from Rear Window During Test H5 ...................................... 48

Figure 4-2 Hydrogen Torch Flow from Side Window During Test H8 ...................................... 49

Figure 4-3 Flame Stagnation in Nozzle Vane During Test H12 .................................................. 50

Figure 4-4 Flame Pulsing into Field of View During Test H13 .................................................. 51

Figure 4-5 High-Frequency Temperature Data for Tests L8 and L16 ......................................... 56

Figure 4-6 High-Frequency Temperature Data from DS TC for CNG Tests .............................. 58

Figure 4-7 High-Frequency Temperature Data from DS TC for 28% C2H6 Tests ...................... 60

Figure 4-8 DS TC Data for 28% C2H6 Tests with Various Hydrogen Torch Flow Durations .... 61

ix

Figure 4-9 High-Frequency Temperature Data from DS TC (Test Duration: 5.7 s) ................... 63

Figure 4-10 High-Frequency Afterburner PCB Data for CNG Tests at 90 ft/s ........................... 64

Figure 4-11 High-Frequency Afterburner PCB Data for 28% C2H6 Tests at 55 ft/s ................... 65

Figure 4-12 Hydrogen Torch Flow with 22.5% Ethane-Natural Gas Mixture during Test L4 ... 67

Figure 4-13 Time Evolution of a Flame Pulsing into the Nozzle Vane during Test L8 .............. 68

Figure 4-14 Time Evolution of Flame Pulsing into Nozzle Vane during Test L16 ..................... 69

Figure C-1 Test Chamber Exhaust Tube Insert Part Drawing ..................................................... 81

Figure C-2 Diagnostic Hub Part Drawing ................................................................................... 82

Figure C-3 Hydrogen Inlet Hub Part Drawing ............................................................................ 83

Figure C-4 Hub Assembly Drawing ............................................................................................ 84

Figure C-5 Hydrogen Injection Tube Part Drawing for Gundrilling ........................................... 85

Figure C-6 Part Drawing for Afterburner Nozzle with 0.8-in. Throat Diameter ......................... 86

Figure C-7 Part Drawing for Afterburner Nozzle with 0.59-in. Throat Diameter ....................... 87

x

LIST OF TABLES

Table 2-1 Properties and Chemical Composition of CNG Used ................................................. 14

Table 3-1 Test Matrix for Afterburner Flame Stabilization Study ............................................... 36

Table 3-2 Stabilization Test Results with 1.05-in. Inlet Flow Diameter ...................................... 38

Table 3-3 Stabilization Test Results with 0.76-in. Inlet Flow Diameter ...................................... 38

Table 4-1 Target Flameholding Test Conditions in the Nozzle Vane ......................................... 44

Table 4-2 High Velocity Flameholding Test Results ................................................................... 46

Table 4-3 Test Section Velocities for Various Ethane-Natural Gas Mixtures ............................. 52

Table 4-4 Low Velocity Flameholding Test Results .................................................................... 54

Table A-1 Orifices for Afterburner Flame Stabilization .............................................................. 76

Table A-2a Orifices for High Velocity Flameholding Tests ........................................................ 77

Table A-2b Orifices for High Velocity Flameholding Tests (Continued) ................................... 77

Table A-3 Orifices for Low Velocity Flameholding Tests with CNG Only ................................ 78

Table A-4 Orifices for High Velocity Flameholding Tests with Added C2H6 ............................. 78

Table B-1 Properties and Chemical Compositions of All Fuels Used ......................................... 79

Table D-1 Conditions Achieved During Tests H1-H7 ................................................................. 89

Table D-2 Conditions Achieved During Tests H8-H13 ............................................................... 90

Table D-3 Conditions Achieved During Tests L1-L9 .................................................................. 91

Table D-4 Conditions Achieved During Tests L10-L18 .............................................................. 92

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NOMENCLATURE

Abbreviations

AB Afterburner

ABCW Afterburner Cooling Water

ABP Afterburner Pressurization

CCL Cryogenic Combustion Laboratory

CFD Computational Fluid Dynamics

CLES Combustion Large Eddy Simulation

CNG Compressed Natural Gas

DS Downstream

ET Exhaust Temperature

FFT Fast Fourier Transform

GE General Electric

GTCL Gas Turbine Combustion Laboratory

ID Inner Diameter

IFC Inlet Flow Conditioner

LNG Liquefied Natural Gas

MA Main Air

MCNG Main Compressed Natural Gas

OD Outer Diameter

PA Pilot Air

PCNG Pilot Compressed Natural Gas

PIV Particle Image Velocimetry

xii

PMI Premixed Inner Fuel

PMO Premixed Outer Fuel

PD Photodiode

PLIF Planar Laser-Induced Fluorescence

PSD Power Spectral Density

PT Pressure Transducer

TC Thermocouple

US Upstream

VI LabVIEW Virtual Instrument

WI Wobbe Index

xiii

Symbols

Φ Equivalence Ratio

CD Discharge Coefficient

Mass Flow Rate

P1 Pressure Upstream of Metering Orifice

P3 Pressure Downstream of Metering Orifice

PC Chamber Pressure

PPP Peak-to-Peak Variation in Pressure

PS Static Pressure

t Time

T1 Temperature Upstream of Metering Orifice

T3 Temperature Downstream of Metering Orifice

T4 Temperature Immediately Upstream of Nozzle Injection

V Velocity

xiv

ACKNOWLEDGEMENTS

I would like to start by thanking my advisor, Dr. Robert Santoro, for his guidance and

support throughout my graduate career at Penn State. He has been a source of inspiration and

encouragement in all aspects of my graduate education, both academic and non-academic.

Having the opportunity to work with Dr. Santoro on the research presented in this thesis has been

an extremely enjoyable and educational experience for me. Additionally, Dr. Geraldine Mouis,

Dr. Sibtosh Pal and Dr. Roger Woodward have been crucially important to both the current

research effort and my development as an engineer. I am truly honored to have worked closely

with them during this study. I would also like to take the opportunity to thank Dr. Domenic

Santavicca for his time and effort in reviewing this thesis.

The research presented herein was funded by General Electric (GE) Energy. My sincerest

gratitude to Dr. Hasan Karim, Dr. Kwanwoo Kim and Ms. Christine Zemsky for their technical

and financial support for the project. Dr. Christopher Mordaunt has played an integral role in my

decision to pursue graduate studies at Penn State, as well as in the inception and continued

efforts of the research conducted. As my professor at Bucknell University and as an adjunct

professor for the current work, his contributions have been truly invaluable. I would also like to

thank Dr. Seong-Young Lee for his contributions to the project, and Ms. Jenny Houser, Mr. John

Raiser and Mrs. Ginny Smith for their outstanding administrative assistance. Mr. Larry Horner

and Mr. Larry Schaaf have been extremely helpful in providing technical assistance in all aspects

of my research, and their commitment and the quality of their work is greatly appreciated.

Finally, I would like to thank my family for their never-ending love and support.

Throughout my entire life, they have provided me with unparalleled opportunities and

understanding, which have been essential to both my personal and professional development.

1

CHAPTER 1 – INTRODUCTION

Stationary gas turbine engines have long been used a means of power generation.

However, stricter emissions standards and a changing energy market require that standard

operating practices for existing systems and design and operation of future systems adapt

accordingly. In order to achieve lower emissions, swirl-stabilized, lean premixed combustion has

commonly been used in gas turbine combustion systems. Although these systems simultaneously

reduce soot, nitrogen oxides (NOx) and carbon monoxide (CO) emissions [1], they remain

susceptible to several detrimental combustion phenomena by operating near the lean

flammability limit. Additionally, due the potential future decline of available natural gas

supplies, a need has developed for the thorough understanding of variability in fuel composition

and its effects on gas turbine engine operation.

This chapter aims to provide a foundation for the current research effort by reviewing

relevant publications and recent studies investigating these issues as well as establishing the

motivation for and approach to the current work.

1.1 – Review of Relevant Literature

1.1.1 – Lean Premixed Combustion

Early industrial gas turbine engines used strictly non-premixed systems that operated at

near-stoichiometric conditions. Additional air streams were added to ensure complete

combustion, and to cool the combustion products to the target temperature prior to entering the

turbine [1]. However, operating at these conditions produced high temperatures in the

combustion zone and led to high-levels of NOx formation via the Zeldovich (thermal)

Mechanism [1,2,3].

2

In 1990, significant amendments were made to the Clean Air Act which imposed

stringent regulations on the emission control standards from stationary sources, including

industrial gas turbines burning natural gas [4]. These amendments prompted the innovation of

low NOx burners, as decreasing NOx production in the combustor is more cost effective than

implementing post-combustion cleaning techniques [4]. By premixing the fuel and air injected

into the combustion chamber and combusting the mixture closer to the lean flammability limit, it

becomes possible to attain lower temperatures in the reaction zone, and thus, dramatically reduce

NOx emissions.

Several problems arise when using lean premixed combustion systems as systems

operating at these conditions become susceptible to severe combustion oscillations, blowout,

auto-ignition in the premixing section, flashback from the reaction zone into the premixer, and

flameholding in premixing passages. In order to avoid these potentially destructive combustion

phenomena, it is necessary to impose tight operability restrictions and limit the variability in fuel

composition for these systems.

Each of the aforementioned issues associated with lean premixed combustion has a

unique physical-chemical mechanism at its source. Combustion oscillations, as stated by the

Rayleigh Criterion, arise from a coupling of heat release and pressure fluctuations, and are

sustained when the two are roughly in phase [5,6]. As defined by Lieuwen et al. [7], blowout is

when the flame lifts off its anchored location and is physically blown out. Flashback refers to the

flame propagating upstream from its designated anchor location through the burner tube without

being quenched, and typically occurs when the flame speed is greater than the incoming mixture

air [1]. Auto-ignition occurs when the residence time and temperature of the fuel-air mixture in

the premixing section are sufficient to cause combustion of the mixture without the need of an

3

external ignition source. Flameholding, for the purposes of this research effort, will be defined as

a flame becoming disgorged, and anchoring somewhere in the premixing passage. Flashback,

auto-ignition and flameholding all have the potential to cause serious damage to the premixer

hardware as these phenomena expose the surrounding hardware to hot gases at temperatures

greater than the tolerable operating limits.

To achieve both minimal NOx emissions and stability in the combustor without flashback

or auto-ignition occurring in the premixer, careful premixing is required. Complete premixing of

the fuel and air leads to ultra-low emissions, but is difficult to realize in practical application.

Therefore, it is important to understand the effects incomplete premixing. As determined by the

research of Shih et al. [8], incomplete fuel-air mixing in the premixer can lead to increased

combustion oscillations, NOx emissions and lean blowout limit, which dramatically reduces the

range of stable combustor operation.

A commonly used technique for improving both mixing characteristics and flame

stability in the combustor is through the use of a swirler downstream of the point of fuel

injection. A comprehensive review of swirl-stabilization for lean premixed combustion is

provided by Huang and Yang [9]. In their review, they give an overview of technological

advances in low-emission industrial gas turbine engines, flow characteristics through swirled fuel

injector nozzles, combustion instabilities and instability mitigation techniques.

Several studies have also been conducted to investigate the effects of swirl-stabilization

on premixed flames. Sivasegaram and Whitelaw [10] conducted experiments to investigate

combustion oscillations in various ducted, swirled flows, including flames stabilized behind

disks, sudden expansions and annular rings. These experiments, however, were conducted with

4

ambient air inlet temperatures and focused on rough combustion rather than the discrete

frequency oscillations typically observed in fuel-lean combustion.

A study by Richards and Janus [6] was performed at industrial gas turbine relevant

conditions to characterize oscillations experienced during lean premixed combustion. Tests were

conducted at pressures up to 10 atm and temperatures up to 600°F. As found in their results,

combustion oscillations during lean premixed operation can be characterized by a nozzle

reference velocity and oscillation amplitudes appeared to scale with pressure. In their

conclusions, however, they emphasize that small changes in both nozzle geometry and operating

conditions can significantly affect combustor stability.

Fritz et al. [11] investigated flashback in premixed, swirl burners and identified four

distinct mechanisms that can cause flashback into the premixer: (1) flashback in the boundary

layer, (2) upstream flame propagation through the core region of flow, (3) combustion

instability-induced flashback, and (4) vortex breakdown leading to flashback. Flashback in the

boundary layer arises from the low velocities near the solid-wall boundaries and is typical of

non-swirled, low-turbulence flows. Flashback through the core region of flow occurs when the

flame speed is greater than the local flow velocity and is more common to swirling flames where

considerable swirl increases the turbulent flame speed. Combustion instabilities, driven by

changes in heat release, can also cause flashback through either the boundary layer or core flow.

Combustion-induced vortex breakdown occurs when a recirculation zone is formed and

propagates upstream, stabilizing in the mixing zone. This occurs at excessive swirl numbers and

is strongly geometry and flowfield dependent.

Nauert et al. [12] also investigated flashback in swirl-stabilized, lean premixed flames.

The intent of this research was to experimentally identify conditions near flashback, and to

5

capture the transition from stable burning to flashback using various diagnostic techniques,

including particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF). The

effects of increasing swirl were also examined. The results from this study were to be used for

validation of numerical simulations, such as combustion large eddy simulation (CLES).

As compared to combustion instabilities and flashback, fewer studies have been

conducted to investigate flameholding for application in industrial gas turbine engines. The

majority of flameholding research conducted to date has been focused on methods to anchor a

flame in a combustor, such as an aircraft afterburner, rather than on the avoidance of

flameholding in the premixing passages of a stationary gas turbine engine.

Samuelsen et al. [13] provided a comprehensive study to examine the mechanisms

involved in flameholding in fuel-air premixing passages at conditions typical of stationary gas

turbine engine operation. The aim of their research was to establish flameholding tendency as a

function of equivalence ratio, temperature, pressure, velocity and sudden expansion step height.

During cases where flameholding was observed only at equivalence ratios near stoichiometric,

the weak extinction limit (the lean limit below which flameholding was no longer observed) was

determined and used for comparison with previous studies. According to the results obtained

during this study, several conclusions were made by the authors that are relevant to the current

research effort: (1) the weak extinction limit for flameholding is significantly dependent on both

velocity and pressure, and is largely affected by velocity at higher pressures, (2) lower weak

extinction limits are observed at higher temperatures, higher pressures and lower velocities, and

(3) flameholding was only observed at expansion step heights greater than 0.0375 in.

An additional parameter not investigated in the aforementioned flameholding study is

fuel composition. Due the pressing need for fuel flexible systems in the near future, it is essential

6

to develop an understanding the effects of fuel variability on combustion phenomena in lean

premixed combustion systems. This is explored further in the following section.

1.1.2 – Variations in Natural Gas Composition

As current supplies of natural gas are exhausted and become insufficient to meet the

growing energy demand, it will be necessary to rely on both alternate fuels, such as syngas, as

well as bio-derived sources of natural gas, including coal bed methane and imported liquefied

natural gas (LNG) [14]. Existing sources of natural gas also vary widely in composition across

the globe, and as such, the development of fuel-flexible gas turbine systems tolerant to variations

in fuel composition is currently of great interest to the power generation community [15].

One primary concern associated with the use alternate sources of natural gas, specifically

imported LNG, is the presence of higher molecular weight hydrocarbons, such as ethane,

propane and butane. These heavier hydrocarbons have the potential to negatively impact the

performance of stationary gas turbine engines, particularly those that operate at lean premixed

conditions. For example, heavier hydrocarbons are known to have considerably lower auto-

ignition temperatures, risking auto-ignition in the fuel and air premixing section, and, in turn,

severe hardware damage due to excessive heating. Therefore, it is essential to develop a thorough

understanding of the effects of fuel variability on gas turbine engine performance. Some

performance characteristics directly affected by variations in fuel composition include emissions,

combustion dynamics, turndown ratio and tendency for auto-ignition or flashback [14].

Several studies have been conducted to specifically examine the effects of fuel variations

on lean premixed combustor performance. Lieuwen et al. [7] conducted a thorough review of

contemporary research to determine how variability in fuel composition affects combustion

phenomena previously described in Section 1.1.1. In their conclusions, the authors emphasized

7

that small changes in mixture composition can have substantial effects on operability issues, and

the behavior of mixtures can be noticeably different than that of its constituents. Therefore, it is

necessary to understand fundamental combustion properties, such as flame speed and ignition

delay times, of the mixture compositions [7].

Bourque et al. [15] performed such a study to investigate fuel blends with heavier

hydrocarbons and their respective ignition characteristics and laminar flame speeds. Three

natural gas fuel mixtures (containing as little as 62.5% methane) were examined. Results from

the study indicated that ignition delay times were shorter for fuels with greater higher-order

hydrocarbon content, and were largely dependent on pressure.

Flores et al. [16] conducted a series of experiments with ethane- and propane-natural gas

blends to examine the effect of higher hydrocarbon fuel mixtures on NOx and CO emissions

when compared to natural gas. Experiments were conducted for a model gas turbine at

atmospheric pressure. As shown in the results, heavier hydrocarbon blends led to increased NOx

and CO emissions, with NOx formation exhibiting a subtle dependence on the amount of fuel-air

premixing.

Janus et al. [17] also performed a study examining not only the effects of fuel variability,

but also changes in ambient conditions on combustion stability in swirl stabilized, lean premixed

combustors. Stability maps were generated that evaluated the effect of fuel composition, inlet air

temperature and humidity. Natural gas, propane and a natural gas/hydrogen blend were tested.

Both variations in ambient conditions and fuel compositions were shown to have an effect on the

instability region on a map of equivalence ratio versus air flow rate; and the authors advised that

gas turbine manufacturers and end-users should be aware of these effects.

8

Bosheck et al. [18] studied the effects of fuel variability on turbulent, lean premixed

combustor performance. Pure methane, methane/hydrogen and methane/propane fuels were

tested at high temperature (~750°F) and pressures (5-10 bars). Lean blowout limits were reduced

with both hydrogen and propane addition, while NOx emissions increased for both additives at

equivalence ratios greater than 0.5 with respect to pure methane. Additionally, turbulent flame

speeds were greater with both hydrogen and propane enriched fuels versus methane.

Traditionally, gas interchangeability has been defined by the Wobbe Index (WI), a figure

of merit used to determine the volumetric energy density of a gas. In short, as long as two gases

have a similar WI, they are considered interchangeable. In order to optimize the performance and

operability of industrial gas turbines, manufacturers would typically establish an acceptable WI

range. While maintaining a reasonable level of fuel consistency has been possible in the past, this

may prove difficult as alternative fuels and new sources for natural gas are utilized [14]. It also

should be noted that WI alone is not sufficient in fully describing fuel variability effects [7], but

is still relevant as a basis for fuel comparison.

In a study, performed by Rosal and Di Scipio [19], the WI of five different LNG reserves

in Venezuela were examined and compared to average WI ranges in potential major consumer

markets, including Spain, Argentina and the United States. Ultimately, it was found that while

the WI of several of these reserves fell within acceptable ranges, expensive gas enrichment

techniques would be required for three out of the five sites [19]. Although this study is only one

example, it highlights the need for fuel-flexible power generation in order to accommodate for

the necessary deviation from traditional energy sources in the near-future.

9

1.2 – Motivation for Current Work

While numerous studies have been conducted to study combustion instabilities, auto-

ignition and flashback during lean premixed operation of industrial gas turbine engines, there are

fewer that have investigated flameholding tendencies in premixing passages, and as such, the

phenomenon is not very well understood; especially, at gas turbine relevant conditions.

Additionally, with the increasing demand for alternate sources of natural gas, it will be essential

to establish a clear understanding of fuel variability effects on flameholding to both prevent

potentially catastrophic hardware damage during stationary gas turbine engine operation, and

develop design considerations that guard against the many operability issues described in the

previous sections.

The current research effort arose out of a desire to address these challenges in fuel-

flexible burner development for application in industrial gas turbine engines. The primary goals

of this effort were to: (1) determine fuel penetration and mixing characteristics in a swirl-

stabilized fuel injector nozzle for use in lean premixed combustion, (2) establish a range of

operating conditions and fuel compositions where flameholding is observed in the interior of the

nozzle, and (3) provide high-speed visual records of the combustion phenomena for future

analysis.

1.3 – Approach

To accomplish these goals it was necessary to design an optically-accessible combustor

chamber used to house a single vane of a multi-vane, swirl-stabilized, lean premixed fuel injector

nozzle. A clear view of the interior of the nozzle vane was required to capture any combustion

phenomena observed during testing. Non-reacting tests were first conducted to understand fuel-

10

air mixing characteristics inside the nozzle vane as well as relevant flowfield properties at inlet

air temperatures typical of gas turbine engine operation. During hot-fire flameholding tests, inlet

air velocities were used as a reference parameter and were extended to provide a thorough range

of possible operating conditions experienced in a single vane of a fuel injector nozzle as uniform

air distribution through the multi-vane nozzle is not guaranteed due to the complex nature of the

nozzle design. For all combustion tests, inlet air temperature, chamber pressure and equivalence

ratio were maintained at constant target levels, which were representative of lean premixed

combustion conditions in industrial gas turbines.

The following report aims to provide a thorough account of the various studies conducted

during this research effort. Chapter 2 provides a complete overview of the hardware used during

the current work as well as a discussion of several problems encountered during tests. Chapter 3

reviews a study performed to mitigate those same issues, and provides a discussion of the

experimental results from that series of tests. Chapter 4 gives an analysis of the results from the

hot-fire flameholding tests conducted on the single nozzle vane. These tests were divided into

two regimes: high velocity (air reference velocities between 100 and 190 ft/s) and low velocity

(test section air velocities between 55 and 100 ft/s). Chapter 5 provides relevant conclusions and

suggestions for future work.

11

CHAPTER 2 – EXPERIMENTAL APPARATUS

2.1 – Experimental Facilities

Two test facilities, located at the University Park campus of the Pennsylvania State

University, were used during this study: the Cryogenic Combustion Laboratory (CCL) and the

Gas Turbine Combustion Laboratory (GTCL). Use of the CCL was necessary for the

flameholding experiments described herein, as the CCL could deliver air, preheated to 1000°F, at

mass flow rates upwards of 0.75 lbm/s. Alternately, the GTCL provided a supplementary test

facility for conducting smaller-scale experiments with lower air supply demands. These tests

included an acetone-PLIF experiment to examine mixing characteristics in the nozzle, an

afterburner ignition check to test the functionality of the afterburner before use at the CCL, and

the afterburner flame stabilization study. Typically, these experiments were conducted at half of

the mass flow rate and pressure as compared to the operating conditions at the CCL.

All studies conducted at both facilities will be discussed in detail in subsequent sections

of this report.

2.2 – Air Supply Systems

Air delivery systems at the CCL and GTCL include oil filters to remove moisture from

the air supply, dome loaders and hand loaders for flow regulation, and pneumatic fire valves for

starting and ending air flow. Air flow at both facilities is metered using critical orifices, which

are choked during operation. By using choked orifices, mass flow rates are fixed by the

stagnation properties and independent of changes in conditions downstream of the orifice.

Therefore, only fluid properties of the gas and upstream temperature and pressure are required to

12

determine mass flow rates. Upstream temperature and pressure are measured using static

temperature (TC) and pressure (PT) transducers as shown in Figure 2-1.

Figure 2-1 provides a schematic for the air supply system employed during the

afterburner flame stabilization study, which was conducted at the GTCL. Main air (MA), pilot

air (PA) and afterburner pressurization (ABP) delivery lines were used during this study. The

Acetone-PLIF experiments and the afterburner ignition check only required use of the main air

supply line, and as such, the PA and ABP supply lines were disconnected. For the main air

supply, one or more critical orifices were used in parallel in order to achieve the desired air flow

rate. Main air lines not in use were closed during testing using manual two-way ball valves

located upstream of the orifices as shown. A table of critical orifices used during the afterburner

flame stabilization study is provided in Appendix A.1.

Figure 2-1 Air Supply System at Gas Turbine Combustion Laboratory

13

Figure 2-2 provides a schematic of the air delivery system used during the flameholding

experiments conducted at the CCL. Because these experiments operated at chamber pressures of

222 psia, twice that of the afterburner flame stabilization study, the ABP supply line was divided

into two legs (as shown in Figure 2-2); one of which opened at the start of each test to prevent

combustion products from entering the pressurization supply lines.

Critical orifices used for the high velocity (100-190 ft/s) and low velocity (55-100 ft/s)

flameholding experiments are provided in Appendices A.2 and A.3, respectively.

Figure 2-2 Air Supply System at Cryogenic Combustion Laboratory

2.3 – Gaseous Fuel Supply Systems

The experimental apparatus for the current study was designed to handle several different

types of gaseous fuels. All fuels were supplied to the test rig by either individual compressed gas

14

cylinders or clusters of cylinders. Compressed Natural Gas (CNG) was used as the primary fuel

supplied to the test chamber and as the pilot fuel for the afterburner. Commercial natural gas

from the building natural gas supply line was pressurized to approximately 2000 psig using a

Fuelmaker Vehicle Refueling Appliance (Model FM4). Gas chromatography was performed

prior to the high velocity flameholding experiments to determine the chemical composition of

the commercially-supplied natural gas used during experimentation. Results from this test

(shown below in Table 2-1) were used for analysis of all data collected during this research.

Table 2-1 Properties and Chemical Composition of CNG Used

Compressed Natural Gas (CNG)

Composition C1.075H4.149

Molecular Weight 17.09

Gamma 1.295 Stoichiometric Air

to Fuel Ratio 16.971

Compounds Percent by

Volume

Methane (CH4) 94.895

Ethane (C2H6) 4.028

Propane (C3H8) 0.515

Isobutane (C4H10) 0.082

Butane (C4H10) 0.104

In addition to flameholding experiments using natural gas, tests were conducted with

various mixtures of ethane and natural gas. For the ethane-natural gas experiments, ethane was

supplied to the test chamber via independent lines and mixed with the natural gas supply prior to

injection into the test section. Ethane concentrations and mixed fuel compositions were different

for each test. The chemical compositions for these fuels, along with relevant mixed fuel

properties, are provided in Appendix B.

15

Gaseous hydrogen was used for two distinct purposes: to provide a strong ignition event

immediately upstream of the fuel injector nozzle test section, and to assist with flame

stabilization in the afterburner. Hydrogen lines to the test chamber and the afterburner chamber

were supplied by separate compressed gas cylinders and were controlled independently.

Similar to the MA and ABP supply systems, gaseous fuel flows were regulated by dome

loader valves, and metered through the use of choked orifices. Fuel flow was started or stopped

using pneumatic fire valves. Because of safety concerns with using extremely combustible fuels,

such as hydrogen, provisions were made so that all fuels lines could be immediately vented and

nitrogen-purged in the event of an emergency. Gas detectors were also installed in both

laboratories to notify the operators of any gas leaks in the test cell. To clear the lines of any

combustible gas or air, all fuel lines were vented and purged with nitrogen at the beginning and

end of each test day.

2.4 – Combustor Apparatus

A test apparatus was designed and constructed in order to investigate flameholding in a

single vane of a swirled, cross-flow fuel injector nozzle with wall-mounted fuel injection ports.

The combustor apparatus (shown in Figure 2-3) consisted of two chambers: a test chamber and

an afterburner chamber. In addition to housing the fuel injector nozzle, the test chamber provided

optical access to the test section and delivered fuel and air to the fuel injector nozzle. The

afterburner chamber combusted the fuel-air mixture injected into the test chamber, and helped to

suppress the rapid rise in chamber pressure caused by hydrogen torch firing.

16

Figure 2-3 Schematic of the Combustor Apparatus

2.4.1 –Test Chamber

2.4.1.1 –Test Chamber Setup

The test chamber was designed to provide optical access to the test section through the

rear and side windows (as shown in Figure 2-3). The rear window allowed for an unobstructed,

upstream view of the nozzle vane. The side window provided direct access to the interior of the

vane, where fuel was injected through ports located in the nozzle side walls. High-speed videos

were taken from both of these locations during the flameholding experiments.

As in land-based gas turbine engine operation, preheated air was introduced upstream of

the fuel injector nozzle. The target temperature inside the fuel injector nozzle during testing was

861°F. To account for cooling of the air prior to entering the test section, air was preheated to

approximately 1000°F using an electric heater (Watlow Process Systems, 265 BTU/s (280 kW)).

17

Air from the heater was delivered to the combustor chamber through two 1-in. insulated, flexible

lines, and injected into a plenum within the outer shell of the chamber through air inlets at the

top and bottom of the chamber (as shown in Figure 2-4). After entering the plenum, air was fed

through an inlet flow conditioner (IFC). The IFC was a perforated cylinder of 2.16-in. inner

diameter (ID), and was used to evenly distribute the air prior to entering the fuel injector vane.

Figure 2-4 Section View of the Test Chamber

The test section comprised a single vane of a multi-vane, swirled fuel injector nozzle,

where inlet air mixed with fuel injected through ports located in the side walls of the vane. The

nozzle vane was machined to allow for optical access to the vane passage, while maintaining a

flow area similar to that during stationary gas turbine engine operation. The cross-sectional area

at the point of fuel injection in a single vane of the original multi-vane nozzle was 1.372 in2,

whereas the cross-sectional area at this same axial location of the trimmed, test vane was 1.276

in2. This resulted in a 7% reduction in area. Since air velocity through the vane was specified as a

18

similarity parameter, air and fuel mass flow rates were adjusted to account for this difference in

flow area.

Fuel to the injector nozzle was supplied by two separate fuel lines: the premixed inner

(PMI) fuel line and the premixed outer (PMO) fuel line (shown in Figure 2-3). The PMI line

delivered fuel to the injection ports nearest the central hub of the fuel injector nozzle, whereas

the PMO line delivered fuel to the ports nearest the outer edges of the nozzle vane (closest to the

side window of the test chamber). Ratios of inner-to-outer fuel flow rates were established, and

fuel lines and critical orifices were sized accordingly.

The gaseous mixture of fuel and air was then swirled and passed through an exit flow

passage. The design of the exit flow passage was based on a computational fluid dynamics

(CFD) calculation performed by General Electric (GE) Energy. The purpose of the CFD model

was to determine the flow field of the lean premixed mixture downstream of a single fuel injector

nozzle vane. After exiting the flow passage, the mixture flowed downstream to the afterburner

chamber by way of an exhaust tube of 2.32-in. ID.

To provide optical access to the fuel injector vane, a series of two windows were used to

provide an unobstructed view inside the vane from the side of the test chamber. A 1-in. thick

quartz window was mounted on the outer shell of the chamber and a 1/2-in. thick quartz window

was mounted on the inner chamber. The difference in thickness between the inner and outer

windows was due to the much larger pressure difference across the outer window. The rear

window was made of 1-in. thick quartz and was mounted on the outer shell of the test chamber.

In order to force ignition in the test section, a hydrogen torch was installed in the test

chamber. The torch was mounted on the outer shell of the test chamber and penetrated through

the IFC. The hydrogen torch consisted of an annular hydrogen gas flow passage around a

19

ceramic-filled, center-body spark source. During testing, a spark was created between the outer

wall of the torch and the center-body, and was fired approximately every 0.45 s. Due to the

intermittency of the spark source, proper timing was essential to ensure that at least one spark

coincided with the hydrogen flow. This guaranteed ignition of the hydrogen upstream of the

nozzle vane, and, in turn, of the premixed fuels injected into the test section.

The use of a hydrogen torch is a common technique in gas turbine engine performance

testing as it introduces a severe combustion event upstream of the point of fuel injection inducing

a significant pressure disturbance in the flow through the test section. The purpose of this

disturbance is to disgorge the flame and evaluate the possibility of flashback and, ultimately,

flameholding at an undesired location, such as within the fuel injector nozzle.

2.4.1.2 – Acetone-PLIF Experiments

Prior to the author‟s involvement in the current research effort, a non-reacting, cold-flow

study was performed at the GTCL to experimentally determine fuel jet penetration and mixing

characteristics in the test section under high pressure, high temperature conditions. The goal of

this study was to form a basis for comparison between flow characteristics in a single-vane of the

swirled fuel injector nozzle and any combustion phenomena observed during the flameholding

experiments conducted at the CCL.

The diagnostic technique used for this experiment was Planar Laser-Induced

Fluorescence (PLIF), with acetone chosen as the tracer [20]. Through the acetone-PLIF

experiments, fuel jet penetration was visualized at thirty-two distinct test points and temperature

and fuel mole fractions were measured. Results from this study were provided to GE Energy as a

basis for a CFD model of the flow field within the nozzle vane.

20

2.4.2 – Afterburner Chamber

2.4.2.1 – Afterburner Chamber Hardware

As mentioned in section 2.4, the afterburner chamber combusted the fuel-air mixture

injected into the test chamber, and minimized the pressure fluctuations in the test section after

the hydrogen torch was ignited. The afterburner chamber was designed as a dump combustor,

where any burned and unburned fuel-air mixture from the test chamber is fed to the afterburner

through a 1.8-in. ID afterburner inlet tube. Pilot injectors used to supply the afterburner chamber

with pilot fuel and additional air were installed in four of six 0.187-in. diameter holes bored at

60° intervals through the upstream flange of the afterburner chamber. The remaining two 0.187-

in. holes were capped and not used during testing. Pilot natural gas (PCNG) injectors were flush

with the combustor dump plane as shown in Figure 2-5. PCNG was fed through four 1/8-in.

tubes, while pilot air was fed through the annular region between the 1/8-in. injector tubes and

the 0.187-in. pilot holes. Pilot air was added to provide a steady supply of air to the afterburner

during hydrogen torch firing. An Autolite Copper Core automotive spark plug was used as the

ignition source for the afterburner chamber during start-up.

21

Figure 2-5 Section View of the Afterburner Chamber

The afterburner also served as a way to pressurize the test section to target pressure. This

was done through the use of a choked nozzle and an additional gas pressurization circuit. Two

nozzles were used during testing: a 0.8-in. throat diameter nozzle for the high-velocity

flameholding tests and a 0.59-in. throat diameter nozzle for the low velocity flameholding tests.

Both nozzles converged to the specified throat diameter from the core chamber inner diameter of

4-in. The depth of convergence was 1.85 in. and 1.95 in., respectively. Appendix C provides

drawings for both nozzles as well as other relevant parts and assemblies used during the current

research effort.

Additional pressurization was achieved by injecting air directly into the afterburner

chamber through six 1/4-in. ports bored through the face of the downstream nozzle (shown in

Figure 2-5). By introducing air into the test apparatus at the downstream end of the afterburner, it

was possible to increase the pressure in the test section to the desired value independent of

22

upstream air and fuel flow rates. By utilizing this method of pressurization, the need for an

expensive, high temperature-resistant throttling valve was eliminated. At the CCL, the ABP

circuit was separated into two legs. Flow through the pressurization legs was controlled remotely

by a single Tescom ER 3000 Electronic Pressure Controller coupled with a UI3000 User

Interface. Pneumatic fire valves were placed on each leg, so that they could be opened and closed

independently of one another. The first leg was opened prior to testing in order to maintain a

minimum flow through the six ports to prevent backflow through the afterburner pressurization

lines. The second leg was opened during testing to further pressurize the test chamber to the

desired target pressure.

To protect afterburner chamber hardware in the high heat flux environment, three

approaches were used. First, both the inner surface of the core chamber and face of the nozzle

were coated in zirconia (ZrO2) by Hayden Corporation. Second, cooling passages parallel to the

direction of flow were installed to cool the chamber walls, prevent the afterburner from

overheating, and avoid damage to the afterburner hardware. The six cooling passages were 1/2-

in. in diameter and were bored at even 60° intervals through the annular region surrounding the

core combustion chamber. This provided sufficient heat transfer from the combustion chamber to

the cooling fluid, while maintaining the structural integrity of the afterburner. As shown in

Figure 2-3, cooling fluid entered the afterburner cooling circuit through two inlet pipes welded to

the upstream flange of the afterburner. Cooling fluid was then distributed in an annular channel

(shown in Figure 2-5) and fed through the six cooling passages.

During the initial phases of testing, air was used as the afterburner cooling fluid. After

determining that air provided insufficient cooling for the afterburner, water was used. The

cooling water was pumped through a closed loop circuit from a 150-gallon reservoir using a

23

Franklin Electrics pump that delivered between 20 and 24 gpm of water to the afterburner. Cold

water was supplied to the 150-gallon tank by a separate line at a rate of approximately 6 gpm.

The cold water fill line was used to maintain a steady reservoir volume and temperature.

Lastly, the downstream surface of the afterburner nozzle was water cooled to maintain a

sufficiently low exhaust temperature and prevent damage to the nozzle hardware. Three 0.18-in.

holes (shown in Figure 2-5) were bored at 120° intervals through the nozzle mount to allow the

nozzle cooling fluid to impinge directly on the back surface of the nozzle. The nozzle cooling

fluid and combustion products were then expelled from the testing apparatus though an exhaust

stack.

2.4.2.2 – Afterburner Ignition Check

Prior to any flameholding experiments, it was necessary to test the functionality of the

afterburner chamber. An ignition check was performed in the GTCL to ensure that the fuel-air

mixture in the afterburner could not only ignite, but maintain a steady flame during testing. For

this study, air was preheated to 700°F using an electric heater (Watlow Process Systems, 133

BTU/s (140 kW)) and passed through the afterburner chamber. It should be noted that no pilot

air was used during this check.

Equivalence ratios used for afterburner ignition were between 0.5 and 0.6; comparable to

the target equivalence ratio during flameholding experiments. The equivalence ratio in the

afterburner was then progressively lowered in a stepwise fashion, by either increasing the main

air flow rate or decreasing the pilot fuel flow rate. The purpose being to determine the

equivalence ratio at which the flame became unstable. Ultimately, it was determined that a

steady flame was maintained in the afterburner for equivalence ratios greater than 0.15. At

equivalence ratios less than 0.15, the flame became very unstable or blew out entirely.

24

2.4.2.3 – Afterburner Flame Stabilization Test Rig

After the high velocity flameholding experiments were conducted, a study was designed

to examine large pressure fluctuations observed in the afterburner at lower reference air

velocities. This section aims to provide a discussion of the afterburner assembly used during this

series of tests. Specific details about the afterburner flame stabilization experiments and a

discussion of relevant results are presented later in Chapter 3.

Figure 2-6 provides a picture of the test apparatus at the GTCL. As shown, preheated

main air (MA) is introduced to the afterburner through a 1-in. flexible line. MA is mixed with

main compressed natural gas (MCNG) injected into the air stream through six 1/8-in. lines. The

target MCNG and MA mass flow rates were such that the equivalence ratio of the mixture

entering the afterburner was the same as that during flameholding experiments conducted at the

CCL. To evenly distribute fuel and air from individual supply lines to multiple injection lines,

fuel and air manifolds were constructed (as shown in Figure 2-6).

25

Figure 2-6 Afterburner Flame Stabilization Test Rig

Hydrogen was injected into the afterburner at the dump plane through a hydrogen

injection system consisted of an inlet hub welded to an injection tube. The hub allows for

distribution of hydrogen from four 1/4-in. inlet lines to twelve 9.15-in. long, 0.125-in. diameter

hydrogen flow passages in an 1.78-in. outer diameter (OD) insert tube. Special machining was

required to drill the narrow hydrogen flow passages. Therefore, the hydrogen injection tube was

shipped to Thompson Gundrilling, Inc. to have the flow passages gun-drilled. Hydrogen was

injected to the dump combustor through twelve 0.1-in. long, 0.016-in. ID outlet passages. Rapid

contraction at the exit served to increase the momentum of the hydrogen jets to ensure sufficient

strength of the pilot. The hydrogen injection tube was inserted into the afterburner inlet tube, as

26

shown in Figure 2-7. The injection tube has an ID of 1.05 in. A 0.44-in. thick spacer was

required such that the hydrogen injection plane was flush with the combustor dump plane.

Figure 2-7 Section View of the Hydrogen Injection System

Because the original PCB port in the afterburner inlet tube was blocked by the hydrogen

injection tube, a diagnostic hub was constructed. As shown in Figure 2-7, the diagnostic hub

housed ports for static pressure and temperature transducers as well as a high-speed pressure

transducer. A discussion of specific diagnostic equipment used during experimentation is

presented in Section 2-5.

Not shown in Figure 2-7 is an additional insert used to further decrease the flow area of

premixed gas to the afterburner. This insert was installed between the diagnostic and hydrogen

inlet hubs. The insert comprised a mount with a short converging section to reduce the flow

27

diameter from 1.05 in. to 0.76 in. welded to a 1-in. OD straight pipe with an ID of 0.76 in. The

insert was installed in the hydrogen injection tube, such that the downstream face of the insert

was flush with the dump plane of the combustor. Tests were conducted both with and without

this insert to determine the effect of increasing the main stream inlet velocity by reducing the

main jet flow area.

2.4.3 – Hardware Modifications Made During High Velocity Flameholding Tests

Several modifications to the test apparatus were made during experimentation to

troubleshoot problems encountered during the high velocity flameholding tests. During the early

stages of testing at the CCL, large pressure fluctuations were observed in the afterburner when

increasing fuel flow rates to the test section. Various corrective measures were employed to both

attenuate the amplitude of those fluctuations, and introduce a pressure drop between the

upstream and downstream chambers to prevent downstream oscillations from affecting flow

through the test section.

During the high velocity flameholding test period, three concepts were investigated: (1)

converging-diverging nozzles, (2) an auger apparatus, and (3) a multi-vane swirler. Each one of

the three devices was installed between the test section exhaust tube and the afterburner inlet

tube.

Two converging-diverging nozzles of differing throat diameters were tested: 0.841 in.

and 0.652 in. For tests with the 0.841-in. nozzle, the afterburner flame flashbacked into the inlet

tube when fuel was injected into the test section. For tests with the 0.652-in. nozzle, the pilot

flame was difficult to maintain and repeatedly blew out.

The auger device consisted of a 3-in. long, 1.75-in. diameter stainless steel auger with a

3-in. long, 1/2-in. diameter molybdenum rod welded at the end to act as a center-body

28

flameholder. The auger was mounted to a hub installed between the two chambers. The auger

was positioned such that the downstream face of the molybdenum flameholder was flush with

the dump plane. The auger not only induced a pressure drop across the apparatus, but also

introduced swirl to the flow, which is a common method for enhancing burner stability [21]. The

swirl induced by the 1.75-in. diameter auger was too great and blew out the pilot flame. A

smaller auger, with an OD of 1.375-in., was also tested, but did not survive the harsh combustion

environment.

The third tested solution, a multi-vane swirler, was successful in reducing pressure

fluctuations in the afterburner, and was used during all high velocity flameholding tests. The

swirler consisted of several vanes around a conical downstream-facing center post. It was

inserted into a mounting flange with six 3/8-in. bypass holes allowing for unswirled air flow in

the annular region between the outer diameter of the swirler and inner wall of the afterburner

inlet tube. A diverging section downstream of the swirler expanded the swirled flow, providing a

smooth transition to the 1.8-in. ID afterburner inlet tube. After a week of testing, the center post

of the swirler was dislodged leaving only the outer swirl vanes intact. Even without a center post,

however, this apparatus was effective in both reducing pressure oscillations in the afterburner

and stabilizing the afterburner flame during the high velocity experiments.

When testing at lower air velocities (~100 ft/s), there was insufficient pressurization air to

reach the target pressure in the test section. Therefore, the 0.59-in. throat diameter nozzle was

required during the low velocity flameholding experiments. Additionally, peak-to-peak pressure

fluctuations greater than 7.5% of the afterburner chamber pressure were observed at these test

section air velocities. Because of these pressure oscillations, a more in-depth study was

conducted at the GTCL to investigate alternate methods of afterburner flame stabilization at test

29

section air velocities below 100 ft/s. This study is described in detail in Chapter 3. In addition to

the installation of the 0.59-in. afterburner nozzle, a converging tube was welded to the diagnostic

hub and inserted into the exhaust tube of the test chamber allowing for smooth transition to the

smaller diameter afterburner inlet tube.

2.5 – Diagnostics and Data Acquisition

Two separate data acquisition systems were required: one for high-frequency temperature

and pressure data (recorded at 40 kHz) and one for low-frequency temperature and pressure data

(recorded at 10 Hz). To monitor high-frequency pressure oscillations in both the afterburner and

connecting tube, two water-cooled PCB Piezotronics Integrated Circuit Piezoelectric (ICP®

)

Pressure Sensors (model 113A21) were used. Data from the PCB transducers was collected and

processed using a LabVIEW program. Data was recorded for a twenty second period, during

which the hydrogen torch was fired. This allowed for data to be recorded before, during and after

hydrogen ignition in the test chamber. The program also allowed for real-time observation of any

pressure fluctuations prior to achieving target mass flow rates and pressure.

For the low-frequency system, all parameters were monitored simultaneously on a

separate computer using a custom-made LabVIEW Virtual Instrument (VI) shown in Figure 2-8.

All pressures and temperatures were recorded to a log file for the duration of any given

flameholding experiment. The VI also featured a mass flow rate calculator that monitored air and

fuel mass flow rates as well as test section equivalence ratio while testing.

30

Figure 2-8 LabVIEW VI Used for Flameholding Tests

In addition to the two data acquisition systems, several optical diagnostic devices were

used. These included a Logitech Quickcam Pro 9000 to monitor the water level in the 150-gallon

afterburner cooling tank, a Thorlabs DET110 photodiode (PD) to establish ignition in the

afterburner, a JVC Everio camcorder for low-speed (30 fps) observation of any combustion

phenomena in the test section, and a Photron FASTCAM SA1 for high-speed (5400 fps)

visualization inside the fuel injector nozzle. The high-speed Photron camera was used to show

31

the temporal evolution of any observed combustion phenomena and to determine whether or not

a flame held within the nozzle vane after hydrogen flow cessation.

Due to the high cost of the high-speed camera and its proximity to the test section, the

Photron camera was encased in a protective aluminum shell with a Lexan window installed on

the front of the casing. As shown in Figure 2-9, the camera was mounted at a 90° angle from the

direct line of view of the side window to further protect the camera and to adhere to spatial

constraints in the test cell.

Figure 2-9 Test Chamber with Photron Camera Mounted at the Side Window

2.6 – Abort Systems

In addition to the safety measures discussed in Section 2.3, automated abort systems were

used to prevent damage to the hardware and maximize the safety of the operators. For both the

afterburner flame stabilization study at the GTCL (discussed in Chapter 3), and the flameholding

tests at the CCL (discussed in Chapter 4), a six-channel abort monitor, fabricated at the

Chemistry Electronics Shop at the Pennsylvania State University, was used to ensure selected

pressures and temperatures stayed within specified ranges. Temperature and pressure ranges

32

were selected such that the system would automatically abort in the event of flame extinction in

the afterburner or flashback into the connecting tube. A manual trigger was also connected to the

abort monitor. This allowed operators to abort the test in case of a gas leak in the test cell or any

monitored temperature or pressure reaching an unsafe level.

When either an automatic or manual abort was tripped, all fuel fire valves were

automatically shut, ceasing fuel flow to the combustor apparatus. In addition, fuel vents were

opened to vent off any combustibles left in the fuel lines. After an abort, the test was brought to a

partial shutdown condition to safely evaluate the source of the abort.

In addition to the abort monitor, a timed trigger system was used to automatically record

PCB data, trigger the Photron camera and automatically shutdown the test after a pre-determined

amount of time during flameholding tests. Immediately prior to hydrogen torch ignition, the

abort monitor was deactivated and the timed trigger system was turned on. For all flameholding

tests, hydrogen flow to the torch was started two seconds after the high-speed camera began

recording. Both hydrogen flow duration and total test time were extended for several tests during

both the high and low velocity flameholding tests and will be discussed in further detail in

Sections 4.1 and 4.2, respectively.

33

CHAPTER 3 – AFTERBURNER FLAME STABILIZATION

This chapter provides an overview of the motivation, relevant operating conditions and

results of the afterburner flame stabilization study conducted at the Gas Turbine Combustion

Laboratory (GTCL).

3.1 – Introduction

As discussed in Section 2.4.3, the afterburner chamber was unstable at test section air

velocities around100 ft/s. Therefore, a separate experiment was designed at the GTCL to

investigate various methods of flame stabilization for the afterburner. Lewis and von Elbe, in

Chapter 6 of Combustion, Flames and Explosions of Gases, discussed various techniques for

stabilizing turbulent flames [22]. Common methods included the insertion of a bluff-body into

the gas stream and the addition of a sufficiently strong pilot flame to prevent blow-off of the

flame.

Introduction of a bluff-body into the gas stream creates a recirculation zone immediately

downstream of the body allowing for nearly complete combustion in this region [23]. Unburned

free stream gas mixes with hot gases in the recirculation zone creating a stable flame in the wake

region of the bluff-body. As hot gases in the recirculation zone are passed downstream, they

provide a steady ignition source for the free stream mixture [23]. As summarized concisely by

Lefebvre [24], a flameholder provides “a sheltered zone of low velocity in which flame speeds

are greatly enhanced by imparting a high level of turbulence to the primary air jets and by

arranging for hot combustion products to recirculate and mix with the incoming air and fuel.”

This stabilization technique was not employed during testing, however, due to issues regarding

34

bluff-body material and cooling, as well as the high cost associated with retrofitting the

afterburner chamber with a cross-stream, bluff-body flameholder.

Due to the low burning velocity and susceptibility to blow-off when burning natural gas,

an annular pilot flame is commonly used to anchor the main flame at the burner face and help to

prevent lift-off and potential blow-off of the flame [25]. The annular pilot flame aids in both

reducing wall quenching and providing a heat source for ignition of the main jet mixture [25]. In

order for the pilot flame to be effective, it must be of sufficient momentum to avoid the inlet

mixture from completely bypassing the pilot flame [22].

For this study, hydrogen was chosen as the pilot gas due to its fast combustion kinetics

and favorable stability characteristics. As an example, a hydrogen pilot flame was used by Lovett

to stabilize an unpulsed, turbulent propane jet flame [26]. To sufficiently stabilize the flame

during this study, only a small amount of hydrogen was required (0.5-1.5% of the total propane

mass flow rate) [26]. Using this as a basis for the current effort, experiments were conducted

both without hydrogen and with 1% by mass hydrogen to determine the effect of using hydrogen

as a supplementary pilot on afterburner flame stability.

In addition to the stabilization methods described above, the flow area of the afterburner

inlet tube was also decreased to increase the inlet velocity of the main gaseous mixture. From

data collected during the high velocity flameholding experiments (to be discussed in Section

4.1.2), it was found that the afterburner became unstable for afterburner inlet velocities less than

70 ft/s. Therefore, an area reduction from the original 1.8-in. ID inlet tube was required.

Installation of the hydrogen injection system (described in Section 2.4.2.3) reduced the main jet

inlet ID to 1.05 in. With the additional insert (also described in Section 2.4.2.3), the inlet tube ID

35

was further reduced to 0.76 in. Afterburner stabilization tests were conducted both with and

without the smaller insert tube.

3.2 – Operating Conditions and Test Matrix

Afterburner flame stability experiments were conducted at half the pressure and mass

flow rates of the CCL tests due to air compressor limitations at the GTCL. Therefore, the target

mean chamber pressure was 111 psia (as opposed to 222 psia for flameholding experiments).

Nominal test section air velocity was chosen as a parameter for comparison between the

flameholding experiments and the afterburner flame stabilization study. As such, main air (MA)

mass flow rates were calculated at the lower target pressure to match test section air velocities

during flameholding tests. Additionally, air immediately upstream of the afterburner was heated

to 800°F to ensure similar afterburner inlet air temperature. The target main natural gas (MCNG)

mass flow rate for each case was selected to achieve the same target equivalence ratio upstream

of the afterburner as in the flameholding experiments.

A test matrix (as shown in Table 3-1) was developed for the afterburner flame

stabilization study. Four test section velocities between 55 and 100 ft/s were tested with and

without 1% by mass hydrogen injected at the dump plane. Tests were also conducted with and

without the 0.76-in. ID insert, resulting in a total of sixteen tests.

36

Table 3-1 Test Matrix for Afterburner Flame Stabilization Study

Test Section

Velocity

(ft/s)

1% by

mass

Hydrogen

AB Inlet Flow

Diameter

(in.)

55 Yes 1.05

55 Yes 0.76

55 No 1.05

55 No 0.76

70 Yes 1.05

70 Yes 0.76

70 No 1.05

70 No 0.76

85 Yes 1.05

85 Yes 0.76

85 No 1.05

85 No 0.76

100 Yes 1.05

100 Yes 0.76

100 No 1.05

100 No 0.76

3.3 – Results and Discussion

Pressure fluctuations in the flow were measured at a sample rate of 10,000 Hz using PCB

pressure sensors installed in the diagnostic hub and the afterburner chamber. High-frequency

pressure data was recorded to a log file several times during each test. This was done to collect

data at various chamber pressures and equivalence ratios ranging from 0.475 to 0.625.

For each log file, a fast Fourier transform (FFT) was performed on the raw data (in volts)

to develop a power spectral density (PSD) useful in determining dominant frequencies of

pressure oscillations in the flow. The PSD was integrated across the entire range of frequencies

(0-5000 Hz) to calculate the variance in the oscillations at each location in the flow. The square

root of the variance (standard deviation) was multiplied by (for sinusoidal waves) to

37

calculate the peak-to-peak variations. A voltage-to-pressure conversion factor, specific to each

PCB transducer, was applied to change the peak-to-peak calculations to units of pressure (psi).

Peak-to-peak variations were divided by the mean static pressure at the time of each high-

frequency pressure recording to establish a non-dimensional figure of merit, PPP/PS.

Tables 3-2 and 3-3 provide results at target operating conditions for all tests shown in

Table 3-1. Percent pressure oscillations in the flow at both locations are given. „Hub‟ refers to

data collected from the pressure sensor installed in the diagnostic hub (Figure 2-7), while

„Chamber‟ refers to data from the pressure sensor installed in the wall of the afterburner core

chamber (Figure 2-5). Shaded rows in Tables 3-2 and 3-3 represent tests where 1% by mass

hydrogen was injected at the dump plane.

As shown in the tables, there was little change in static pressure between the hub and the

afterburner. However, since there were considerably larger differences in pressure fluctuations

exhibited in the chamber than in the hub for the various cases, percent oscillations in the

chamber were used for analysis and case comparison. As expected from the results of the high

velocity flameholding experiments (discussed in Section 4.1.2), tests more stable at higher

reference air velocities. It should be noted that full target pressure was not reached for the 55 ft/s

cases due to air compressor limitations at the GTCL, as more air was required for afterburner

pressurization than could be supplied.

38

Table 3-2 Stabilization Test Results with 1.05-in. Inlet Flow Diameter

When comparing results presented in Table 3-2 and Table 3-3, it is evident that pressure

oscillations in both the hub and chamber were appreciably reduced when testing with the 0.76-in.

ID insert. This can be attributed to the fuel-air mixture velocity at the dump plane being

increased by a factor of approximately two. For the 100 ft/s cases, percent pressure oscillations

were greater when testing with the 0.76-in. ID insert tube, suggesting an upper stability limit in

dump plane velocity. For these cases, the addition of the hydrogen pilot decreased pressure

oscillations in both the hub and chamber, indicating a positive impact on afterburner flame

stability at higher velocities. This effect was most likely due to the hydrogen pilot providing a

stable flame anchor to inhibit lift-off of the primary flame.

Table 3-3 Stabilization Test Results with 0.76-in. Inlet Flow Diameter

Test Section

Velocity

(ft/s)

Afterburner

Inlet Velocity

(ft/s)

1% by

mass

Hydrogen

Inlet T

(°F)

Upstream

Φ

Hub P

(psi)

Chamber P

(psi)

Hub

Oscillations

(%)

Chamber

Oscillations

(%)

55 99.78 No 810.50 0.569 86.6 86.1 2.294 7.897

55 89.59 Yes 816.15 0.565 95.82 96.56 2.478 6.637

70 97.9 No 768.86 0.576 106.02 106.37 2.615 12.383

70 96.56 Yes 787.40 0.567 110.5 111.46 2.165 6.874

85 118.03 No 794.08 0.575 109.66 110.5 1.037 0.820

85 117.94 Yes 791.58 0.571 111.15 111.38 0.988 0.951

100 134.19 No 786.13 0.577 111.84 112.39 0.651 0.675

100 136.21 Yes 792.53 0.578 110.08 111.13 0.622 0.653

Test Section

Velocity

(ft/s)

Afterburner

Inlet Velocity

(ft/s)

1% by

mass

Hydrogen

Inlet T

(°F)

Upstream

Φ

Hub

Pressure

(psi)

Chamber

Pressure

(psi)

Hub

Oscillations

(%)

Chamber

Oscillations

(%)

55 158.06 No 817.82 0.565 105.73 105.33 0.406 0.933

55 155.8 Yes 810.47 0.559 107.12 106.9 0.464 1.090

70 187.04 No 813.55 0.568 112.42 111.63 0.399 0.697

70 185.87 Yes 812.94 0.57 112.35 111.85 0.372 1.183

85 224.06 No 802.47 0.573 112.16 111.2 0.432 1.002

85 224.47 Yes 812.49 0.574 112.52 111.52 0.485 0.921

100 266.9 No 811.71 0.57 112.31 110.68 2.228 1.281

100 270.67 Yes 818.72 0.568 111.43 109.91 0.377 0.901

39

Figure 3-1 provides percent pressure oscillations in the chamber plotted against inlet

equivalence ratios for the 55 ft/s test section velocity cases. During tests conducted with the

0.76-in. ID insert tube (indicated by circular markers), pressure oscillations were approximately

equal to or less than 2% at all equivalence ratios. For tests without the insert (indicated by

triangular markers), pressure oscillations were greater than 2% for numerous test conditions. At

equivalence ratios near 0.5 and 0.6, the hydrogen pilot reduced pressure oscillations in the

chamber.

Figure 3-1 Percent Pressure Oscillations in the Chamber for Tests at 55 ft/s

0

2

4

6

8

10

12

14

16

0.475 0.500 0.525 0.550 0.575 0.600 0.625

% P

resu

sre

Osc

illat

ion

s

Equivalence Ratio, Φ

0.76 in. ID (-H2) 0.76 in. ID (+H2)

1.05 in. ID (-H2) 1.05 in. ID (+H2)

40

In Figure 3-2, percent pressure oscillations in the afterburner chamber are presented for

tests at conditions corresponding to a 70 ft/s test section velocity. Again, oscillations were less

than 2% of the afterburner chamber pressure for a range of equivalence ratios when testing with

the insert tube. Severe pressure oscillations (>10%) were only experienced at equivalence ratios

less than 0.5 for tests without the hydrogen pilot. Therefore, the addition of 1% by mass

hydrogen injected at the dump plane reduced pressure oscillations at lower equivalence ratios,

and, thus, increased the operability range. This allowed for greater flexibility when increasing

fuel and air mass flow rates to their respective target values.

Figure 3-2 Percent Pressure Oscillations in the Chamber for Tests at 70 ft/s

0

2

4

6

8

10

12

14

16

0.475 0.500 0.525 0.550 0.575 0.600 0.625

% P

resu

sre

Osc

illat

ion

s

Equivalence Ratio, Φ

0.76 in. ID (-H2) 0.76 in. ID (+H2)

1.05 in. ID (-H2) 1.05 in. ID (+H2)

41

Figure 3-3 provides a graphical representation of pressure oscillations in the afterburner

chamber for a range of equivalence ratios during the 85 ft/s tests. For tests conducted with an

inlet flow diameter of 1.05 in. and no hydrogen pilot, the afterburner chamber was highly

unstable (>7% oscillations) at equivalence ratios near 0.5 and 0.6. Again, instabilities

experienced at these border equivalence ratios were reduced when adding hydrogen pilot. Also,

the use of the insert tube helped to further reduce pressure oscillations observed in the chamber.

Figure 3-3 Percent Pressure Oscillations in the Chamber for Tests at 85 ft/s

0

2

4

6

8

10

12

14

16

0.475 0.500 0.525 0.550 0.575 0.600 0.625

% P

resu

sre

Osc

illat

ion

s

Equivalence Ratio, Φ

0.76 in. ID (-H2) 0.76 in. ID (+H2)

1.05 in. ID (-H2) 1.05 in. ID (+H2)

42

Figure 3-4 presents pressure oscillations for the 100 ft/s cases. As shown, oscillations in

the chamber were less than 3% at all equivalence ratios for all cases. For this test section

velocity, both the addition of hydrogen and installation of the 0.76-in. ID insert tube had little

effect on the stability in the chamber. As shown in Table 3-3, use of the insert tube without the

hydrogen pilot increased pressure oscillations, demonstrating the benefit of the supplementary

hydrogen pilot flame.

Figure 3-4 Percent Pressure Oscillations in the Chamber for Tests at 100 ft/s

0

2

4

6

8

10

12

14

16

0.475 0.500 0.525 0.550 0.575 0.600 0.625

% P

resu

sre

Osc

illat

ion

s

Equivalence Ratio, Φ

0.76 in. ID (-H2) 0.76 in. ID (+H2)

1.05 in. ID (-H2) 1.05 in. ID (+H2)

43

As found during this study, the use of both the 0.76-in. ID insert and the hydrogen pilot

yielded minimum instabilities for the widest range of test conditions. By decreasing the

afterburner inlet flow area, inlet velocities to the afterburner were increased, showing favorable

stability in afterburner. However, for the highest velocity case with the insert installed, percent

oscillations in both the hub and chamber were substantially larger when compared to lower

reference velocity cases. These oscillations were dramatically reduced through the use of the

hydrogen pilot, as the hydrogen flame most likely provided a stable anchoring region for the

main flame at these higher inlet velocities. Additionally, the hydrogen pilot reduced combustion

instabilities in the afterburner at fuel-lean conditions (Φ < 0.5), increasing the stable operating

range of equivalence ratios while reaching target conditions during flameholding tests.

Therefore, for the low velocity flameholding experiments (described in Section 4.2), use

of both the hydrogen pilot and the 0.76-in. ID insert tube was employed. For all tests conducted,

the supplementary hydrogen pilot flame was lit prior to introducing fuel to the nozzle vane.

Discussion of the operating conditions and relevant results from both the high and low velocity

flameholding experiments is presented in the following chapter.

44

CHAPTER 4 – FLAMEHOLDING EXPERIMENTS

This chapter describes flameholding experiments conducted at the Cryogenic

Combustion Laboratory (CCL). High velocity flameholding tests were classified as tests with

nominal test section air velocities between 100 and 190 ft/s. Low velocity flameholding

experiments consisted of tests with nominal test section air velocities from 55 to 100 ft/s.

4.1 – High Velocity Flameholding Tests

4.1.1 – Operating Conditions and Test Plan

Initially, it was presumed that flameholding would occur within the fuel injector nozzle

near the point of fuel injection. To establish a threshold for flameholding, a range of reference

velocities were tested. For the high velocity flameholding experiements, test section air velocities

ranged from 100 to 190 ft/s. Since no flameholding was observed in the nozzle vane during this

series, the velocity range was further extended to include lower velocities (discussed in Section

4.2) as uniform air distribution through the multi-vane fuel injector nozzle may not be achieved

under practical operating conditions due to the complexities of the nozzle design.

Other parameters, premixed equivalence ratio, test section mixture temperature and

pressure had set target values (provided in Table 4-1) that were maintained across both the high

and low velocity flameholding tests. These target values were based on conditions typical of

full-scale, lean premixed gas turbine engine operation.

Table 4-1 Target Flameholding Test Conditions in the Nozzle Vane

Equivalence Ratio, Φ 0.5-0.6

Pressure, P (psia) 222

Mixture Temperature, T (°F) 861

45

For all flameholding tests, the afterburner was lit using pilot natural gas and pre-heated

main air at half of the target air mass flow rate. After the afterburner pilot flame was lit and

stable, fuel to the fuel injector nozzle was introduced. Premixed inner and outer fuel mass flow

rates were alternately increased with the main air mass flow rate in a step-wise fashion to

maintain a test section equivalence ratio between 0.5 and 0.6. Staying within this equivalence

ratio range ensured that the afterburner flame did not blow out or become unstable (see Chapter

3). Once the premixed fuel and main air flow rates reached their target values, afterburner

pressurization air was increased to pressurize the test chamber to the target pressure. When all

conditions were within ±5% of their respective targets, the hydrogen torch was fired, and the

high-frequency data acquisition system and high-speed camera were triggered to record.

4.1.2 – Results and Discussion

Test section air velocities for this testing period were decreased in 10 ft/s increments

from 190 to 140 ft/s with compressed natural gas (CNG) as the fuel. Lower velocities were also

tested, but target pressure was not achieved due to air compressor limitations when testing with

the 0.8-in. throat diameter afterburner nozzle. In addition to natural gas tests, experiments were

conducted with an ethane-natural gas mixture (30% by volume ethane) at nominal test section air

velocities of 140 ft/s and 115 ft/s.

In all, thirteen independent tests were conducted where the hydrogen torch was fired. The

test conditions reached prior to hydrogen torch ignition for each of the high velocity

flameholding experiments are summarized in Table 4-2. For more detailed tabulated results,

including air and fuel mass flow rates for each test, refer to Appendix D.1. Prior to the tests listed

in Table 4-2, a series of preliminary tests were conducted without the hydrogen torch to develop

optimal testing practices, and to test the various pressure drop devices discussed in 2.4.3.

46

Although no flameholding was observed in the test section for any velocity or fuel

composition, results from this series of experiments provided the motivation for the afterburner

flame stabilization study (Chapter 3) and the low velocity flameholding tests (Section 4.2).

Table 4-2 High Velocity Flameholding Test Results

Test Number

Date Test

Section V (ft/s)

Test Section P

(psia)

Test Section T

(°F)

Test Section

Φ Fuel Type

Camera Mount

Chamber Oscillations

(%)

H1 10/21/08 190.12 218.06 851.29 0.594 CNG Rear 4.45

H2 10/21/08 177.01 217.9 853.71 0.599 CNG Rear 2.48

H3 10/21/08 168.42 214.21 857.59 0.597 CNG Rear 7.23

H4 10/22/08 158.09 217.24 863.35 0.599 CNG Rear 6.38

H5 10/23/08 145.9 218.01 873.05 0.606 CNG Rear 3.25

H6 10/23/08 147.66 217.85 900.3 0.636 CNG Rear 2.57

H7 10/27/08 157.32 219.01 867.61 0.594 CNG Side 4.68

H8 10/27/08 148.81 228.13 866.13 0.603 CNG Side 4.34

H9 10/27/08 140.09 218.95 861.46 0.613 CNG Side 3.30

H10 10/28/08 142.34 150.28 869.69 0.562 30% C2H6 Side 1.96

H11 10/29/08 116.44 184.8 866.62 0.576 CNG Side 7.88

H12 10/30/08 103.4 205.93 897.65 0.58 CNG Side 7.93

H13 10/31/08 114.92 182.8 860.71 0.584 30% C2H6 Side 4.43

As shown in the Table 4-2, tests H1-H5 were conducted within 5% of the target

conditions. For test H6, target temperature was increased to 900 °F and premixed fuel mass flow

rates were increased by 10% to provide more favorable flameholding conditions. For tests H7-

H13, the Photron camera was moved to the side window. Tests H7 and H8 were at identical

target conditions as tests H4 and H5, respectively, to evaluate the reproducibility of the results

once the camera was moved. During test H7, the Photron camera was not triggered, but high-

and low-frequency data were collected.

Test H10 was the first test where an ethane-natural gas mixture was injected into the

nozzle vane. Due to concerns with hardware damage during this initial test, target pressure was

47

reduced to 150 psia, while the target velocity of 140 ft/s was maintained. Test H13 was

conducted using the same 30% by volume ethane-natural gas mixture, but was run at a lower test

section velocity and the maximum attainable pressure (in this case, 182.8 psia). Similar

combustion phenomena were observed on the high-speed videos for both ethane-natural gas

tests, and are discussed further in Section 4.1.3.

For the first twelve tests, hydrogen flow to the torch igniter was set for a duration of 0.5

seconds with the total test time of 3.4 seconds. For the final test, hydrogen torch flow and total

test time were extended by 0.3 seconds to 0.8 seconds and 3.7 seconds, respectively. This was to

allow the test section hardware to heat up due to the increased time of burning in the nozzle

vane, raising the temperature of the walls and increasing the likelihood of observing

flameholding in the test section. However, no flameholding was detected within the nozzle vane

for any of the thirteen tests.

For tests H11 and H12, low-frequency (~75 Hz) pressure fluctuations in excess of 7.5%

of the afterburner chamber pressure were observed in the afterburner. These fluctuations raised

the issue of afterburner flame stability at lower test section air velocities, and prompted the

afterburner flame stabilization study, previously discussed in Chapter 3. Several attempts were

made to conduct additional experiments at test section velocity lower than 115 ft/s, but were

aborted due to exceedingly high temperatures in the test section exhaust tube. It was speculated

that these high temperatures were caused by the afterburner flame flashing back into the

afterburner inlet tube during pressurization of the combustor apparatus. In order to protect

afterburner hardware and prevent flashback into the inlet tube during subsequent tests, the

afterburner flame stabilization study was developed to investigate methods to improve flame

stability and arrest flashback at low test section air velocities (< 100 ft/s).

48

4.1.3 – High-Speed Video Summary

All high-speed videos began recording two seconds before the hydrogen torch was

ignited and continued recording for a pre-determined amount of time after hydrogen flow to the

torch was shut-off. For both high and low velocity flameholding tests, no burning was observed

prior to hydrogen torch ignition. Once the hydrogen torch was ignited, hydrogen fed to the torch

burned steadily with the fuel-air mixture in the test section as shown in Figures 4-1 and 4-2.

For the higher velocity cases (190 ft/s to 140 ft/s) the Photron camera was mounted at the

rear window to protect the camera during the early stages of testing. From this perspective, it was

difficult to discern the exact location of combustion events occurring within the nozzle due to

depth-of-field issues as shown in Figure 4-1. The snapshot in Figure 4-1 was taken while

hydrogen was flowing to the torch igniter after hydrogen torch ignition during test H5.

Figure 4-1 Hydrogen Torch Flow from Rear Window during Test H5

49

As stated in Section 4.1.2, the camera was moved to the side window for the lower

velocity (150 to 100 ft/s) tests to capture high-speed recordings of any phenomena in the interior

of the nozzle vane. Figure 4-2 provides a view from the side window towards the end of

hydrogen flow during test H8 (same target conditions as test H5). It should be noted that the tip

of the hydrogen torch was out of the field of view from the side window.

Figure 4-2 Hydrogen Torch Flow from Side Window during Test H8

Although the fuel-air mixture burned consistently throughout the duration of hydrogen

flow to the torch, no distinct flame anchoring was observed at the locations of the fuel injection

ports in the nozzle vane during this series of tests. Illuminated particles seen in Figure 4-2 can

possibly be attributed to melted metallic particles in the test chamber or contaminants in the

incoming air flow from the compressor. Though these particles were not intended, their presence

aided in distinguishing flow direction in high-speed recordings of the flameholding experiments.

Once hydrogen supply to the torch was shut-off, any flame within the nozzle vane was

immediately blown downstream by the incoming flow. There were, however, several flow

reversals that occurred after shutting off the hydrogen flow due to pressure oscillations in the

chamber. The significant pressure spike first introduced by hydrogen torch ignition and the

50

subsequent decrease in total mass flow rate when the hydrogen is shut-off is the mostly likely

cause of oscillations observed in the test chamber. During these flow reversals, back flow would

stagnate either upstream of the nozzle vane or in the nozzle vane itself (as shown in Figure 4-3)

leading to ignition of the fuel-air mixture in the vane. Despite many flameholding opportunities

during these repeated ignition events, any burning in the vane itself was immediately blown

downstream by the incoming flow or upstream by a subsequent flow reversal.

Figure 4-3 Flame Stagnation in Nozzle Vane during Test H12

Figure 4-3 was taken after hydrogen flow cessation (time stamp: 3.016667 s) during test

H12. The flame shown briefly stagnates on the pressure side of the nozzle vane (bottom curved

surface of the vane) before being forced upstream of the nozzle vane by a flow reversal. The

ignited natural gas-air mixture in the vane was then instantly blown out by incoming flow.

During both ethane-natural gas tests (H10 and H13), a faint glow was observed in the

upper-right-hand corner of the field of view for much of the duration of the high-speed video

recordings. This was possibly indicative of a flameholding condition occurring downstream of

the nozzle vane. In addition, periodic flame pulsations in and out of the field of view were

clearly observed after shutting off the hydrogen flow to the torch for both tests H10 and H13. A

51

snapshot of a flame pulsing into the field of view during Test H13 (time stamp: 3.208333 s) is

provided in Figure 4-4.

Figure 4-4 Flame Pulsing into Field of View during Test H13

Because no flameholding was observed in the vane during the high velocity experiments,

it was recommended to further extend the test section air velocity range to include velocities

between 55 and 100 ft/s. Events similar to the pulsing flame shown in Figure 4-4 were also

observed during the ethane-natural gas mixture tests in the low velocity range. An extensive

discussion of the low velocity flameholding experiments is provided in Section 4.2 below.

4.2 – Low Velocity Flameholding Tests

4.2.1 – Operating Conditions and Test Plan

For the low velocity flameholding tests, various fuel compositions were studied at test

section air velocities ranging from 100 to 55 ft/s. In order to achieve the target test section

pressure of 222 psia at lower nominal air velocities, a 0.59-in. throat diameter afterburner nozzle

was used (as opposed to the 0.8-in. throat diameter nozzle used previously). Both the hydrogen

injection tube and 0.76-in. insert from the afterburner flame stabilization study were installed in

52

the afterburner inlet tube to reduce pressure fluctuations in the afterburner when increasing fuel

and air mass flow rates to their respective target values prior to hydrogen torch ignition.

The same target test section conditions (pressure, temperature and equivalence ratio)

were set for both the high and low velocity flameholding tests (see Table 4-1 for target values).

Natural gas tests were conducted at air velocities between 95 and 65 ft/s. Hydrogen torch

duration and total test time were extended for natural gas tests at 90 ft/s in order to provide a

comparison with extended duration ethane-natural gas tests. Several mixture compositions of

ethane-natural gas were tested: 18, 22.5, 24 and 28% by volume ethane. Due to the limited

availability of critical orifices used for flow metering, potential test section air velocities were

constrained by the composition of the ethane-natural gas blend. Table 4-3 provides the

corresponding test section velocity for each ethane-natural gas composition used during this test.

Critical orifices used during the flameholding tests are given in Appendix A.

Table 4-3 Test Section Velocities for Various Ethane-Natural Gas Mixtures

Percent by Volume Ethane (%) Test Section V (ft/s)

18 90

22.5 75

24 65

28 55

The time period for both hydrogen flow to the torch igniter and the length of each test

(from camera triggering to fuel flow shut-off) were extended for several cases. Hydrogen flow

duration was extended to further heat the test section hardware, and increase the likelihood of

flameholding in the nozzle vane. Total test duration was increased to allow for sufficient time

after the hydrogen flow was shut-off to observe flameholding anywhere in the test chamber. To

avoid damage to the hardware due to excessive heating, hydrogen torch flow duration was

53

increased in small increments. Once a complete set of tests was performed with the Photron

camera mounted at the side window, hydrogen flow duration was increased to 2.0 s for a single

test with a 28% ethane-natural gas blend at 55 ft/s. Since no hardware was damaged during this

test, the high-speed camera was moved to the rear window, and certain tests of interest were

repeated. Run conditions, timing and results for all tests are discussed in the following section.

4.2.2 – Results and Discussion

During the low velocity flameholding tests, a total of eighteen tests were conducted

where the hydrogen torch was ignited. Throughout all of these tests, no flameholding was

observed in the test section. Despite extending the test duration for the natural gas experiments,

no steady burning in the test chamber was observed once the hydrogen flow to the torch was

shut-off for any velocity condition. The only ignition events that occurred during natural gas

tests were re-ignitions in the vane resulting from flow reversals similar to those seen during the

high velocity flameholding tests. As in the high velocity tests, these flames were immediately

blown out by the incoming flow. During the ethane-natural gas tests, extending the total test

duration allowed for marked improvement in the observation of combustion phenomena in the

test chamber after hydrogen flow was shut-off. As will be discussed later in this section, there

were several indications of steady burning occurring downstream of the test section during all

experiments conducted with ethane-natural gas mixtures.

As shown in Table 4-4, low-frequency data was collected for all tests except for test L14.

However, both high-frequency data and a high-speed video were recorded during test L14. For

completeness, these conditions were repeated during test L17 after the Photron camera had been

moved to the rear window. It should be noted, then, that high-speed videos were recorded from

both the side and rear window for this case, as well as natural gas at 90 ft/s (Tests L13 and L18),

54

and 28% by volume ethane-natural gas at 55 ft/s (Tests L8 and L16). More detailed tables of

results from the low velocity flameholding tests are provided in Appendix D.2.

Table 4-4 Low Velocity Flameholding Test Results

As shown in Table 4-4, pressure oscillations in the afterburner prior to hydrogen torch

ignition appeared to be a direct function of test section velocity with little to no dependence on

mixture composition. Low-frequency (~75 Hz) pressure fluctuations on the order of 10% were

observed for 55 ft/s cases, suggesting that while the addition of the hydrogen pilot and insert tube

were effective flame stabilization methods at higher velocities in this range, they were not

sufficient for the lowest velocity case. Since there was evidence of burning downstream of the

test section for all ethane-natural gas mixtures at velocities from 55 ft/s up to 90 ft/s, it was

determined that this behavior was not driven by combustion instabilities in the afterburner.

Test

NumberDate

Test

Section V

(ft/s)

Test

Section P

(psia)

Test

Section T

(°F)

Test

Section

Φ

Fuel Type

Test Duration

(H2 Duration)

(s)

Camera

Mount

Chamber

Oscillations

(%)

L1 12/09/09 96.25 217.32 860.15 0.567 CNG 3.7 (0.8) Side 1.61

L2 12/16/09 75.51 216 838.79 0.574 CNG 3.7 (0.8) Side 1.70

L3 12/17/09 66.29 203.08 869.98 0.577 CNG 3.7 (0.8) Side 5.47

L4 12/18/09 76.59 216.82 860.87 0.57 22.5% C2H6 3.7 (0.8) Side 2.22

L5 12/22/09 64.77 221.9 860.41 0.579 24% C2H6 3.7 (0.8) Side 5.10

L6 12/22/09 56.25 217.9 871.32 0.592 28% C2H6 3.7 (0.8) Side 11.52

L7 12/23/09 57.11 219.27 877.04 0.581 28% C2H6 4.7 (0.8) Side 10.67

L8 12/23/09 57.16 218.22 872.65 0.577 28% C2H6 5.7 (0.8) Side 10.81

L9 01/04/10 58.32 211.85 865.81 0.569 28% C2H6 5.0 (1.1) Side 9.12

L10 01/04/10 58.84 212.75 867.33 0.562 28% C2H6 5.2 (1.3) Side 10.19

L11 01/05/10 88.22 229.19 861.37 0.576 24% C2H6 5.0 (1.1) Side 1.32

L12 01/06/10 89.8 224.87 862.54 0.581 CNG 4.7 (0.8) Side 1.10

L13 01/06/10 91.65 219.54 864.14 0.566 CNG 5.7 (0.8) Side 1.09

L14 01/06/10 n/a n/a n/a n/a 18% C2H6 5.7 (0.8) Side n/a

L15 01/06/10 56.28 218.62 859.7 0.587 28% C2H6 5.9 (2.0) Side 11.41

L16 01/07/10 56.82 214.35 859.65 0.583 28% C2H6 5.7 (0.8) Rear 11.15

L17 01/08/10 89.03 225.51 859.97 0.574 18% C2H6 5.7 (0.8) Rear 1.14

L18 01/08/10 90.63 221.814 866.35 0.571 CNG 5.7 (0.8) Rear 1.09

55

High-frequency temperature and pressure data are displayed in the following series of

figures. Although both hydrogen torch duration and total test duration varied for most tests, all

flameholding tests began at the same point. Time “zero” refers to the time when the Photron

camera was triggered. The hydrogen torch was ignited 2.3 seconds later. For Figures 4-5 through

4-8, any instantaneous spikes in the temperature data represent noise from spark firings, and

should not be considered as relevant temperatures.

Temperature data shown in the following figures was collected by the downstream (DS)

thermocouple (TC) located in the diagnostic hub, unless otherwise indicated. Upstream (US) TC

measurements refer to those made by the thermocouple installed in the test chamber exhaust

tube. During all low velocity flameholding tests, temperatures in the connecting tube between the

test chamber and the afterburner chamber exceeded the thermocouple saturation temperature of

2300°F for the duration of hydrogen flow to the torch igniter. For some of the ethane-natural gas,

these high temperatures were maintained in the test chamber exhaust tube until premixed fuel

flow to the nozzle vane was shut-off. Therefore, temperature data collected by the DS TC were

used for comparison between natural gas and ethane-natural gas tests.

Figure 4-5 provides high-frequency temperature data collected from both the US and DS

connecting tube TC‟s to demonstrate the differences during a natural gas test at 90 ft/s (Test L18)

and an ethane-natural gas test at 55 ft/s (Test L8). Hydrogen flow duration and overall test

duration for both tests shown were 0.8 s and 5.7, respectively. It should be noted that the US TC

failed during hydrogen flow in Test L13 (other natural gas test at 90 ft/s), and temperatures

measured by the US TC exceeded 2300°F for the entirety of Test L16 (other ethane-natural gas

test at 55 ft/s). As such, these tests were not shown in Figure 4-5. However, for the tests

displayed in Figure 4-5, temperatures for both cases were higher in the test chamber exhaust

56

tube. For the natural gas test, increased temperatures can be attributed to proximity of the US TC

to burning in the nozzle vane during hydrogen flow. As shown, once the hydrogen flow to the

torch was shut-off, temperatures throughout the connecting tube steadily decreased to the steady

state temperature of roughly 800°F. For the ethane-natural gas test, maintained temperatures in

the test chamber exhaust tube were approximately 150°F greater than that in the diagnostic hub

after hydrogen flow to the torch ended. This provided insight into the possible location of a

flame anchoring downstream of the nozzle vane as the temperatures observed in the test chamber

exhaust tube were indicative of products of localized combustion occurring somewhere

upstream.

Figure 4-5 High-Frequency Temperature Data for Tests L8 and L18

57

Figures 4-6 through 4-9 display high-frequency temperature data collected by the DS

TC. These figures are given to examine the effect of extending total test duration for natural gas

and ethane-natural gas tests, extending hydrogen torch flow duration for 28% ethane natural gas

tests, and to provide a comparison between the temperature traces for three fuel compositions

(natural gas, 18% ethane-natural gas, and 28% ethane-natural gas) with similar hydrogen flow

and overall test durations.

Figure 4-6 (shown on next page) provides temperature data for natural gas tests at various

test section air velocities. Dashed lines represent tests with an overall test duration of 3.7 s, while

solid lines indicate those with an overall test duration of 5.7 s. Hydrogen duration was 0.8 s for

all tests shown. Tests L1, L2 and L3, were at velocities of 95 ft/s, 75 ft/s and 65 ft/s,

respectively, while tests L13 and L18 were both conducted at air velocities of 90 ft/s. As shown,

all natural gas tests exhibited similar behavior. A sharp decline in temperature was observed

immediately after hydrogen flow to the torch was shut-off for all tests. Aside from occasional

fluctuations in temperature due to re-ignitions in the nozzle vane, temperatures steadily

decreased to approximately 800°F over a second prior to shutting off fuel flow to the nozzle vane

during tests L13 and L18, indicating that no flameholding occurred anywhere in the test chamber

after hydrogen flow ended for tests with natural gas as the fuel.

58

Figure 4-6 High-Frequency Temperature Data from DS TC for CNG Tests

59

Figure 4-7 (next page) shows the high-frequency temperature data for tests conducted

with a 28% by volume ethane-natural gas mixture. Again, solid lines represent tests with an

overall test duration of 5.7 s, while dashed lines indicate tests with test durations less than 5.7 s.

For convenience, overall test duration is provided in the legend. As shown, temperatures in the

diagnostic hub were maintained at approximately 2000°F until fuel flows were shut-off and the

test was ended during all 28% ethane-natural gas mixtures. This behavior was only explicitly

observed once total test duration was extended to greater than 3.7 s. As previously discussed, this

was indicative of a flame holding downstream of the nozzle vane as the temperatures measured

in the diagnostic hub were consistent with the temperature of product gases. It should also be

noted that temperatures measured by the US TC for tests L6 and L7 were similar to US TC

temperatures during test L8 (shown in Figure 4-5), and were uniformly greater than temperatures

collected at the DS TC.

60

Figure 4-7 High-Frequency Temperature Data from DS TC for 28% C2H6 Tests

61

Figure 4-8, portrays the effect of increasing hydrogen torch flow duration during tests

with 28% ethane-natural gas mixtures. Total test durations for each test are provided in the

legend. Hydrogen duration for tests L9, L10 and L15 were 1.1, 1.3 and 2.0 s, respectively, while

hydrogen flow duration for tests L8 and L16 was 0.8 s. As shown, temperatures in the diagnostic

hub were greater than 2300°F during hydrogen flow for all tests. Once hydrogen flow to the

torch igniter was shut-off, DS TC temperatures exhibited an immediate decline to approximately

2000°F for all hydrogen torch flow durations. Although no flameholding was observed in the

nozzle vane for any of these cases, temperatures established once hydrogen flow ended were

once again indicative of a flame anchoring some distance downstream of the test section.

Figure 4-8 DS TC Data for 28% C2H6 Tests with Various H2 Torch Flow Durations

62

Figure 4-9 (next page) shows DS TC temperature data for tests with three different fuel

compositions. Tests L13 and L18 were conducted with natural gas at 90 ft/s, tests L14 and L17

with 18% ethane-natural gas at 90 ft/s, and tests L8 and L16 with 28% ethane-natural gas at 55

ft/s. Hydrogen torch flow and overall test durations for all tests were 0.8 and 5.7 s, respectively.

As shown, tests with ethane-natural gas mixtures exhibited similar behavior despite differing fuel

compositions and distinct test section air velocities. This similarity supports the hypothesis that

the downstream flameholding condition seen during the ethane-natural gas tests was not driven

by combustion oscillations in the afterburner as this behavior was observed for all ethane-natural

gas mixtures at velocities ranging from 55 to 90 ft/s. DS TC data from the natural gas tests at 90

ft/s are provided in Figure 4-9 for comparison with the four ethane-natural gas tests. As also

shown previously in Figure 4-6, once hydrogen flow to the torch was shut-off, temperatures in

the connecting tube immediately declined to the pre-heated natural gas-air mixture temperature

seen prior to hydrogen torch ignition.

63

Figure 4-9 High-Frequency Temperature Data from DS TC (Test Duration: 5.7 s)

(– – – CNG; – ∙ – 18% ethane-natural gas; –––– 28% ethane-natural gas)

Figures 4-10 and 4-11 provide high-frequency pressure data for natural gas tests at 90 ft/s

and ethane-natural gas tests at 55 ft/s, respectively. Dashed lines in the figures denote the time of

hydrogen torch ignition, hydrogen flow shut-off and pre-mixed fuel flow shut-off, labeled as

“End of Test” in the legend of each figure. Again, overall hydrogen flow and total test durations

for all tests shown in Figures 4-10 and 4-11 were 0.8 s (hydrogen flow) and 5.7 s (test duration).

64

Figure 4-10, below, displays high-frequency data from the PCB pressure sensor located

in the afterburner chamber wall for the two natural gas tests at 90 ft/s (tests L13 and L18). The

large pressure fluctuations observed after the end of hydrogen flow to the torch directly

correspond to the re-ignition events observed on the high-speed video recordings (discussed

further in Section 4.2.3). As shown in the figure, substantial pressure fluctuations were no longer

observed after ~4.5 s. This is consistent with the temperature data for these natural gas cases

(provided previously in Figures 4-5, 4-6 and 4-9), as the temperatures decreased to the premixed

natural gas-air mixture temperature observed prior to hydrogen torch ignition at approximately

this time.

Figure 4-10 High-Frequency Afterburner PCB Data for CNG Tests at 90 ft/s

65

Figure 4-11 shows the high pressure data collected by the afterburner PCB sensor during

the two 28% ethane-natural gas tests at 55 ft/s with test durations of 5.7 s (tests L8 and L16). As

shown, large pressure fluctuations (> ±10 psi) are seen in the afterburner chamber prior to

hydrogen torch ignition. Once the hydrogen torch was ignited and the fuel-air mixture in the

nozzle vane was completely combusted, only product gases entered the afterburner, significantly

reducing pressure oscillations in the afterburner. When hydrogen flow to the torch was shut-off,

pressure fluctuations slightly increased, but still remained substantially lower than those seen

prior to hydrogen torch ignition. It was speculated that this behavior was caused by steady, local

combustion somewhere upstream of the afterburner once hydrogen flow to the torch was ended.

Figure 4-11 High-Frequency Afterburner PCB Data for 28% C2H6 Tests at 55 ft/s

66

While no flameholding was observed in the test section for both the high and low

velocity tests, there was evidence of a flame anchoring at some distance downstream of the

nozzle vane for tests with ethane-natural gas mixtures. This behavior was exhibited in all ethane-

natural gas tests, regardless of specific ethane concentration or test section air velocity.

Conversely, any combustion events that occurred during natural gas tests were blown out prior to

shutting off the fuel flow to the test section as shown in both the temperature and pressure plots

provided. These results were consistent with trends observed in the high-speed video recordings

of the low velocity flameholding tests. These recordings are discussed in the following section.

4.2.3 – High-speed Video Summary

As in the high velocity flameholding tests, no burning was observed prior to hydrogen

ignition. Therefore, all events discussed in this section refer to those that occurred during or after

hydrogen flow to the torch igniter. For cases where the velocity was greater than 75 ft/s, burning

in the nozzle vane during hydrogen torch flow was nearly identical to that shown in Figure 4-2.

For tests close 75 ft/s, more localized combustion near the curved bottom surface of the nozzle

vane was observed during hydrogen flow to the torch. This phenomenon was more pronounced

during the 22.5% ethane-natural gas test (shown in Figure 4-12 on the next page) than during the

natural gas test at this velocity. For completeness, it should also be noted that during natural gas

and ethane-natural gas tests at nominal air velocities less than 75 ft/s, several flow reversals,

where the flame would appear to briefly propagate upstream in the test section, were observed

during hydrogen flow to the torch.

67

Figure 4-12 Hydrogen Torch Flow with 22.5% Ethane-Natural Gas Mixture during Test L4

After hydrogen flow to the torch igniter was ceased, the types of combustion events in the

test chamber appeared to be more dependent on fuel composition than reference velocity. As

previously mentioned in Section 4.2.2, occasional re-ignition events in the nozzle vane occurred

during tests with natural gas. Eventually, these events would die out, and no further combustion

phenomena would be observed in the test section. When the total test duration was extended for

the natural gas case at 90 ft/s, no combustion events were observed during the last ~1.5 s of

testing from either the side window (in test L13) or the rear window (in test L18).

For all ethane-natural gas experiments (including those during the high velocity

flameholding tests), a flame was seen pulsing in and out of the field of view for the majority of

the test after the hydrogen flow was shut-off. During the low velocity flameholding tests, these

flame pulsations were both more frequent and pronounced at lower velocities and higher ethane

concentrations. This behavior was thought to be indicative of a flame anchoring some distance

downstream of the nozzle vane, and was consistent with the high-speed pressure and temperature

data discussed in Section 4.2.2.

68

A series of snapshots from test L8 are provided in Figure 4-12. This is given to show the

time evolution of a flame pulsing into view, partially igniting the fuel-air mixture in the nozzle

and receding out of view from the side window during an 28% ethane-natural gas test at 55 ft/s.

Time stamps are provided at the bottom of each snapshot.

Figure 4-13 Time Evolution of a Flame Pulsing into the Nozzle Vane during Test L8

69

For tests L16-L18, the camera was moved to rear window to further investigate

flameholding downstream of the fuel injector nozzle vane as the upstream-most region of the exit

flow passage was visible from this perspective. It was speculated that if a flame anchored in the

region immediately downstream of the vane then either the flame or a reflection of a flame

would be seen from the rear window for the duration of the test after hydrogen flow cessation.

For the test with natural gas at 90 ft/s, only occasional flow reversals resulting in re-ignition in

the vane were observed, as in the high-speed video from the side window. For the test with 28%

by volume ethane-natural gas mixture, a faint reflection off of the bottom (pressure side) of the

vane is visible for the much of the test with frequent flame pulsations in and out the field of

view. A series of snapshots from the rear window are presented in Figure 4-14, and show a flame

pulsing into and out of the field of view. Time stamps for each snapshot are again provided in

Figure 4-14.

Figure 4-14 Time Evolution of Flame Pulsing into Nozzle Vane during Test L16

70

From the rear window, it was difficult to discern how far the flame propagated into the

nozzle vane during each of these flame pulses, and where the flame appeared to hold in the

region downstream of the nozzle. Nonetheless, between the temperature and pressure data

(discussed in 4.2.1) and the high speed videos collected during the low velocity flameholding

tests, it became evident that while no flameholding was observed in the test section for any tests

conducted, a flame did appear to anchor at some distance downstream of the nozzle vane during

the ethane-natural gas tests.

71

CHAPTER 5 – CONCLUSIONS AND FUTURE WORK

5.1 – Conclusions for Current Study

A series of flameholding studies were conducted on a single vane of a multi-vane, swirl-

stabilized cross-flow fuel injector nozzle similar to those used stationary gas turbine engines. The

goal of these studies was to develop a more thorough understanding of flameholding, and to

determine if the flame would anchor in the interior of the nozzle vane near the location of fuel

injection. A range of test section air velocities were tested (from 190 ft/s to 55 ft/s) using

compressed natural gas and several ethane-natural gas mixtures (18, 22.5, 24, 28 and 30% by

volume ethane). Target values for test section parameters (pressure, temperature and equivalence

ratio) corresponded to typical operating conditions during lean premixed combustion in

industrial gas turbine engines.

Two series of tests were conducted during the current research effort. The first

investigated test section velocities between 190 ft/s and 100 ft/s, while the second series

investigated velocities between 100 and 55 ft/s. During both series of tests, no flameholding was

observed in the nozzle vane for any velocity or fuel composition. During the natural gas tests,

several re-ignitions of the fuel-air mixture in the nozzle vane occurred due to flow reversals after

the hydrogen flow to the torch igniter was shut-off. These re-ignition events in the vane were

immediately blown downstream by the incoming flow. For natural gas tests during the second

series of experiments where the total test duration was extended, any combustion events in the

test chamber died out well before premixed fuel flow to the test section was shut-off and the tests

were ended.

For ethane-natural gas tests during the first series of tests, a flame was observed pulsing

in and out of the field of view on the high-speed video recordings. This behavior was also seen

72

during ethane-natural gas tests at velocities between 100 and 55 ft/s. When the total test duration

was extended for several cases during the second series of experiments, it was found that flow in

the connecting tube would remain at temperatures consistent with combustion product gases for

the duration of the test after hydrogen torch flow was shut-off. This was the case for both 18%

ethane-natural gas tests at 90 ft/s and 28% ethane-natural gas tests at 55 ft/s. From these results,

it was hypothesized that a flame would anchor at some distance downstream of the nozzle vane

for any ethane-natural gas mixture composition tested at velocities between 55 and 90 ft/s.

Without optical access to this region in the flow, however, it is difficult to discern the exact

location of the hold.

Therefore, although no flameholding was observed in the interior of the nozzle vane for

any test section air velocity or fuel composition, a flame did appear to anchor at some

downstream location at lower velocities for ethane-natural gas blends. Further studies and a

complete re-design of the test chamber would be required to determine the exact location of any

downstream flameholding event.

5.2 – Suggestions for Future Work

Although the current research effort was successful in showing that no flameholding

occurs within the fuel injector nozzle at industrial gas turbine relevant test conditions for a wide

array of fuel compositions, there are several areas of interest that would require further

investigation. The test chamber could be re-designed to allow for optical access downstream of

the current test section to determine the specific location of flameholding. This would be a direct

continuation of the current study as tests could be conducted at similar conditions to those where

a flame appeared to anchor downstream of the vane. However, this would involve an expensive

and time consuming re-design of the entire test chamber.

73

Without requiring a complete re-design of the existing hardware, the combustor apparatus

could be used to examine flameholding phenomena for various swirl vane configurations.

Specifically, vanes with different fuel injection port configurations could be tested to help

investigate potential causes of flameholding in stationary gas turbine engines. In the event of

observed flameholding within of any one of the nozzle vanes, chemiluminescence measurements

could be made to provide spatially and temporally resolved images of the flame zones within the

vane interior.

74

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76

APPENDIX A – ORIFICES USED

As discussed in sections 2.2 and 2.3, critical orifices were used for flow metering for both

the air delivery and gaseous fuel delivery systems, respectively. The following tables provide

both the orifice diameters and discharge coefficients (CD) for the critical orifices used in each air

and fuel line for every test run during the various studies. Appendix A-1 provides orifices used

during the afterburner flame stabilization study. These tests were run at four distinct upstream air

velocities: 100 ft/s, 85 ft/s, 70 ft/s and 55 ft/s, as shown in Table A-1. Appendix A.2 and A.3

provide critical orifices for both natural gas and ethane-natural gas flameholding experiments

conducted at the Cryogenic Combustion Laboratory (CCL). Appendix A.2 provides the orifices

used during the high velocity tests (in the range of 190 ft/s to 95 ft/s), while Appendix A.3

provides the orifices used during the low velocity tests (in the range of 95 ft/s to 55 ft/s). Two

orifices listed for an individual circuit indicate that these orifices were run in parallel.

Appendix A.1 - Orifices for Afterburner Flame Stabilization

Table A-1 Orifices for Afterburner Flame Stabilization

Line Diameter (in.) Orifice CD Diameter (in.) Orifice CD Diameter (in.) Orifice CD Diameter (in.) Orifice CD

0.033 0.9483 0.029 0.9446

0.033 0.927 0.033 0.927

0.9223

0.0125 0.9018 0.0125 0.9018

0.0055 0.9701 0.0055 0.9701

0.973 0.15 0.971 0.291 0.9223 0.291

0.291 0.9223 0.195 0.9315 0.195 0.9315

0.112

0.0125 0.9018PCNG 0.0125 0.9018

AB H2 0.0055 0.9701 0.0055 0.9701

0.9122 0.03 0.9122

0.936 0.033 0.9483

ABP

MCNG 0.03 0.9122 0.03 0.9122 0.03

PA 0.04

Compressed Natural Gas Only

100 ft/s 85 ft/s 70 ft/s 55 ft/s

MA 0.195 0.9315

77

Appendix A.2 - Orifices for High Velocity Flameholding Tests

Table A-2a Orifices for High Velocity Flameholding Tests

Table A-2b Orifices for High Velocity Flameholding Tests (Continued)

Line Diameter (in.) Orifice CD Diameter (in.) Orifice CD Diameter (in.) Orifice CD Diameter (in.) Orifice CD Diameter (in.) Orifice CD

0.075 0.9542 0.075 0.9542 0.075 0.9542 0.1 0.946 0.15 0.93880.15 0.9388 0.15 0.9388 0.15 0.9388 0.18 0.9383 0.18 0.9383

1.0094 0.059 1.0094

PCNG 0.025 0.93 0.025 0.93 0.025

0.059 1.0094 0.059CNG Outer 0.059 1.0094 0.059 1.0094

0.03 1.009 0.03 1.009 0.03 1.009

ABP

CNG Inner 0.03 1.009 0.03 1.009

0.052 1.124 0.052 1.124 0.052 1.124

0.9405 0.35 0.9405 0.35 0.9405

PA 0.052 1.124 0.052 1.124

MA 0.35 0.9405 0.35 0.9405 0.35

Compressed Natural Gas Only

187 ft/s 175 ft/s 165 ft/s 155 ft/s 140 ft/s

0.93 0.025 0.93 0.025 0.93

C2H6 Inner n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a

C2H6 Outer n/a n/a n/a n/a n/a n/a n/a n/a n/a n/a

H2 Torch 0.018 0.9079 0.018 0.9079 0.018 0.9079 0.025 0.93 0.025 0.93

Line Diameter (in.) Orifice CD Diameter (in.) Orifice CD Diameter (in.) Orifice CD Diameter (in.) Orifice CD

0.15 0.9388 0.15 0.93880.18 0.9383 0.18 0.9383

Compressed Natural Gas Only 30% C2H6 by Volume115 ft/s 95 ft/s 140 ft/s 95 ft/s

MA 0.247 0.9516 0.247 0.9516 0.35 0.9405 0.247 0.9516

PA 0.052 1.124 0.052 1.124 0.052 1.124 0.052 1.124

ABP 0.15 0.9388 0.15 0.9388

CNG Inner 0.03 1.009 0.03 1.009 0.025 0.93 0.02 0.94

CNG Outer 0.0375 1.102 0.0375 1.102 0.0375 1.102 0.03 1.009

PCNG 0.025 0.93 0.025 0.93 0.02 0.94 0.025 0.93

C2H6 Inner n/a n/a n/a n/a 0.018 0.9079 0.018 0.9079

0.03 0.92

C2H6 Outer n/a n/a n/a n/a 0.025

0.03 0.92

0.988 0.025 0.988

H2 Torch 0.03 0.92 0.03 0.92

78

Appendix A.3 - Orifices for Low Velocity Flameholding Tests

Table A-3 Orifices for Low Velocity Flameholding Tests with CNG Only

Table A-4 Orifices for Low Velocity Flameholding Tests with Added C2H6

Line Diameter (in.) Orifice CD Diameter (in.) Orifice CD Diameter (in.) Orifice CD Diameter (in.) Orifice CD

0.15 0.9388 0.18 0.9383 0.15 0.9388

0.18 0.9383 0.125 0.9464 0.125 0.9464

0.085 0.939 0.1 0.946 0.085 0.939 0.19 0.95

0.125 0.946 0.15 0.9388 0.23 0.95 0.23 0.95

0.015 0.982 0.012 1.033 0.012 1.033

0.0105 0.869 0.0105 0.869 0.005 0.932

0.0055 0.9701 0.0055 0.9701

0.0045 1.008 0.0035 1.186

0.03 0.920.03 0.92

0.018 0.9079

0.03 0.92 0.03 0.92

0.025 0.988

0.0375 1.102 1.102

0.05 0.949

H2 Torch

PA

ABP

CNG Inner

CNG Outer

PCNG

AB H2

95 ft/s 75 ft/s 65 ft/s90 ft/s

0.18MA

0.0055 0.9701

0.03 1.009

0.0055 0.9701

0.05 0.949

0.018 0.9079

0.03 1.009

0.0375 1.102

0.018 0.9079

0.9383

0.0375 1.102

0.025 0.988

0.0375

Compressed Natural Gas Only

Line Diameter (in.) Orifice CD Diameter (in.) Orifice CD Diameter (in.) Orifice CD Diameter (in.) Orifice CD Diameter (in.) Orifice CD

0.18 0.9383 0.15 0.9388 0.18 0.9383 0.15 0.9388 0.15 0.9388

0.125 0.9464 0.125 0.9464 0.125 0.9464 0.125 0.9464 0.125 0.9464

0.1 0.946 0.1 0.946 0.19 0.95 0.19 0.95

0.15 0.9388 0.15 0.9388 0.23 0.95 0.23 0.95

0.015 0.982

0.0105 0.869

0.0105 0.869 0.0105 0.869 0.0105 0.869

0.012 1.012 0.015 0.982 0.012 1.012

0.018 0.9079 0.018 0.9079 0.018 0.9079 0.018 0.9079

0.012 1.033 0.012 1.033 0.012 1.033 0.012 1.033

0.0055 0.9701 0.0055 0.9701 0.0055 0.9701 0.0055 0.9701 0.0055 0.9701

0.0035 1.186 0.0035 1.186 0.0035 1.186 0.0035 1.186 0.0035 1.186

75 ft/s 90 ft/s 65 ft/s

0.012

MA

PA 0.05 0.949 0.0375 1.102 0.05 0.949 0.0375 1.102

28% C2H6 by Volume

55 ft/s

0.0375 1.102

90 ft/s

0.982 0.018 0.9079 0.015 0.982

ABP

CNG Inner

AB H2

H2 Torch 0.03 0.92

1.009 0.03

PCNG

0.01 1.112

C2H6 Outer

24% C2H6 by Volume

0.01 1.112 0.01 1.112

0.03 0.92 0.03 0.92 0.03

C2H6 Inner 0.01 1.112 0.012 1.033

18% C2H6 by Volume 22.5% C2H6 by Volume 24% C2H6 by Volume

CNG Outer 0.0375

0.025 0.988

1.012

0.03 1.009

0.03 0.920.92

1.0090.03

0.02 0.94 0.015 0.982

0.23 0.95

1.102 0.03 1.009

0.015

79

APPENDIX B – PROPERTIES AND CHEMICAL COMPOSITIONS OF FUELS

In addition to the natural gas flameholding tests, several tests were conducted with

various ethane-natural gas blends. For these tests, gaseous ethane was added upstream of the fuel

injector nozzle and mixed with natural gas prior to injection into the test section. This appendix

provides a table of properties and compositions of all fuels tested. The composition for natural

gas used in analysis is based on a gas chromatography study briefly discussed in Section 2.3.

Table B-1 Properties and Chemical Compositions of All Fuels Used

Natural

Gas

18%

Ethane by

Volume

22.5%

Ethane by

Volume

24%

Ethane by

Volume

28%

Ethane by

Volume

30%

Ethane by

Volume

Composition C1.075H4.149 C1.209H4.419 C1.253H4.506 C1.267H4.534 C1.306H4.612 C1.325H4.650

Molecular Weight 17.090 18.980 19.588 19.791 20.332 20.603

Gamma 1.295 1.273 1.266 1.264 1.259 1.256

Stoichiometric Air

to Fuel Ratio 16.971 16.744 16.680 16.659 16.607 16.582

Compounds Percent by Volume

Methane (CH4) 94.895 81.080 76.630 75.147 71.192 69.214

Ethane (C2H6) 4.028 18.000 22.500 24.000 28.000 30.000

Propane (C3H8) 0.515 0.440 0.416 0.408 0.387 0.376

Isobutane (C4H10) 0.082 0.070 0.066 0.065 0.062 0.060

Butane (C4H10) 0.104 0.089 0.084 0.083 0.078 0.076

80

APPENDIX C – AFTERBURNER PART DRAWINGS

The following pages provide part drawings for several pieces of hardware used during the

current research effort. Parts included are the test chamber exhaust tube insert, diagnostic hub,

hydrogen hub, hydrogen injection tube and the two afterburner nozzles. Figure C-1 provides the

part drawing for the test chamber exhaust tube insert used only during the low velocity

flameholding tests. The test chamber exhaust tube insert was welded to the upstream end of the

diagnostic hub (drawing shown in Figure C-2) prior to testing. Figure C-3 displays the part

drawing for the hydrogen inlet hub used to evenly distribute hydrogen to the twelve hydrogen

flow passages in the hydrogen injection tube (shown in Figure C-5). As shown in Figure C-4, the

two hubs were designed such that they formed an air tight seal using an Inconel C-ring. An 1/8-

in. Swagelok fitting was welded to the diagnostic hub to serve as a static pressure and

temperature port, and four 1/4-in. stainless steel tubes were welded to the hydrogen inlet hub as

shown. The upstream-most 1.25 inches of the hydrogen injection tube was trimmed to an outer

diameter of 1.27 in. to fit into the hydrogen hub. The hydrogen injection tube was welded to the

hydrogen hub using circumferential welds on both the upstream and downstream faces of the

hydrogen hub. Figures C-6 and C-7 provide part drawings for the 0.8-in. and 0.59-in. throat

diameter afterburner nozzles, respectively. The 0.8-in. nozzle was used in the high velocity

flameholding experiments, while the 0.59-in. nozzle was required to reach target pressure during

the low velocity tests.

81

Figure C-1 Test Chamber Exhaust Tube Insert Part Drawing

82

Figure C-2 Diagnostic Hub Part Drawing

83

Figure C-3 Hydrogen Inlet Hub Part Drawing

84

Figure C-4 Hub Assembly Drawing

85

Figure C-5 Hydrogen Injection Tube Part Drawing for Gundrilling

86

Figure C-6 Part Drawing for Afterburner Nozzle with 0.8-in. Throat Diameter

87

Figure C-7 Part Drawing for Afterburner Nozzle with 0.59-in. Throat Diameter

88

APPENDIX D – DETAILED RESULTS FOR FLAMEHOLDING EXPERIMENTS

The following tables provide detailed results for both the high and low velocity

flameholding tests conducted at the Cryogenic Combustion Lab (CCL). Additional information

provided in the following tables, but data not presented in Tables 4-2 and 4-4, include the main

air, premixed natural gas and ethane mass flow rates, afterburner chamber pressure, pressure

oscillations in the afterburner and the dominant frequency of those oscillations for each

flameholding test. All tabulated results show the conditions achieved prior to hydrogen torch

ignition during each test. Table D-1 gives the test results for high velocity flameholding tests H1-

H8, while Table D-2 provides data for tests H8-H13. Table D-3 gives detailed data for low

velocity flameholding tests L1-L9, and Table D-4 provides data for tests L10-L18.

89

D.1 – Results from High Velocity Flameholding Experiments

Table D-1 Conditions Achieved During Tests H1-H7

Test Number H1 H2 H3 H4 H5 H6 H7

Date 10/21/08 10/21/08 10/21/08 10/22/08 10/23/08 10/23/08 10/27/08

Test Section V

(ft/s) 190.12 177.01 168.42 158.09 145.9 147.66 157.32

Test Section P

(psia) 218.06 217.9 214.21 217.24 218.01 217.85 219.01

(lbm/s)

0.7565 0.7025 0.6551 0.6210 0.5709 0.5658 0.6210

(lbm/s) 0.0265 0.0248 0.0230 0.0219 0.0204 0.0212 0.0217

(lbm/s) n/a n/a n/a n/a n/a n/a n/a

Test Section

Φ 0.594 0.599 0.597 0.599 0.606 0.636 0.594

Fuel Type

(% by Volume) CNG CNG CNG CNG CNG CNG CNG

Test Duration

(H2 Duration)

(s)

3.4 (0.5) 3.4 (0.5) 3.4 (0.5) 3.4 (0.5) 3.4 (0.5) 3.4 (0.5) 3.4 (0.5)

Camera Mount Rear Rear Rear Rear Rear Rear Side

AB Chamber P

(psia) 208.88 209.73 207.59 211.59 212.57 210.07 213.61

Chamber Oscillations

(psi)

9.30 5.20 15.00 13.50 6.90 5.40 10.00

Chamber

Oscillations

(%)

4.45 2.48 7.23 6.38 3.25 2.57 4.68

Dominant

Frequency

(Hz)

84 82 92 91 81 81 87

90

Table D-2 Conditions Achieved During Tests H8-H13

Test Number H8 H9 H10 H11 H12 H13

Date 10/27/08 10/27/08 10/28/08 10/29/08 10/30/08 10/31/08

Test Section V

(ft/s) 148.81 140.09 142.34 116.44 103.4 114.92

Test Section P

(psia) 228.13 218.95 150.28 184.8 205.93 182.8

(lbm/s) 0.6144 0.5554 0.3849 0.3881 0.3752 0.3806

(lbm/s)

0.0218 0.0200 0.0078 0.0132 0.0128 0.0081

(lbm/s) n/a n/a 0.00523 n/a n/a 0.00535

Test Section

Φ 0.603 0.613 0.562 0.576 0.58 0.584

Fuel Type

(% by Volume) CNG CNG 30% C2H6 CNG CNG 30% C2H6

Test Duration (H2 Duration)

(s)

3.4 (0.5) 3.4 (0.5) 3.4 (0.5) 3.4 (0.5) 3.4 (0.5) 3.4 (0.5)

Camera Mount Side Side Side Side Side Side

AB Chamber P (psia)

223.30 215.05 147.97 181.57 200.53 176.27

Chamber

Oscillations

(psi)

9.70 7.10 2.90 14.30 15.90 7.80

Chamber

Oscillations

(%)

4.34 3.30 1.96 7.88 7.93 4.43

Dominant

Frequency

(Hz)

80 81 81 75 76 87

91

D.2 – Results from Low Velocity Flameholding Experiments

Table D-3 Conditions Achieved During Tests L1-L9

Test Number L1 L2 L3 L4 L5 L6 L7 L8 L9

Date 12/09/09 12/16/09 12/17/09 12/18/09 12/22/09 12/22/09 12/23/09 12/23/09 01/04/10

Test Section V

(ft/s) 96.25 75.51 66.29 76.59 64.77 56.25 57.11 57.16 58.32

Test Section P

(psia) 217.32 216 203.08 216.82 221.9 217.9 219.27 218.22 211.85

(lbm/s) 0.3791 0.3005 0.2422 0.3008 0.2604 0.2203 0.2241 0.2240 0.2230

(lbm/s) 0.0127 0.0102 0.00824 0.00724 0.00609 0.00490 0.00485 0.00484 0.00470

(lbm/s) n/a n/a n/a 0.00303 0.00295 0.00294 0.00299 0.00294 0.00293

Test Section

Φ 0.567 0.574 0.577 0.57 0.579 0.592 0.581 0.577 0.569

Fuel Type

(% by

Volume)

CNG CNG CNG 22.5%

C2H6

24%

C2H6

28%

C2H6

28%

C2H6

28%

C2H6

28%

C2H6

Test Duration

(H2 Duration)

(s)

3.7 (0.8) 3.7 (0.8) 3.7 (0.8) 3.7 (0.8) 3.7 (0.8) 3.7 (0.8) 4.7 (0.8) 5.7 (0.8) 5.0 (1.1)

Camera

Mount Side Side Side Side Side Side Side Side Side

AB Chamber

P (psia) 212.50 212.25 201.34 212.52 219.36 215.61 217.86 215.31 210.05

Chamber

Oscillations

(psi)

3.43 3.62 11.02 4.71 11.19 24.84 23.24 23.27 19.15

Chamber

Oscillations

(%)

1.61 1.70 5.47 2.22 5.10 11.52 10.67 10.81 9.12

Dominant

Frequency

(Hz)

77.1 78.0 73.9 78.6 76.9 77.4 77.8 77.2 79.3

92

Table D-4 Conditions Achieved During Tests L10-L18

Test Number L10 L11 L12 L13 L14 L15 L16 L17 L18

Date 01/04/10 01/05/10 01/06/10 01/06/10 01/06/10 01/06/10 01/07/10 01/08/10 01/08/10

Test Section V

(ft/s) 58.84 88.22 89.8 91.65 n/a 56.28 56.82 89.03 90.63

Test Section P

(psia) 212.75 229.19 224.87 219.54 n/a 218.62 214.35 225.51 221.814

(lbm/s) 0.2256 0.3661 0.3653 0.3636 n/a 0.2231 0.2208 0.3640 0.3627

(lbm/s) 0.00475 0.00845 0.0125 0.0121 n/a 0.00503 0.00481 0.00960 0.0122

(lbm/s) 0.00288 0.00422 n/a n/a n/a 0.00286 0.00294 0.00288 n/a

Test Section

Φ 0.562 0.576 0.581 0.566 n/a 0.587 0.583 0.574 0.571

Fuel Type

(% by

Volume)

28%

C2H6

24%

C2H6 CNG CNG

18%

C2H6

28%

C2H6

28%

C2H6

18%

C2H6 CNG

Test Duration

(H2 Duration)

(s)

5.2 (1.3) 5.0 (1.1) 4.7 (0.8) 5.7 (0.8) 5.7 (0.8) 5.9 (2.0) 5.7 (0.8) 5.7 (0.8) 5.7 (0.8)

Camera

Mount Side Side Side Side Side Side Rear Rear Rear

AB Chamber

P (psia) 210.02 224.07 219.89 215.81 n/a 216.89 213.95 221.08 217.27

Chamber

Oscillations

(psi)

21.40 2.95 2.42 2.36 2.84 24.76 23.86 2.51 2.36

Chamber

Oscillations

(%)

10.19 1.32 1.10 1.09 n/a 11.41 11.15 1.14 1.09

Dominant

Frequency

(Hz)

76.8 91.1 94.9 93.5 93.4 76.6 76.9 94.1 69.4