ILASS Americas 27th Annual Conference on Liquid Atomization and Spray Systems, Raleigh, NC, May 2015

Study of Sprays Generated by Impinging Liquid Jets from Unlike Doublet Injectors

Hannah E. Bower, Scott B. Leask, and Vincent G. McDonell*

UCI Combustion Laboratory

Mechanical & Aerospace Engineering

University of California, Irvine

Irvine, CA 92697-3550 USA

Abstract

This paper explores the application of optical diagnostics for the study of the structure of sprays generated by unlike

doublet injectors. The cold-flow spray characteristics can be compared to corresponding hot-fire tests on prototype

thrusters. A set of conditions, including differing injector impingement angles, were tested using high speed cinema-

tography, laser diffraction, and a novel dual wavelength laser induced fluorescence system (DLIF). The DLIF system

involved the development of appropriate dye/solvent mixtures that can be optically isolated through use of two exci-

tation wavelengths and corresponding optical filters for isolating the fluorescence from the liquid stream from each of

the two injectors. Additional analysis was required to assess the extent to which wavelength dependent scattering by

the droplets impacted the relative intensity of the fluorescence from each stream. This was accomplished using cal-

culated extinction coefficients from Mie scattering theory for each excitation wavelength combined with the measured

size distribution from laser diffraction. The high speed cinematography captures the highly dynamic process induced

by the impinging jets. The periodic nature of the atomization process for this type of injection strategy is clearly

indicated and quantified in terms of dominant wavelengths and frequencies. High speed video is also used to quantify

the resulting spread angle of the spray plume and the average spray breakup generated by the impingement. Addi-

tionally, the laser diffraction size distribution results indicated larger Sauter Mean diameters and volume distribution

diameters for smaller impingement angles. The visualization results from the DLIF system indicated the effects of

impingement angle on overall mixing of the two liquid streams. The results from the various optical measurement

methods clearly demonstrate the strong dependence of the spray structure on impingement angle.

Keywords: Spray, atomization, measurement, drop size, imaging diagnostics

*Corresponding author: mcdonell@ucicl.uci.edu

Introduction

Unlike doublet injectors are commonly used for liq-

uid rocket combustion technologies. Unlike doublet in-

jectors are used most often for propellant combinations

that have approximately equal fuel and oxidizer injection

orifice areas and momentum ratios [1]. Unlike doublet

injectors also produce finer atomization elements and

have higher performance efficiencies, but usually pro-

duce less stable flames [1]. However, experiments

aimed at understanding the cold flow (i.e., atomization,

dispersion, mixing, and evaporation) behavior of avia-

tion and rocket fuel using impinging jets have not been

sufficiently studied to date. Most tests on rocket injector

engines, such as the Lunar Module Decent Engine

(LMDE) and the SENTRY Pinch and Yew engine con-

sist of only hot fire tests, as mentioned in the “TRW Pin-

tle Heritage and Performance Characteristics” paper by

G.A. Dressler [2]. Few hot fire tests have been coupled

with cold flow tests. R.J Santoro and C.L. Merkle per-

formed cold flow tests to determine spray angles as TMR

varied and coupled them with hot fire test to examine ig-

nition and combustion characteristics [3]. Furthermore,

J. Houseman performed a study on mixing of hypergolic

propellants in unlike doublet injectors, which displayed

large concentration gradients in the chamber 5 inches

downstream from the impingement point. J. Houseman’s

study also depicts images of the relative mixing of the

fuel and oxidizer along with the corresponding combus-

tion efficiencies for penetrated, mixed and separated liq-

uid propellants [4]. However, Santoro and Merkle’s and

Houseman’s cold flow tests did not examine any other

fuel characteristics, such as droplet size and velocity,

which could have further enhanced their study of the RP-

1 fuel. Therefore, further research in the flow character-

istics of unlike doublet injectors could lead to better de-

signed and more efficient combustion systems. This pa-

per explores multiple experimental techniques to better

classify and describe the spray structure produced from

two unlike doublet injectors, to help advance combustion

technologies for many applications, such as the aero-

space or automotive industries.

Approach

The approach taken was to experimentally study the

structure of sprays from two unlike doublet injectors. A

test rig was constructed to allow the injectors to be posi-

tioned at precise locations, creating different spray im-

pingement angles, while advanced diagnostic measure-

ments were taken to characterize the spray structure.

High speed cinematography, Malvern laser systems, Mie

scattering modeling and Dual Wavelength Laser Induced

Fluorescence (DLIF) techniques were used for spray

characterization. Water was used for all experiments as

a replacement for RP-1 and other liquid propellants.

Experimental and Analytical Methods

Mie Scattering

A Mie scattering code (Light Lab: Far Field Mie

Scattering) was used to model extinction coefficients in

relation to droplet size. The parameters used to perform

the Mie scattering calculations are displayed in Table 1.

Laser Diffraction

To study the droplet size distribution of the spray, a

Malvern RTS-100 laser system was used. The Malvern

encompasses a line of sight laser diffraction method to

measure and calculate droplet size distributions of a line

within the spray. For the Malvern droplet distribution

tests, three different injector impingement angles (50°,

70° and 90°) were tested. Data taken for all three angles

were taken at a constant Z-axis position, 6mm down-

stream from the impingement point, and Y-axis position,

with varying X-axis positions ranging from -6.000mm to

8.000mm by increments of 2.000mm, with the X-posi-

tion 0.000mm at the center of the spray cone, as dis-

played in Figure 1. The flow rates for the oxidizer and

fuel were kept at 15.3kg/hr and 11.5kg/hr, respectively.

In order to determine the scattering efficiency de-

pendence on wavelength, the Malvern distribution was

analyzed using a Rosin-Rammler distribution function,

which then was coupled to the Mie scattering data. A

MATLAB tool, created by Ivan Brezani, et.al., was used

to transform the Malvern Gaussian size distribution to fit

a Rosin-Rammler distribution function [5]. Coupling the

calculated Mie scattering extinction coefficients with the

size distribution allowed for the calculation of the extinc-

tion coefficient for each wavelength (447nm and 655nm)

at each tested X-axis position to be calculated. Finally,

ratios of the calculated extinction coefficients for each

laser were compared and the dependence of wavelength

on scattering efficiency was determined.

Table 1. Parameters used for Mie scattering calculations

Parameter Value Droplet Diameter Variable

Range of Droplet Diame-ter

0.0 to 300 m

Index of Refraction- Real Part

1.33

Index of Refraction- Im-aginary Part (oxazine)

7.19x10-5

Index of Refraction- Im-aginary Part (fluores-

cein)

3.24x10-8

Wavelength of Beam 0.447um, 0.655um Beam Waist Diameter 1.00E6 um

Scattering Angle 0.00 Degrees Number of Data Points 1500

3

Figure 1. Malvern measurement schematic

High Speed Cinematography

High speed shadowgraph videos were captured us-

ing a Vision Research Phantom v7.1 monochrome digi-

tal high speed camera. Videos were recorded in 256x256

pixel resolution at 26143 frames per second. Snapshot

images were taken from the videos and used to depict the

structure of the sprays at the three nozzle impingement

angles tested (50°, 70° and 90°).

MATLAB was utilized for image analysis from the

data gathered by the high speed cinematography. The

code was written to analyze the pixel intensity distribu-

tions of the spray videos which then in turn produced av-

erage spray structure characteristics. The average diverg-

ing spray angle and periodic spray breakup points were

the characteristics considered. The average diverging

spray angle was calculated by producing linear lines

which corresponded to the spray plume edges. These

lines were found for 100 frames of the high speed video

which were then averaged. The angle between these lines

resulted in the average diverging spray angle. The peri-

odic spray breakup points were found through vertical

line intensity profiles. These profiles located the maxi-

mum and minimum intensity locations which produced

the distances between breakups.

LaVision DaVis 7.2.2.429 software was used to ex-

tract velocity information from the high speed videos of

the sprays. The video files were imported into the soft-

ware while maintaining the correct pixel size and time

between each frame via import options. With the im-

ported data, a PIV time series operation was utilized to

produce a vector field for each frame of the video. Proper

Orthogonal Decomposition (POD) was then applied to

the PIV time series data which produced an average vec-

tor field for each impinging jet angle.

DLIF

The two lasers used in the DLIF system were a

447nm OEM MDL-III-447 laser and a 655nm OEM

MRL-III-FS-655 laser. Each beam was collimated at the

laser exit using an Opto Engine 400 micron multimode

fiber coupler SMA905 connector. The fiber couplers

were then attached to an Ocean Optics Split400-UVVIS

400uM Fiber splitter/combiner. Using the combiner, the

447nm and 655nm beams were combined to a single fi-

ber. A 40mm collimating lens was then used at the end

of the fiber to collimate both beams together. For this

experiment, line measurements were taken to gain un-

derstanding about the relative concentration of the liquid

originating from each of the doublet injectors. An Andor

i-Star Intensified CCD camera was placed at a 90° angle

from the DLIF lasers and a 675nm -10nm and a 532 -

2nm optical bandpass filters were placed on the camera

to isolate the concentrations of both the oxazine (“oxi-

dizer”) and fluorescein (“fuel”) could be determined, re-

spectively. Figure 2 displays the DLIF instrumentation

schematic.

Figure 2. DLIF instrumentation schematic

Furthermore, the DLIF setup was used to confirm

the experimental scattering efficiency dependence on

wavelength. The initial intensity (I0) of each laser was

measured via a power meter. Then the incident laser in-

tensity of the laser beam after it traveled through the

spray plume (I) was measured. The I0/I at each X-axis

position were calculated and the two lasers were then

compared to each other.

Results

Mie Scattering

In order to model the Mie scattering of the spray, the

imaginary part of the index of refraction was calculated.

The absorption coefficient for each wavelength was

found from tests performed using the two lasers that

make up the DLIF system. Two calibration curves were

constructed, one for each laser used. The absorption co-

efficient for each wavelength is described by the equa-

tion

𝛾−1 =𝜆

4𝜋𝑛′ (1)

where 𝛾 is the absorption coefficient, λ is the laser wave-

length and n’ is the imaginary part of the index of refrac-

tion [6,7].

A series of solutions of differing concentrations of

oxazine in water and fluorescein in water were prepared

for the 655nm and 447nm calibration curves, respec-

tively. The oxazine and fluorescein solution concentra-

tions tested are displayed in Table 2.

4

Table 2. Fluorescein and oxazine concentrations used

for the calibration curves.

Fluorescein solutions (mM) Oxazine solutions (mM)

2.13 1.37 2.19 1.95 5.33 2.93

10.65 4.88 15.97 9,.75 21.3 14.6

- 19.5

For each concentration, the solution was placed in a

quartz cuvette 6mm from the laser. A power meter was

placed behind the cuvette and recorded the intensity of

the light exiting the cuvette, as displayed in the experi-

mental schematic shown below in Error! Reference

source not found.. The initial intensity of each laser was

recorded via the power meter in the absence of the cu-

vette. Furthermore, a “blank”, containing deionized wa-

ter, was measured before each concentration data point

was collected for each wavelength.

Figure 3. Laser system schematic for the creation of the

fluorescein and oxazine calibration curves.

Calibration curves were then constructed from the

data points collected. Two additional calibration curves

using the same solutions were created using a Shimadzu

UV-1700 Spectrometer. The UV-Vis spectrometer cali-

bration curves were then used in comparison with the la-

ser induced fluorescence (LIF) curves to determine the

accuracy and precision of the LIF calibration curves.

Furthermore, using the calibration curves and Beer’s

Law, the absorption coefficient was determined for both

oxazine and fluorescein.

Calibration curves to determine the absorption coef-

ficient of both the 447nm laser with differing concentra-

tions of fluorescein and the 655nm laser with differing

concentrations of oxazine were constructed. Figure 4 dis-

plays the oxazine calibration curve from the LIF system

with concentrations ranging from 1.37 M to 19.5 M.

Three calibration curves were constructed at 655nm

using the LIF method and the same solutions. An aver-

age absorption coefficient (slope) was calculated, equal

to 0.0725.

Figure 4. Laser induced fluorescence calibration curve

for oxazine.

An additional calibration curve was constructed us-

ing the Shimadzu UV-1700 Spectrometer to determine

the precision and accuracy of the LIF systems. The UV

Vis spectrometer calibration curve for oxazine is dis-

played in Figure 5.

The UV Vis spectrometer calibration curve for oxa-

zine exhibits an absorption coefficient equal to 0.0744,

which differs from that of the LIF system by 2.55%.

Figure 6 displays the fluorescein calibration curve

from the LIF system with concentrations ranging from

2.13 M to 21.3 M. Three separate calibration curves

were made using the laser induced fluorescence method

and the same solutions, for precision. An average absorp-

tion coefficient (slope) was calculated, equal to 0.0091.

An additional calibration curve was constructed for

the fluorescein solutions using the UV Vis spectrometer

at 446nm, which is represented in Figure 7. The UV Vis

spectrometer calibration curve for fluorescein depicts an

absorption coefficient equal to 0.0126, which differs

from that of the LIF system by 27.8%.

Figure 5. UV Vis spectrometer calibration curve for

oxazine.

y = 0.0724x + 0.0111R² = 0.9999

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 5 10 15 20 25

Ab

sorb

an

ce

Concentration

Absorbance vs. Concentration (uM) of Oxazine at 655nm-LIF system

y = 0.0744x - 0.0037R² = 0.999

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 5 10 15 20 25

Ab

sorb

an

ce

Concentration (uM)

Absorbance vs. Concentration (uM) of Oxazine at 655nm- UV Vis Spectrometer

5

Figure 6. Laser induced fluorescence calibration curve

for Fluorescein

Figure 7. UV Vis spectrometer calibration curve for flu-

orescein

The calculated absorption coefficients for both the

oxazine and fluorescein experiments were used in the

imaginary part of the index of refraction calculations.

The imaginary part for 447nm and 655nm were deter-

mined via Equation 1 to be 3.24E-8 and 7.19E-5, respec-

tively. The calculated Mie scattering results from the

Light Lab software were then coupled with the Malvern

distribution data that was transformed using a Rosin-

Rammler distribution function to determine the extinc-

tion coefficients for each laser at the X-axis positions

tested. The ratio of the calculated 447nm and 655nm ex-

tinction coefficients at the varying X-axis positions are

displayed in Table 3.

Based off of the data collected and analyzed, all

three impingement angles tested display ratios close to

1.0 for all X-axis positions tested. Based on these results,

no correction factor is needed to account for the differ-

ences in scattering efficiency when using the two wave-

lengths in the DPLIF system.

Table 3. Calculated extinction coefficient ratios

(447nm/655nm)

X-axis Po-sition

Impingement Angle (degrees)

50 70 90 -6 1.00 1.05 1.06

-4 1.05 1.05 1.06

-2 1.04 1.05 1.06

0 1.04 1.05 1.06

2 1.07 1.05 1.06

4 1.04 1.05

1.06

6 1.05 1.06 1.06

8 1.05 1.06 1.06

Experimental data were collected by measuring the

initial intensity and the intensity of the laser after it trav-

eled through the spray for each laser at a 90° impinge-

ment angle. The ratio of I/I0 for each laser at each X-axis

position were then compared and are displayed in Table

4. The experimental data and modeled Mie scattering

data displays extinction coefficient ratios near 1.0 for all

X-axis positions tested. Therefore, the experimental data

and calculated results support the wavelength having no

dependence on the scattering efficiency.

Table 4. Experimental extinction coefficient ratios

(447nm/655nm)

X-axis Position Extinction Coeffi-cient Ratio

(447nm/655nm)

-6 1.04

-4 0.99

-2 0.99

0 0.99

2 0.98

4 1.01

6 0.98

8 0.97

High Speed Imaging

Video results were obtained for each impingement

angle tested. In order to understand the dynamic struc-

ture of the sprays, the video recordings were taken at

26143 frames per second. As a result, snapshot pictures

are shown to illustrate and support the conclusions drawn

from the videos. Figure 8 displays pictures from the three

impingement angles tested taken at the camera angles of

0°, 45° and 90° in relation to the impinging unlike dou-

blet injectors’ plane.

y = 0.0093x + 0.0024R² = 0.9944

0

0.05

0.1

0.15

0.2

0.25

0 5 10 15 20 25

Ab

sorb

an

ce

Concentration (uM)

Absorbance vs. Concentration (uM) of Fluorescein at 447nm-LIF System

y = 0.0126x + 0.0037R² = 0.9992

0

0.05

0.1

0.15

0.2

0.25

0.3

0 5 10 15 20 25

Ab

sorb

an

ce

Concentration (uM)

Absorbance vs. Concentration (uM) of Fluorescein at 446nm-UV Vis Spectrometer

6

Impinge-ment an-gle

Camera An-gle= 90°

Camera Angle= 45°

Camera Angle= 0°

50°

70°

90°

Figure 8. Images from the three impingement angles

tested taken at differing camera angles

The videos taken at the 0° camera angle for the three

respective impingement angles were read into the

MATLAB code to calculate the average spray angle. The

results from the code are found in Table 5

Table 5. Spray angle versus impingement angle.

Impingement Angle Average Spray Angle

50° 49.9° 70° 72.5° 90° 51.5°

It can be noted from this that the 70° impingement

angle produces the largest average spray angle.

Following this, the same impingement angles were

then read into the code for the average distance between

spray breakup points. The results are found in Table 6

and illustrate that the breakup distance increases with

larger impingement angles.

Table 6. Average distance between spray breakup

points at different impingement angles.

Impingement Angle Average Breakup Dis-tance (microns)

50° 297.44 70° 325.16 90° 358.60

The videos taken at the 0° camera angle for the three

respective impingement angles were imported into the

PIV software and POD was implemented on each video.

Figures 9-11 show the average vector field for the 50°,

70° and 90° impinging jet angles, respectively.

Figure 9. Average vector field for the 50° impingement

angle.

Figure 10. Average vector field for the 70° impinge-

ment angle.

Figure 11. Average vector field for the 90° impingement

angle.

7

Laser Diffraction

To help further understand the spray structure at the

differing impingement angles, droplet size measure-

ments were taken. Profiles of the Sauter mean diameter

(SMD, D32), DV90, and the distribution span ([Dv90-

Dv10]/Dv50) were calculated and are represented in Fig-

ures 12-14.

Note that the SMD trends for the impingement an-

gles tested display similar trends, with an increase in

droplet diameter near the 0mm X-axis position and

smaller SMD diameters at the -6mm and +8mm X-axis

positions. The 50° impingement angle also demonstrates

the largest SMD diameters at all X-axis positions. Also,

all three impingement angles tested show similar volume

distribution trends to that of SMD. The span distribution

for the 70° impingement angle exhibits the widest distri-

bution of droplet sizes compared to the other impinge-

ment angles tested.

Figure 12. The Sauter Mean Diameter distributions for

the spray at various impingement angles.

Figure 13. The volume distributions for the spray at var-

ious impingement angles.

Figure 14. The span distribution for the spray at various

impingement angles.

DLIF

To further understand the mixing of the fuel and ox-

idizer, DLIF line measurements were performed. Images

from each laser at each X-axis position were collected.

Since the camera was placed at a 90° angle from the laser

beam entering the spray, a Y-axis intensity profile was

constructed via a MATLAB code. Furthermore, the max-

imum intensities of pure oxazine and fluorescein solu-

tions at 655nm and 447nm, respectively, were measured

and used for normalization. The relative intensities of the

oxazine to fluorescein at a 50° impingement angle were

calculated and are demonstrated in Figure 15.

The plot shows a steep peak between the Y-axis po-

sitions of 100-150 pixels for each of the X-axis positions

tested. Note the low trough displayed between Y-axis

positions 150-200 pixels for each line tested. These

troughs suggest that the blue laser (fluorescein) is domi-

nant in this region. This could be due to the fact that the

injector spraying fluorescein is present at the 200 pixel

Y-axis position.

Also, take note that the relative intensity values be-

tween 50-100 pixel Y-axis positions are close to 5. This

suggests that the red laser (oxazine) is dominant in this

region, which could be due to the oxazine injector’s

placement at the 0 pixel Y-axis position. The differences

in relative intensities between the 50-100 pixel and 150-

200 pixel Y-axis positions could also be due to a slight

offset of the jets in the Z-axis direction.

Finally, this plot could suggest that the optimal mix-

ing of the fluorescein and oxazine is at the full-width-

half-max of the displayed peaks. Further testing and data

analysis will be done to confirm this.

50

70

90

110

130

150

170

190

210

230

-7 -2 3 8

Dro

ple

t D

iam

ete

r (u

m)

X-axis Position

Sauter Mean Diameter Distribution for Varying Spray Impingement Angles

90 degrees

70 Degrees

50 Degrees

150

250

350

450

550

650

750

-7 -2 3 8

Dro

ple

t D

iam

ete

r (u

m)

X-axis Position

Volume Distribution for Varying Spray Impingement Angles

90degrees

70Degrees

50Degrees

1.4

1.5

1.6

1.7

1.8

1.9

2

2.1

2.2

2.3

2.4

-7 -2 3 8

Dro

ple

t S

pa

n

X-axis Position

Span Distribution for Varying Spray Impingement Angles

90Degrees

70Degrees

50Degrees

8

Red to Blue Laser Relative Intensities for the 50° Impingement

angle

Figure 15. 3D relative intensity plots for the 50° im-

pingement angle.

Summary

An experimental investigation of spray structure

from unlike doublet injectors was conducted. Based on

the high speed cinematography data analysis performed,

the 70° impingement angle displayed the highest spray

cone angle, while the 90° impingement angle displayed

the largest average sheet breakup distance. Furthermore,

the drop size data collected suggests that the 50° im-

pingement angle produces the largest droplet Sauter

Mean diameter and volume distribution droplet size for

each X-axis position tested, demonstrating a dependence

of the spray structure on impingement angle. Addition-

ally, the Mie scattering calculations coupled with the

Malvern and light intensity data suggest that there is no

wavelength dependence on scattering efficiency

throughout the experiments ran. Lastly, the DLIF data

collected suggests that there may be unequal mixing of

the oxidizer and fuel throughout the spray plume.

Additional experiments need to be conducted to help

better understand the dynamic spray structure produced

by two unlike doublet injectors. In the future, planar

measurements will be taken with the DLIF system. Also,

image splitter instrumentation will be incorporated in the

DLIF system so that the concentration/intensity of both

fluorescein and oxazine can be measured at the same

time, reducing systematic and human errors. Addition-

ally, more droplet size and high speed cinematography

data will be collected to help better understand the dy-

namics present in the sprays. All of these results can help

to better interpret the role of fuel/oxidizer injection on

thrust performance results in associated hot fire tests.

Acknowledgements

The authors would like to acknowledge the assis-

tance of the University of California, Irvine Spectros-

copy Laboratory and BioTel Laboratory for access to

their laboratories. The authors and Mr. Leask in particu-

lar also acknowledge the University of Glasgow Engi-

neering Department for assisting in the collaboration

with the University of California, Irvine. Mr. Leask is at

UC Irvine as part of an international education oppor-

tunity made possible by the University of Glasgow.

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