ILASS Americas 28th Annual Conference on Liquid Atomization and Spray Systems, Dearborn, MI, May 2016

The Effect of Fuel Temperature on the Penetration of a Liquid Jet in Crossflow

Heather K. Wiest∗ and Stephen D. HeisterDepartment of Aeronautics and Astronautics

Purdue UniversityWest Lafayette, IN 47907 USA

AbstractThe increased thermal demands of advanced aeroengines necessitates the use of fuel as a primary coolant.The resulting higher fuel temperatures cause a reduction in fuel density, viscosity, and surface tension, whichcan have an effect on the overall spray breakup, trajectory, and penetration of the liquid jet in crossflow. Anexperimental study was conducted to assess the effects of elevated fuel temperatures on the penetration of aheated liquid jet in crossflow. Experiments were run with a liquid jet of Jet-A fuel at injection temperaturesfrom ambient to over 320◦C in a steady air crossflow that could be adjusted to simulate gas turbine combustorconditions. Based on the vapor pressure curve of Jet-A, flashing conditions for the jet were assumed to bereached at the highest fuel temperatures. High speed backlit imaging as well as Mie scattering was usedto capture the reduction in penetration caused by the large increase in fuel temperature. Image processingtechniques were employed to extract the upper edge trajectory of the jet in an effort to develop a correlationfor predicting penetration changes for the heated liquid jet in crossflow. Typically, trajectory correlationsare based on the momentum flux ratio, aerodynamic Weber number, and axial distance downstream fromthe injection point. For a heated liquid jet in crossflow, the vaporization of a multi-component fuel as wellas changes in fuel properties need to be incorporated into a correlation in order to effectively predict thetrajectory.

∗Corresponding Author: hwiest@purdue.edu

Introduction

The liquid jet injected transversely into agaseous crossflow is an extensively researched flowfield with numerous applications in airbreathingpropulsion devices including low NOx gas tur-bine combustors, turbojet afterburners, and scram-jet/ramjet engines. In comparison with a free jet,the liquid jet in crossflow (LJICF) configurationallows for efficient fuel-air mixing as aerodynamicforces on the jet from the crossflow augment atom-ization. Jet trajectory, penetration, breakup andother spray characteristics resulting from the atom-ization process of the LJICF can have significant ef-fects on the performance of combustors. Overall,the jet in crossflow has been widely studied theoret-ically, experimentally, and computationally, whereresearchers have examined both liquid and gaseousjets under reacting and non-reacting conditions forsingle and multi-phases flows [1–3].

In the LJICF configuration, the atomizationprocess of the liquid jet is driven by the growth ofinstability waves along the jet column as a result ofthe passing crossflow. These instability waves canbe initiated from various factors such as imperfec-tions in the injector, fluctuations in the fuel feedpressure, and turbulence in the air or fuel flows [4].The breakup of an unheated liquid jet in an ambienttemperature and pressure crossflow is illustrated inFigure 1. As instability waves grow on the wind-ward side of the jet, the liquid column will beginto fracture into ligaments then drops, also knownas column breakup. Drops and ligaments strippedoff of the intact liquid column, also known as sur-face breakup, are a result of the high shear forcesproduced by the crossflow. Smaller drops are gener-ated from surface breakup as compared with columnbreakup, resulting in larger variations of the dropsize distribution [5].

Heated Fuels

As advancements in gas turbine engine technol-ogy are made, various thermal management chal-lenges have surfaced, and the associated increase inthermal demands creates the need for utilizing fuelas a primary coolant. Operating engines with heatedfuels can have both positive and negative effects.The resulting rise in fuel temperature from exploit-ing the fuel’s heat sink capabilities leads to an in-crease in its sensible enthalpy, allowing less fuel tobe burned to reach a specified level of thrust. Emis-sions can also be positively influenced by employingheated fuels with lower levels observed for carbonmonoxide and unburned hydrocarbons as a resultof higher combustion inefficiencies associated with

Cross ow

Ligaments

Liquid Jet

Drops

Column

Breakup

Instability

WavesSurface

Breakup

Figure 1. Schematic of liquid jet in crossflow atom-ization including instability waves, column vs. sur-face breakup, ligaments, and drops

burning heated fuels [6]. However, the thermal sta-bility of jet fuel at high temperatures can give rise tothe formation of deposits within the fuel lines or in-jector through autoxidative coking of the fuel [7].Typically, current aeroengines limit the incomingfuel temperature to near 160◦C in order to mitigatethe effects of autoxidative coking, but deoxygena-tion methods such as nitrogen sparging, membrane-based fuel deaerators, and catalytic deoxygenationhave been shown to effectively reduce dissolved oxy-gen in the fuel to safe levels of less than 1 ppm [8].

The use of heated fuels in gas turbine combus-tors also creates a potential for flash atomizationto occur. Flashing jets exhibit violent and explo-sive atomization characteristics from voids in thefluid that develop as a result of depressurization andphase change [9]. Flash atomization can occur whenfuel at an elevated temperature is injected into anenvironment with a pressure lower than the fuel’svapor pressure at that given temperature[10]. Interms of the LJICF, increased fuel temperatures canlead to the formation of smaller drops as well as adecrease in overall spray penetration as comparedwith room temperature jets [11]. Flash atomizationof the LJICF is characterized by a rapidly expandingspray that can penetrate upstream prior to bendingover from its interaction with the crossflow . Thebreakup of a heated liquid jet at flashing conditionsin an ambient temperature and pressure crossflow isillustrated in Figure 2.

If the spray penetration is lower and the dropletsproduced are smaller with heated fuels, the emis-sions, ignitability, and performance of gas turbineengines can be influenced. Although issues can arisefrom utilizing heated fuels in combustors such as va-por lock or fuel coking, there is a potential for en-

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Droplet

Stripping

Cross ow High Jet

Bending

Vapor

Cloud

Upstream Jet

Penetration

Figure 2. Schematic of liquid jet in crossflow flashatomization showing the large degree of jet bendingand including the upstream penetration of the jet

hanced mixing as a result of the effects of heatedfuels on spray atomization. Overall, the main bene-fit of flash atomization is the finer droplet sizes thatare formed, which quickly vaporize.

Trajectory Correlations

One of the most important characteristics of theliquid jet in crossflow is its trajectory and resultingpenetration into the crossflow. As the jet enters thecrossflow, it begins to bend over in the direction ofthe crossflow as it undergoes the breakup process.Correlations for the trajectory of a liquid jet in cross-flow have been developed from experimental data bytracking the windward side of the jet by using imageprocessing techniques. By performing a simple forcebalance on a column of liquid in a gaseous crossflow,the trajectory of the jet as it moves downstream fromthe injection location is shown to be a function of thesquare root of the momentum flux ratio [12].

Trajectory correlations and breakup regimeshave been well characterized based on momentumratio (q) and aerodynamic Weber number (We∞)for liquid jets injected into atmospheric crossflowconditions [12–16]. In a few experimental studies,the LJICF has been subjected to elevated crossflowtemperatures and/or pressures as well as varying in-jection fluids in order to simulate the conditions of agas turbine combustor [5, 17–19]. In terms of cross-flow property variations, these experimental studieshave shown that jet penetration and spray area cov-erage reduces with an increase in ambient pressureof the airflow [5, 20, 21]. Changing the air pres-sure or temperature serves to vary the air density,which affects the spray dispersion and level of atom-ization [17, 18]. For a fixed momentum flux ratioand crossflow pressure, an increase in the crossflowtemperature results in a decrease of the penetration,mostly likely due to a high evaporation rate of theliquid ligaments and drops in addition to the increasein crossflow velocity [22].

The typical LJICF correlations predict jet pen-etration as a function of momentum flux ratio and

axial distance from the injection location. As re-searchers varied crossflow properties in their exper-iments, correlations have evolved to include aerody-namic Weber number and gas transport propertiesincluding viscosity. Ragucci et al conducted a se-ries of LJICF experiments with elevated air crossflowtemperatures and pressures, simulating gas turbineengine conditions, and their trajectory correlation isgiven in Equation 1 [17].

y

d= 2.28q0.422We∞

−0.015(µ∞

µair,300K)0.186(

x

d)0.367

(1)

In an effort to characterize the effects of jet bend-ing, Stenzler et al included a viscosity term of theliquid as shown in Equation 2 [19]. These researchersstudied various liquids including water, acetone, and4-Heptanone, which were injected at room temper-ature. High liquid viscosities lead to increased dragforces on the jet causing a larger degree of jet bend-ing in the direction of the crossflow.

y

d= 3.35q0.442We∞

−0.088(µl

µH2O)−0.027(

x

d)0.391

(2)

Since viscosity decreases with an increase in fueltemperature, the Stenzler correlation predicts that arise in fuel temperature will result an increase in jetpenetration; however, the opposite trends have beenobserved for the highest fuel temperatures tested.In order to more accurately predict the trajectory ofheated liquid jets in crossflow, other parameters ofthe liquid need to be considered in the correlations.The objective of this work is to explore the inclusionof other fuel properties in an effort to characterizethe reduction in penetration observed for the heatedliquid jet in crossflow.

Experiment

Experimental testing of a heated liquid jet inan air crossflow was performed in the Gas TurbineTest Cell of the High Pressure Laboratory at Pur-due University’s Maurice J. Zucrow Laboratories.Airbreathing experiments at the Zucrow Laborato-ries operate from the facility’s blow-down air system.The system is equipped with compressed air tanksthat are capable of holding 57 m3 (2,000 ft3) of to-tal air volume at 15 MPa (2,200 psi). A natural gasfired heat exchanger is utilized to preheat a contin-uous air flow of approximately 3.6 kg/s (8 pps) and4.8 MPa (700 psi) to 540◦C (1,000◦F) for the max-imum operating condition. Purges and pneumaticcontrol for experiments are set from the lab’s bulk

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supply of gaseous nitrogen stored in two 7 m3 (250ft3) tanks at 41 MPa (6,000 psi).

The rig employed for testing was previously usedfor studying a premixed nitrogen-hydrogen jet in-jected transversely into an acoustically unstable vi-tiated crossflow [23]. A dimensioned schematic ofthe test rig used in this experiment is shown in Fig-ure 3. A plumbing and instrumentation schematic ofthe rig with air, nitrogen, and fuel supplies is shownin Figure 4. In previous experiments, a natural gasfired dump combustor with a half-wave resonator in-let was designed to produce unstable crossflow con-ditions in the rig. By removing the dump combustorand adding a flow straightening orifice plate with 25x 6.35 mm (0.25”) holes, the rig could be utilizedfor testing with steady air crossflow conditions. Thetest section consisted of a square duct with inner di-mensions of 76.2 x 76.2 mm2 (3 x 3 in2). A troughwas placed downstream of the rig’s exhaust exit inorder to collect the unburned, liquid fuel.

Air Flow

Direction

Ori ce

Plate

Fuel

Injector

Exit

Nozzle

0.9 m0.8 m

0.9 m 0.45 m 0.75 m

Figure 3. Schematic of the test rig with dimensionsand callouts for the air flow direction, orifice plate,fuel injector, and exit nozzle

Figure 4. Schematic of the facility’s air, nitrogen,and fuel supply systems integrated into the currentexperimental set-up. (PT - pressure transducer, T- thermocouple, HFPT - high frequency pressuretransducer)

Jet-A fuel was used for the liquid jet during alltesting and was delivered to the test cell at pressuresup to 10 MPa (1,500 psi) by a high pressure pumpfrom a 1,060 Liter (280 gallon) fuel tank. Fuel flowrates were controlled with remotely actuated meter-ing values and are measured using a MicroMotionCoriolus Flow Meter (ELITE with M2700 transmit-ter). Elevated fuel temperatures were reached bya 25kW electric cartridge heater with a closed-looptemperature control. In order to mitigate the po-tential for autoxidative coking in the fuel lines andinjector, nitrogen sparging was employed. This de-oxygenation method was incorporated into the fuelsystem via a sparge rail within the jet fuel tank. Thelevels of dissolved oxygen in the fuel were monitoredby a Mettler-Toledo Inpro 6850 sensor and were mea-sured to be less than 0.2% of fully saturated valuesbefore fuel heating can begin. The fuel injector wasa plain orifice with a diameter of 0.5 mm (0.020”)and an L/D of 20. The injector transitioned from atube of diameter of 4 mm (0.157”) with a 90◦ taper.The longer L/D was chosen in an effort to producea fully developed turbulent jet exit velocity profilewith a Reynolds number of approximately 10,000 forunheated fuel at the average fuel massflow rate.

All temperatures, pressures, and flow rates weremonitored during testing in addition to collectinghigh speed images of the heated liquid jet in cross-flow atomization via backlit imaging and Mie scat-tering. Temperatures were measured with Type Kthermocouples, low frequency pressures with DruckPMP 1260 and 4060 pressure transducers of vari-ous ranges, and high frequency pressures with KuliteEWCT-312M with a range of 1 MPa and a sam-pling rate of 180 kHz. The uncertainty associatedwith the measured experimental conditions include±0.04% of the full scale range of the low frequencypressures, ±0.1% of the high frequency pressures,±0.75% of the temperatures, and ±0.1% of the fuelflow rates. The error propagation for the calculatedvalues for momentum flux ratio, aerodynamic We-ber number, and air flow rates led to uncertaintiesof ±10%, ±3%, and ±0.5% respectively. The largestuncertainties were associated with the parameterscalculated from the error in the injector diameter in-cluding the momentum flux ratio and aerodynamicWeber number. As the fuel injector’s diameter wasvery small (0.5 mm), the seemingly small error esti-mation (±0.0125 mm) was still 2.5% of the nominalmeasurement.

Optical Diagnostics

Optical access for high-speed imaging and laserbased measurement techniques in the rig was

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achieved with 127 x 76.2 mm2 (5 x 3 in2) quartz win-dows. Backlit imaging and Mie scattering were usedto characterize the changes in overall spray breakup,jet column trajectory, and jet column width as a re-sult of heating the jet. The camera settings for test-ing with either high speed backlit imaging or Miescattering is given in Table 1. Schematics of each di-agnostic method’s configuration are provided in Fig-ure 5. Only the backlit images were studied in thispaper. The Mie scattering diagnostic configurationand application to the heated liquid jet in crossflowis provided as reference. Future work will includethe analysis of the Mie scattering data [24]

Table 1. Camera settings for backlit imaging andMie scattering.

Camera

Setting

Backlit

Imaging

Mie

Scattering

Camera Lens 105 mm 250 mm

Focal Ratio f/2.8 f/4

Camera Exposure 40 µs 14 µs

Imaging Rate 10 kHz 10 kHz

Image Resolution 8 pixelsmm 14 pixels

mm

Raw Image Size

(row x column)

512 x 768

pixels2592 x 640

pixels2

Field of View

(height x width)

64 x 96

mm2

42 x 46

mm2

Backlit images were collected through a Nikon105 mm focal length lens with f/2.8 and recordedwith a Photron SA4 high speed CMOS camera. Thejet was backlit through the test section’s bottomwindow from a diffuse LED light source. The lightreflected off a mirror placed above the rig into thecamera setup. Mie scattering was performed withone laser head of a dual cavity, diode-pumped, solidstate Nd:YAG laser (Edgewave IS811-DE). For thisexperiment, the laser was used to produce 532 nmlight at 0.2 mJ/pulse for a repetition rate of 10 kHz.A collimated laser sheet of 2 inches was created byusing two cylindrical lenses (f1 = -25 mm, f2 = 300mm). The sheet was focused using a third cylindri-cal lens (f3 = 750 mm). The scattered light from theliquid fuel spray was reflected off a mirror located di-rectly above the rig and then collected through 200mm focal length f/4.0 objective lenses (Nikkor IF-ED Micro) with an extension tube and recorded bya Photron SA4 high speed CMOS camera.

Both diagnostic techniques were employed asthey provide different information about the LJICF

Camera

ViewingMirror

Test Section

LightSource

Diffuser

(a) Backlit imaging

Sheet Forming Optics

Camera

Laser532 nm

ViewingMirror

Test Section

(b) Mie scattering

Figure 5. Diagnostic configurations used duringtesting of a heated liquid jet in crossflow

atomization process [25]. Backlit imaging is a line ofsight measurement based on light extinction that canoccur when fuel in either a gas or liquid phase passesbetween the light source and the camera. Therefore,the backlit images were utilized to determine thejet trajectory in addition to the visual study of theoverall atomization process. The Mie scattering im-ages are created from a planar measurement that isa result of light scattering off of the liquid surfaces.Therefore, the Mie scattering images can be used todetermine the location of the liquid column and itsconsequent break point.

After calibration and all test condition imageswere collected, LaVision DaVis 8.2.0 software wasemployed for image preprocessing. Both the backlitand Mie scattering images were calibrated to deter-mine spatial resolution using a dual plane dot target(LaVision Type 05). The preprocessing in the LaV-ision software included mapping the image to thespatial calibration, adjusting for any optical distor-tions, and setting the spatial coordinate origin tothe centerline of the fuel injector face. All process-ing of spatially calibrated images was performed inMATLAB. Each image was rotated and cropped tocapture the desired viewing area. Additionally forbacklit images, each recorded image was adjusted

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by comparing each pixel’s intensity with the corre-sponding intensity of the averaged background im-age. The maximum and minimum intensity valuefor each pixel comparison was determined, and thedifference between these two values was calculatedfor each pixel, resulting in a corrected image.

Average images used in determining the gen-eral trajectory trends of all test cases were takenover 1,000 images. Each test case’s average imagewas converted to a binary image through threshold-ing with Otsu’s method, which chooses a thresh-old based on the intraclass variance of the blackand white pixels [26]. The threshold value basedon Otsu’s method was determined from MATLAB’sgraythresh function. Other researchers have usedthis thresholding method for image processing ofshear-coaxial gaseous jets in crossflow [27]. The edgeof the binary image was determined using MAT-LAB’s edge function.

Due to substantial changes in jet behavior fromthe varying levels of liquid vaporization, severalthresholding techniques were tested included thresh-olding on an optical thickness [28, 29] and image gra-dients [30]. In addition, a range of thresholds aroundthe value determined from Otsu’s method was eval-uated to determine a sensitivity to thresholding. A±10% change in the threshold value resulted in anapproximately ±1% variation in the penetration ofthe jet at 25 diameters downstream of the injectionlocation. Additionally, a source of error is intro-duced from the discretization of the field of viewinto pixel sensors of the CMOS camera. The un-certainty associated with the discretization of themeasurement is one pixel in all directions from thepixel position of interest [31].

Test Conditions

Four test series were conducted to collect datafor the heated liquid jet in crossflow study. Testseries consisted of two different crossflow conditionsfor temperature and pressure (ambient and elevated)with two diagnostic techniques employed for eachcrossflow condition (high speed backlit imaging andMie scattering). The ambient air crossflow condi-tions were set to an average temperature of 17◦C(65◦F) and pressure of 108 kPa (15.7 psi). The slightincrease in the test section’s pressure is attributedto the exhaust traveling through a long pipe section(∼3 m) installed at the rig’s exit, used to deflectthe exhaust down and away from the test cell. Theelevated temperature and pressure crossflow condi-tions were approximately 330◦C (615◦F) and 7 MPa(80 psi). For both crossflow conditions, the air washeld constant at a mass flow rate of 0.45 kg/s (1.0

pps). The jet conditions and resulting momentumratio for each test cases across all four test series isgiven in Table 4. Weber number for the unheatedfuel cases was approximately 100 and increased toabove 400 due to a large reduction in surface ten-sion as the fuel temperature was elevated. In aneffort to characterize the flow field of the air cross-flow, six high frequency pressure transducers wereinstalled in the air flow path and test section. Lowpressure amplitude fluctuations were measured thatproduced negligible changes in momentum flux ratioand aerodynamic Weber number allowing the cross-flow to be considered steady.

While fuel and air flow rates were held constant,the fuel temperature was increased from ambient toover 320◦C (600◦F). If the heated liquid fuel is in-jected from a high pressure into a low pressure en-vironment, rapid vaporization of the fuel or flashingcan occur. The transition point where flashing con-ditions can occur for the chosen test section pres-sures is shown on the Jet-A vapor pressure curve inFigure 6. The transition between a liquid jet and aflashing jet condition occurs at a fuel temperature ofapproximately 190◦C (375◦F) for the ambient cross-flow and 285◦C (545◦F) for the elevated tempera-ture and pressure crossflow. During all conditions,the fuel injection pressure remained high enough toensure vapor lock did not occur in the fuel injec-tor, approximately 860 kPa (125 psi). Data was col-lected at 7-8 points along a fuel temperature sweep.Temperature, pressure, and fuel property measure-ments were recorded for approximately 10 secondsonce the liquid jet was introduced into the crossflow.During that time period, backlit and Mie scatteringimages for their respective test series were acquiredfor about 0.5 seconds.

Figure 6. Vapor pressure curve for Jet-A/JP-8

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Properties of the jet fuel at a given tempera-ture and pressure were determined using the NISTprogram Supertrapp. The fuel was approximated as11 major components, which are given in Table 2along with their respective mole fractions. Proper-ties for Jet-A’s density, viscosity, and enthalpy wereobtained from the Supertrapp analysis. Addition-ally in this program, the feed fraction of the fuelcan be determined, which is defined as a fuel’s splitbetween the liquid and vapor phases at a specifictemperature and pressure. For an ambient pressurecrossflow, the range of temperatures over which theassuming fuel composition is two-phase spans from170 to 215◦C (340 - 420◦F). For an elevated cross-flow pressure of approximately 80 psi, the tempera-ture range is from 255 to 290◦C (490 - 560◦F). For

Table 2. Major components of Jet-A used in Su-pertrapp to estimate properties

ComponentMolecularFormula

MoleFraction

Methylcyclohexane C7H14 0.075

Meta-Xylene C8H10 0.07

N-Octane C8H18 0.13

N-Decane C10H22 0.156

Butylbenzene C10H14 0.055

Isobutylbenzene C10H14 0.055

T-Butylbenzene C10H14 0.055

N-Dodecane C12H26 0.175

1-Methylnaphthalene C11H10 0.052

N-Tetradecane C14H30 0.112

N-Hexadecane C16H34 0.065

a consistent momentum flux ratio calculation acrossall test conditions, the density of the fuel was de-termined at the injection conditions as the pressureremained high enough in the injector to ensure thefuel remained a liquid. When injecting a high tem-perature liquid into the lower pressure test section,the jet can experience a phase change at elevatedtemperatures. Since Jet-A is a multicomponent fuel,not all constituents of the fuel vaporize at the sametemperature, introducing additional uncertainty inthe momentum flux ratio calculation if the densityof the fuel is estimated at the test section pressure.

Results

LJICF Overall Spray Characteristics

By visual inspection of the images collected dur-ing testing, the overall spray characteristics includ-

ing penetration, trajectory, droplet stripping, andcolumn break point location of the LJICF can beidentified. The variations due to the effects of in-creasing fuel temperature or changing crossflow con-ditions can be compared. For a qualitative analysisof the spray characteristics, single images from back-lit, background corrected, high speed videos wereemployed. All distances on images provided in theResults section are normalized by the fuel injectorexit diameter of 0.5 mm. An image of an unheatedliquid jet in an air crossflow at atmospheric tem-perature and pressure is given in top image of Fig-ure 7, and an image of an unheated liquid jet in anair crossflow at elevated temperature and pressure isgiven in the bottom image.

(a) Ambient crossflow conditions (q = 99)

(b) Elevated crossflow conditions (q = 126)

Figure 7. Single frame backlit images of an un-heated liquid jet in crossflow with varying air tem-perature and pressure

The unheated jet in an ambient air crossflowproduces a fine, dense fuel spray downstream of theintact liquid column. The darkest region in the im-age indicates where the bulk liquid is located asthere is a high level of optical extinction in thatarea. Instability waves can be observed along thewindward side of the jet which form and lead to col-umn breakup then into ligaments and further into

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droplets. In addition, remnants of these instabilitywaves can be seen in the spray cloud, just down-stream of the liquid column break point. The inter-action of the crossflow with the jet strips dropletsfrom the liquid column starting very near the exit ofthe fuel injector on the leeward side.

The overall structure of the unheated jet in el-evated crossflow conditions is very similar to thatof an ambient crossflow. As the momentum ra-tios of the unheated jet in the top and bottom im-ages of Figure 7 are different (99 and 126 respec-tively), conclusions regarding penetration differencesdue to varying crossflow conditions cannot be explic-itly made; however, the penetration of the jet in theelevated crossflow penetrates nearly the same dis-tance as the ambient crossflow jet even though themomentum flux ratio is much higher. It is expectedthat a liquid jet in a crossflow with elevated tem-perature and pressure will experience a reduction inpenetration and spray coverage. The main differencefor the unheated jet’s structure under these varyingcrossflow conditions is that the width of the jet inelevated crossflow conditions as it exits the injectorappears narrower than the jet in the ambient cross-flow, which can be a result of increased vaporizationof the fuel from the passing high temperature cross-flow.

The four images in Figure 8 correspond to jettemperatures of approximately 175, 220, 235, and245◦C in an ambient air crossflow. Images corre-sponding to fuel temperatures below approximately180◦C are not provided as very little change in thebehavior of the LICF was observed. In general, asfuel temperature increases, the spray produced bythe LJICF becomes finer and denser as a result ofthe higher vaporization rate. This change in thespray cloud is indicated by the increased darknessof regions downstream of the jet column. In addi-tion, the liquid column widens as compared to theunheated LJICF. For elevated fuel temperatures be-low the transition point to flashing, the trajectory isnot greatly changed from the unheated LJICF.

As fuel heating continues, the multicomponentfuel begins vaporizing the lighter end hydrocarbonsas it approaches the flashing conditions as shown inthe series of images from top to bottom in Figure 8.The jet appears to be mostly vapor as it issues fromthe fuel injector in the bottom image. The flashingjet immediately bends over in the crossflow direc-tion and stays near the wall of the test section forthe length of the viewing window. Most notable isthe upstream penetration of the jet column at thehighest fuel temperature conditions. The increasein the column width or upstream penetration is also

(a) Tj = 173◦C and q = 130

(b) Tj = 219◦C and q = 123

(c) Tj = 233◦C and q = 125

(d) Tj = 244◦C and q = 151

Figure 8. Single frame backlit images of an liquidjet with increasingly elevated fuel temperatures inair crossflow at ambient temperature and pressure

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observed for the second highest fuel temperature.Although there is no momentum imparted in theupstream crossflow direction from the fuel injection,the explosive nature of the near and at flashing jetcauses this column growth.

Figure 9 provides three images of a heated liq-uid jet at temperatures near 290, 315, and 335◦Cas they are injected into an elevated temperatureand pressure crossflow. The jet in the top imagein Figure 9 still produces a similar trajectory anddownstream spray as the unheated jet shown in thebottom image of Figure 7. However, the spray cloudis less dense as the droplets are rapidly evaporatingas indicated by the lighter color downstream of thejet in the image. The increased fuel temperature inconjunction with the elevated crossflow temperatureleads to this higher level of vaporization of the liquidjet. Additionally, the intact liquid column reducesin width before the column break point as the fueltemperature increases while still below the transi-tion to flashing point. Also, the stripped dropletsare almost fully evaporated at a distance very nearthe liquid column.

As the flashing condition point is reached andsurpassed, the structure and breakup behavior of thejet in an elevated pressure and temperature crossflowgreatly changes. The middle and bottom images ofFigure 9 display the jet at flashing conditions. Thejet column is widened and the downstream spraycloud is almost nonexistent in the images. The pen-etration of the jet is also greatly reduced in com-parison with the heated jets below the transition toflashing point. The difference in the flashing jetsas they are injected into the ambient and elevatedcrossflow conditions (bottom image in Figure 8 andFigure 9 respectively) is especially notable as thelarge vapor cloud which remained very near the testsection’s wall is reduced to a short column with theincrease in air temperature and pressure.

Upper Edge Jet Trajectories

The upper edge jet trajectory of the liquid jet incrossflow was calculated for increasing fuel temper-atures at each set of crossflow conditions using thebacklit images. The trajectories were detected fromeach test case’s average image over 1,000 images.The trajectory distances are normalized by the fuelinjector diameter. All trajectories were also shiftedto start at the origin and were detected for approxi-mately 40 jet diameters downstream. This distancecovers the regions of the intact liquid column, breakpoint, and the initial breakup of ligaments into dropsfor most test cases. Trajectories determined fromthe backlit images are displayed in Figure 11 (am-

(a) Tj = 291◦C and q = 148

(b) Tj = 314◦C and q = 173

(c) Tj = 334◦C and q = 158

Figure 9. Single frame backlit images of an liquidjet with increasingly elevated fuel temperatures inair crossflow at elevated temperature and pressure

9

bient crossflow conditions) and Figure 12 (elevatedcrossflow conditions).

For ambient crossflow conditions, as the fueltemperature increases while remaining below thetransition to flashing temperature, the trajectoriestend to penetrate further in the transverse direction.This result is consistent with those of Stenzler et al.A decrease in fuel viscosity occurs with an increasein temperature. Higher viscosity liquids are associ-ated with an increased drag coefficient, which leadsto a greater degree of jet bending in the crossflowdirection [19]. A lower viscosity liquid will exhibit alesser degree of jet bending in the crossflow directionresulting in greater penetration in the transverse di-rection as observed. Although a higher fuel tem-perature increases the vaporization rate of the fuelleading to smaller droplets, the decrease in penetra-tion that accompanies smaller droplets because oftheir lower momentum is not enough to counteractthe viscosity effects.

As fuel temperature increases near and past thetransition to flashing temperature, the rapid increasein vaporization decreases the jet’s penetration in thetransverse direction as the degree of jet bending inthe crossflow direction increases. Similar observa-tions were made by researchers at UCLA studyinggaseous jets in an air crossflow. Decreasing the den-sity ratio of the gaseous jet in crossflow at a constantmomentum flux ratio produces a jet with a higherlevel of expansion and lower penetration [32]. In ad-dition, the flashing jet bends over significantly in thecrossflow direction and remains at the lowest pene-tration height of all test cases. This change in jettrajectory can be attributed to the rapid increasedvaporization rate associated with the heated fuel asit is injected into the atmospheric pressure test sec-tion.

For the elevated crossflow condition test cases,the transverse penetration of the jet decreases asfuel temperature increases for all test cases. Thehigh level of fuel vaporization which is augmentedby the high temperature air crossflow overcomesthe viscosity effects seen for the lower temperaturejets in ambient crossflow conditions. Additionally,a high pressure crossflow conditions also contributesto a lower penetrating jet as observed by other re-searchers [5]. Once the transition to flashing tem-perature is reached and surpassed, the jet completelyevaporates at axial distances increasingly further up-stream. The two highest temperature cases have tra-jectories that end less than 15 jet diameters down-stream from the injection point. The high levels ofvaporization in conjunction with the elevated tem-perature of the crossflow cause the jet to be almost

fully evaporated in this short distance. Again, theflashing jet penetration is greatly reduced in compar-ison with the unheated jet at the elevated crossflowconditions.

These two sets of trajectories were also scaledby momentum flux ratio and are provided in Fig-ure 13 and Figure 14. The effects of fuel temperatureon the atomization of a liquid jet in crossflow werestudied while attempting to keep momentum flux ra-tio constant. Any variation in momentum flux ratiowas due to experimental difficulties in holding thatvalue constant. Therefore, the trajectories scaledby both jet diameter and momentum flux ratio helpto normalize the results, minimizing the effects ofslight changes in the test conditions. These scaledtrajectories will also be utilized in the trajectory cor-relation analysis discussed in the next section. Thetrends observed in Figures 11 and 12 are confirmedby the momentum flux ratio scaled trajectories givenin Figures 13 and 14.

Trajectory Correlation Analysis

As previously mentioned, many researchers havedeveloped trajectory correlations to predict the pen-etration of a liquid jet in a crossflow. Typical vari-ables included in the trajectory correlations are axialdistance (x/d), aerodynamic Weber number (We∞),and momentum flux ratio (q). Some researchers haveincluded other fluid properties such as liquid or airviscosity. However, these correlations do not predictthe changes in penetration that accompany increasesin fuel temperature [33].

A least squares regression analysis was per-formed in R to predict the penetration height of aliquid jet with increasingly elevated temperatures inan air crossflow with temperature and pressure atboth ambient and elevated conditions. Additionally,a statistical analysis was performed in R to eval-uate the goodness of fit of the predicted penetra-tion height equations given in this section. Vari-ables considered in this analysis were axial distance(x/d), aerodynamic Weber number (We∞), momen-tum flux ratio (q), feed fraction (X), gas viscos-ity normalized by ambient air viscosity (µ′∞), liquidfuel viscosity normalized by water viscosity (µ′l), andlevel of superheat (∆Ts).

The momentum flux ratio has been widelyagreed upon by researchers to have the largest effecton the trajectory of a liquid jet in crossflow. By per-forming a simple force balance on the liquid jet, thetrajectory is shown to be dependent on the squareroot of the momentum flux ratio. When the fuel isheated to a point where explosive expansion and va-porization of the jet occurs upon injection, the mo-

10

mentum ratio no longer proves to be the dominantfactor in predicting jet penetration. For the highestfuel temperature case with an ambient air crossflow(Test Case 9), the momentum flux ratio increasedby approximately 20%, while the penetration at 25jet diameters downstream from the injection pointdecreased by almost 30% from that of the previousjet temperature condition (Test Case 8).

Holding all else constant, an increase in fuel tem-perature corresponds to an increase in aerodynamicWeber number due to the decrease in surface tensionof the fuel. Jet trajectory should scale inversely withWeber number as determined by other researchers.At a higher Weber number, smaller droplets areformed from the liquid jet, which penetration lessin the transverse direction. Although the trend inWeber number is in agreement with the observa-tions of lower penetration for a very high temper-ature LJICF, this variable does not explain how therapid fuel vaporization affects the flow field. Ad-ditionally, aerodynamic Weber number usually con-tributes very weakly in trajectory correlations, so itwill not be included in this analysis.

Since Jet-A is a multicomponent fuel, the va-porization process becomes complex. Lighter endhydrocarbons have a lower boiling point with re-spect to the heavier components in the fuel. While inthe high pressure feed line and injector, the elevatedtemperature fuel remains a liquid. Once injectedinto the crossflow, the fuel essentially undergoes arapid distillation, which could be the cause of thelarge reduction in penetration observed at the high-est fuel temperature conditions. As the fuel tem-perature increases from ambient to the point wherecomponents of the fuel begin vaporizing in the testsection, fuel properties such as density and viscosityare hard to determine.

To capture this multicomponent fuel vaporiza-tion process, the feed fractions obtained from Su-pertrapp can be plotted with respect to fuel tem-perature as shown in Figure 10. The feed fractiondescribes the relative quantity of the liquid and va-por phases at a given fuel temperature and crossflowpressure. The feed fraction values do not take intoaccount the effects of the elevated air crossflow tem-perature.

The feed fraction serves as a way to capture thereduction in jet momentum that occurs as a resultof the rapid fuel distillation upon injection. How-ever, test conditions that were predicted by the feedfractions in Supertrapp to be entirely vapor once in-jected into the test section still saw a variation inpenetration. For the ambient crossflow conditions,Test Cases 7-9 were characterized with a feed frac-

tion indicating total vapor, but there was an almost40% reduction in penetration as the temperaturewas raised across the three conditions. Additionally,this reduction in penetration while injecting fuel atconditions conducive to producing an entirely vaporjet was observed for elevated crossflow conditions inTest Cases 16 and 17.

Figure 10. Feed fraction curve for Jet-A/JP-8

While the feed fraction may be beneficial in pre-dicting jet penetration changes through the transi-tion from liquid to vapor phase, it not capable ofcharacterizing the highest temperature conditions.Another parameter, the level of superheat, was cal-culated for each set of test conditions. This pa-rameter compares the difference between the fueltemperature and it’s saturation temperature at thegiven crossflow pressure, normalized by the satura-tion temperature. The saturation temperature wasdetermined from vapor pressure curve given in Fig-ure 6. For both sets of crossflow conditions, anyfuel temperatures lower than the corresponding sat-uration temperature will result in a negative levelof superheat, even if the conditions allow for somecomponents of the fuel to begin vaporizing.

At fuel temperatures lower than the point wherethe fuel begins to be rapidly distilled upon injec-tion, the overall jet structure remains unchanged.It appears that variations in jet penetration at thelower fuel temperatures are a result of fuel propertychanges. Once components of the fuel begin to va-porize, the physical chemistry of the fuel changes.In an effort to correctly characterize the trajectoryof the heated liquid jet in crossflow, the trajectorycorrelation was analyzed below the transition tem-perature to fuel distillation leading to flashing jetconditions separately from the higher temperatureconditions.

11

Large differences were observed in overall jetstructure for high fuel temperatures at the varyingair crossflow conditions. For the elevated crossflowconditions and highest fuel temperatures, fuel vapor-ization was augmented by the increased air temper-ature leading to a very small jet that disappearedwithin 15 jet diameters from the injection point.This observation is in great contrast to that of theambient crossflow conditions and highest fuel tem-peratures, which revealed a largely expanding jetcolumn and a dense vapor cloud downstream of theinjection point. Therefore, the trajectory correla-tions for the near flashing jet conditions for ambientand elevated crossflow conditions will be consideredseparately.

Table 3. Values for feed fraction and level of super-heat for all backlit imaging test conditions

TestCase

FuelTemperature

FeedFraction

Level ofSuperheat

- ◦C % %

1 28 0 -782 61 0 -623 122 0 -334 173 14 -85 212 83 116 214 92 117 219 100 138 233 100 209 244 100 25

10 35 0 -8311 107 0 -5912 221 0 -2113 231 0 -1714 262 17 -715 291 80 216 314 100 917 334 100 16

All linear regressions were performed on dis-tances normalized by fuel injector diameter and mo-mentum flux ratio, as momentum flux ratio was notsystematically varied in the test conditions. For TestCases 1-4 and 10-14, the trajectory correlation isgiven in Equation 3. For these cases at the lowerfuel temperatures and both ambient and elevated aircrossflow conditions, the level of superheat (∆Ts < 0for these cases) was not included as the primary fo-cus was on fluid property changes. All terms werefound to be significant with all p-values lower than2e-16, which is the likelihood of getting the deter-mined exponents by chance. A multiple R2 value of0.931 was determined for this model.

y

qd= 0.673(

x

qd)0.353(µ′∞)−0.312(µ′l)

0.057 (3)

For the conditions corresponding to a level ofsuperheat less than zero, the trajectory correlationgiven in Equation 3 is a function of fluid propertiesas well as axial distance normalized by fuel injectordiameter and momentum flux ratio. The normal-ized gas viscosity was constant across a given cross-flow condition with a higher value for the elevatedcrossflow conditions since air viscosity increases withtemperature. As the temperature of the crossflow in-creases, the fuel vaporization is augmented, leadingto the formation of smaller droplets with lower mo-mentum. The negative exponent on the normalizedgas viscosity term describes this negative relation-ship with decreasing jet penetration and elevatedcrossflow conditions.

Additionally for Equation 3, the normalized liq-uid viscosity decreased over each set of crossflow con-ditions since Jet-A viscosity decreases with temper-ature. Stenzler et al determined that a decrease inviscosity would lead to an increase in jet penetrationdue to a lesser degree of jet bending as a result ofa lower drag coefficient. However, the exponent wasfound to be positive, indicating that a decrease inviscosity would decrease jet penetration. For condi-tions with a negative level of superheat, the liquid jetpenetration was lower for the elevated crossflow con-ditions. When the test section pressure was higher,the fuel temperature at which saturation occurs ismuch higher, so the fuel properties are skewed whencomparing the different crossflow conditions. Theweakly positive relationship between liquid viscosityand jet penetration could be a result of the higherfuel temperature associated with the elevated cross-flow conditions.

For Test Cases 5-9, the trajectory correlation isgiven in Equation 4. For Test Cases 15-17, the tra-jectory correlation is given in Equation 5. All termswere found to be significant with all p-values lowerthan 2e-16. For the ambient and elevated crossflowconditions, a multiple R2 value of 0.925 and 0.891were determined for these models, respectively.

y

qd= 4.58(

x

qd)0.313(∆Ts)

−0.811 (4)

y

qd= 0.729(

x

qd)0.384(∆Ts)

−0.190 (5)

For the conditions corresponding to a level of su-perheat greater than zero, the trajectory correlations

12

given in Equation 4 (ambient crossflow conditions)and Equation 5 (elevated crossflow conditions) arefunctions of the fuel’s level of superheat and the ax-ial distance normalized by fuel injector diameter andmomentum flux ratio. For both correlations, a nega-tive relationship between the jet penetration and thelevel of superheat exists with the effect being muchgreater for the ambient crossflow conditions basedon the exponents. This results agrees with the ob-servations that jet penetration is greatly reduced atthe highest temperature conditions. The exponentassociated with the axial distance is on par with thecorrelation given in Equation 3.

Although the multiple R2 values were high andall included terms in the models were significant,some measured trajectories did not line up well withthe modeled trajectories. An example of two tra-jectory curves for Test Cases 6 and 9 are given inFigure 15 along with the predicted trajectory curvescalculated from Equation 4. While there is goodagreement between data and the model for the lowerfuel temperature test condition, the model slightlyoverpredicts the highest fuel temperature jet trajec-tory for an ambient temperature and pressure cross-flow.

It is also paramount that the model assump-tions are validated through statistical diagnosticsof the residuals. These models produce a slightlyskewed residual histogram revealing that the residu-als may not be normally distributed, which violatesa basic assumption for the linear regression modelto be valid. A qq-plot also illuminates issues withheavy-tailed residuals, also indicating that the errorterms may not be normally distributed. In orderto address these issues, a nonlinear regression modelwould need to be applied to the data.

Conclusions

The effects of elevated fuel temperatures on theatomization process of a liquid jet injected trans-versely into a steady air crossflow at ambient as wellas elevated temperature and pressure conditions wasexamined in this study. Backlit imaging and Miescattering measurements were recorded to study thechanges in overall spray characteristics, jet trajec-tory, and liquid column width as the fuel temper-ature increased. Imaging processing techniques toextract the upper edge of the liquid jet in cross-flow was utilized to compare the variation in tra-jectories across the incresingly elevated fuel temper-atures. A statistical analysis was performed in aneffort to model the jet penetration based on axialdistance from the injection point, fluid properties,and the fuel’s level of superheat.

A visual analysis of backlit images show that in-creased fuel temperatures lead to a finer and denserfuel spray downstream of the intact liquid jet col-umn. At higher temperatures the degree of jet bend-ing in the crossflow direction increases resulting ina lower transverse penetration. With an increase invaporization rates near and at flashing conditions,the jet has a tendency to penetrate in the upstreamaxial direction before bending over as well as ex-hibits an increase in the width of the liquid column.Similar spray behavior was observed for unheatedjets in ambient as well as elevated temperature andpressure air crossflows. The flashing test cases ex-hibit the greatest differences in spray characteristicsbetween the ambient and elevated crossflow condi-tions.

The jet trajectory comparisons revealed thatpenetration decreases with temperature for all testcases under an elevated temperature and pressurecrossflow. Also, the high level of fuel vaporizationfor the high temperature fuel jet in elevated cross-flow conditions caused the jet to be fully vaporizedwithin 15 jet diameters from the injection point. Forthe ambient crossflow conditions, jet penetration forfuel temperatures below the transition to flashingtemperature tended to increase, most likely due to adecrease in liquid viscosity. As fuel temperature in-creased past the transition to flashing temperature,the penetration dropped significantly.

An linear regression analysis of the trajectorieswas performed in an effort to predict jet penetrationas a function of axial distance downstream of the in-jection point, fluid properties, and the fuel’s level ofsuperheat. Three correlations were developed with ahigh level of variance explained by the models. How-ever, issues with invalid assumptions for the normaldistribution of error terms indicates that a nonlin-ear regression model should be explored for this dataset.

Future work should focus on obtaining the heatof vaporization data for Jet-A and adding this pa-rameter to the model in an attempt to increase thevalidity. The heat of vaporization decreases withan increase in fuel temperature, and the inclusion ofthis term could offer a better description of the phys-ical changes that the fuel undergoes during the hightemperature liquid injection process as compared tothe level of superheat. Lastly, upper edge trajecto-ries can be determined from the Mie scattering im-ages in order to allow for addition data points in themodel regression analysis. Variations in trajectoriesdue to the diagnostic method will need to be takeninto account in order for all trajectory comparisonsto be valid.

13

Acknowledgments

This material is based upon work supported bythe National Science Foundation Graduate ResearchFellowship Program under Grant No. DGE1333468.The assistance of my labmates, especially Bob Zhangand Rohan Gejji, during testing is greatly appreci-ated!

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15

Table 4. Jet flow rate, temperature, and injection pressure with resulting momentum ratio for all test cases

Crossflow

Condition

Diagnostic

MethodTest Case Flow Rate Temperature

Injection

Pressure

Momentum

Ratio

- - - g/s ◦C kPa -

Ambient Backlit

1 3.9 28 572 99

2 4.1 61 683 120

3 4.1 122 752 119

4 4.1 173 800 130

5 3.7 212 1191 117

6 4.1 214 887 143

7 3.8 219 1200 123

8 3.8 233 1262 125

9 4.2 244 1055 151

Elevated Backlit

10 2.9 35 814 126

11 2.8 107 827 129

12 2.8 221 800 155

13 2.8 231 800 155

14 2.7 262 896 153

15 2.6 291 1014 148

16 2.7 314 1172 173

17 2.6 334 1351 158

Ambient Mie Scattering

18 3.3 26 427 67

19 3.3 63 786 71

20 3.2 124 820 74

21 3.2 152 848 73

22 3.1 183 903 75

23 3.3 193 1041 81

24 3.2 206 1076 81

25 3.2 233 1227 83

Elevated Mie Scattering

26 2.5 29 717 96

27 2.5 63 724 109

28 3.4 147 855 215

29 2.5 193 724 125

30 2.6 229 752 138

31 2.6 271 979 147

16

x/d-10 -5 0 5 10 15 20 25 30 35 40

y/d

0

10

20

30

40

50

60

70T

j = 28°C, q = 99

Tj = 61°C, q = 120

Tj = 122°C, q = 119

Tj = 173°C, q = 130

Tj = 212°C, q = 117

Tj = 219°C, q = 123

Tj = 233°C, q = 125

Tj = 244°C, q = 151

Figure 11. Upper edge trajectory tracking determined from backlit imaging test cases for liquid jet injectedinto an ambient temperature and pressure air crossflow for increasing fuel temperatures

x/d-10 -5 0 5 10 15 20 25 30 35 40

y/d

0

10

20

30

40

50

60

70T

j = 35°C, q = 126

Tj = 107°C, q = 129

Tj = 221°C, q = 155

Tj = 231°C, q = 155

Tj = 262°C, q = 153

Tj = 291°C, q = 148

Tj = 314°C, q = 173

Tj = 334°C, q = 158

Figure 12. Upper edge trajectory tracking determined from backlit imaging test cases for liquid jet injectedinto an elevated temperature and pressure air crossflow for increasing fuel temperatures

17

x/(qd)-0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 0.3

y/(q

d)

0

0.1

0.2

0.3

0.4

0.5

0.6T

j = 28°C, q = 99

Tj = 61°C, q = 120

Tj = 122°C, q = 119

Tj = 173°C, q = 130

Tj = 212°C, q = 117

Tj = 219°C, q = 123

Tj = 233°C, q = 125

Tj = 244°C, q = 151

Figure 13. Upper edge trajectory tracking determined from backlit imaging test cases scaled by momentumflux ratio for liquid jet injected into an ambient temperature and pressure air crossflow for increasing fueltemperatures

x/(qd)-0.1 -0.05 0 0.05 0.1 0.15 0.2 0.25 0.3

y/(q

d)

0

0.1

0.2

0.3

0.4

0.5

0.6T

j = 35°C, q = 126

Tj = 107°C, q = 129

Tj = 221°C, q = 155

Tj = 231°C, q = 155

Tj = 262°C, q = 153

Tj = 291°C, q = 148

Tj = 314°C, q = 173

Tj = 334°C, q = 158

Figure 14. Upper edge trajectory tracking determined from backlit imaging test cases scaled by momentumflux ratio for liquid jet injected into an elevated temperature and pressure air crossflow for increasing fueltemperatures

18

0 0.05 0.1 0.15 0.2 0.25 0.30

0.1

0.2

0.3

0.4

0.5

0.6

x/(qd)

y/(q

d)

Data − Tj = 212°, q = 117

Model − Tj = 212°, q = 117

Data − Tj = 244°, q = 151

Model − Tj = 244°, q = 151

Figure 15. Experimental trajectory data plotted with the predicted trajectory from Equation 4 for a heatedliquid jet in ambient air crossflow conditions

19