Atomization and Combustion Characteristics of Ethanol/NitrousOxide at Various Momentum Flux RatiosInchul Lee, Min Son, and Jaye Koo**School of Aerospace and Mechanical Engineering, Korea Aerospace University, 76, Hanggongdaehang-ro, Deokyang-gu, Goyang-si,Gyeonggi-do, Republic of Korea

ABSTRACT: The flame structure of ethanol and nitrous oxide combustion was experimentally studied using a tricoaxial injectorat various momentum flux ratios. The objects of the study were to investigate the effects of the additional supplement of nitrousoxide that gas jets inject at the outer annular gap at various injection velocities and to obtain and analyze the flame structure. Thefully developed patterns due to the transfer of momentum and viscosity mixing were similar to that of the axial flow when themeasurement position was increased from Z/d = 1 to Z/d = 10. The correlation equation, using the momentum flux ratio, wasused along with the linear regression method to calculate the breakup length and Sauter mean diameter (SMD). The effects wereclearly observed and significant. The inner gas injection caused the SMD to decrease, and the outer gas injection was able tocreate a boundary layer around the spray jets. As the momentum flux ratio of the inner gas jets increased, the spray angle andflame angle increased. OH radicals extended toward the rear flame region with the increase in momentum flux ratio from theinner gas jets. As the momentum flux of the outer gas jets increased, the boundary of the OH radicals developed and the intensityof the OH radicals generated in the flame region was enhanced. Also, the flame temperature increased as gas was injected fromthe outer-stage.

1. INTRODUCTIONSince the 1960s, toxic propellants with dinitrogen tetroxide(N2O4), hydrazine (N2H4), monomethyl hydrazine (MMH),and unsymmetrical dimethyl hydrazine (UDMH) have beenused for propulsion systems. However, for future aerospaceapplications, due to its capabilities as a green propellant,ethanol (C2H5OH) offers many benefits in both combustionperformance and adherence to pollutant regulations. Ethanoland nitrous oxide (N2O) have nontoxic characteristics and donot need self-contained atmospheric protective ensemble(SCAPE) operations. Nitrous oxide is usually stored as a gasand in cryogenic fluids at an ambient pressure of −91 °C. Thismakes its thermal conditioning handling and managementeasier. As such, nitrous oxide can realize cryogenic states withgreater ease than liquid oxygen or liquid hydrogen.1

Shear coaxial injectors are used in many liquid rocketengines, including space shuttle main engines (SSMEs),Vulcains, Vincis, RL-10s, and J-2s.1 Liquid rocket enginesrequire a high flow injector assembly to supply the excessivepropellants. For example, the gas-generator of a unit elementinjector is limited to a flow rate of 100−200 g/s, and the mainchamber is also limited to 500−1000 g/s. In the case of thepreburners, the oxidizer gas runs the turbo pump and isinjected into the main combustion chamber. In the combustionprocess, flame structures are commonly determined by theatomization performance and relative velocity ratio of eachphase of the propellants. However, in the case of the shearcoaxial injector, mixing creates a problem for the combustionsystem. Shear coaxial injectors have center posts and gas postsinstalled in the injector head of the liquid rocket, and thistranslates to design problems and additional manufacturingcosts. Shear coaxial injectors have drawbacks in terms of theirhigh flow rate conditions and the fabrication process withrespect to the post and sleeve.2 The high velocity gas at the

annular gap affects the shear forces at the center of the liquidcolumn, which undergoes breakup and mixing processes withthe gases.The flow characteristics of the gas jets injected by a shear

coaxial injector have been the subject of active research due tothe fact that the breakup process of the liquid jet is governed bythe shear forces of the gas jets. In a combustion system,diffusion flames with reaction kinetics are controlled by themomentum ratio or by turbulent mixing. To understand themixing phenomenon in the flow field, many studies have beenconducted using cold flow tests with hot-wire and pilot tubesand various velocity conditions. Forstall and Shapiro3

investigated the flow field with focus on the density,temperature, and velocity at various injection velocity ratios.They concluded that it was the most important parameter inchanging the flow configuration and velocity distribution.Champagne and Wyganski4 showed that neglecting the total

and static pressure had an effect on large errors in the velocityfields. In an effort to study the velocity distributions in the nearfield of the coaxial jets, Moon5 investigated static pressure,mean velocity, and turbulent intensity in the developing regionof nonreactive coaxial jets. He concluded that the effects ofpressure gradients could significantly influence the mixing andcombustion process and that the turbulence mixing models didnot appear to be adequate to predict the flow in the developingregion of the coaxial jet. The spray characteristics of theatomized and mixed propellant injected into the combustorvary as a result of the momentum ratio and velocity ratio.Generally, combustion stability and flame structures are

Received: November 15, 2013Revised: March 9, 2014Published: March 12, 2014

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determined by the momentum flux ratio, which changes thehomogeneous droplet distributions and the dynamic process ofthe liquid phase breakup. The atomization process initiated bythe shear coaxial injector is governed by the interaction of theshear forces on the liquid column, which experiences internalinstability in the form of turbulent motion. Combustionperformances are mainly influenced by atomization character-isticsbreakup lengths, angle, mass flux distributions, anddroplet mean diameters. The atomization characteristics ofshear coaxial injectors have been widely studied for theirapplications with liquid rocket engines. Eroglu et al.6 studiedthe potential core length using a shear coaxial injector andshowed an empirical correlation that was derived for thepurpose of predicting the liquid core length. They maintainedthat the Reynolds number and the aerodynamic Weber numberwere the crucial parameters with respect to atomizing the liquidjet. Farago and Chigier7 showed three breakup modestheRayleigh type mode, membrane type mode, and fiber typebreakup modein which the Weber number had anaerodynamic effect that ranged from 100 to 500. Rehab etal.8 investigated the near field flow structure of shear coaxial jetswith high gas/liquid velocity ratios. They demonstrated that theprimary instability mechanism was the jet-preferred mode withhigh frequency oscillation. Lasheras et al.9 studied near and farfield breakup and atomization characteristics. They proposed asimple correlation equation for the liquid core length anddescribed the dependence of shedding frequencies on themomentum ratio. As an effect of the momentum ratio, theliquid and gas velocities were studied in research using a shearcoaxial injector. The momentum ratio is very important due tothe fact that it can be used to compare fluids of densities andvelocities. Sankar et al.10 and Glogowski et al.11 concluded thatincreasing the gas velocity helped to improve the breakupprocess.Spray behavior related to combustion has been studied as

one of the important characteristics controlling the combustionperformance of liquid propellant sprays. A lot of research hasmade significant contributions to the understanding of theeffects of flame and spray behavior. However, combustioncharacteristics using a tricoaxial injector configured with aninner- and outer-stage have rarely been studied. Previousstudies by Oefelein on atomization and combustion character-istics with a shear coaxial injector have looked into the shearcoaxial injection process in liquid rocket combustion.12 In hisresearch, Oefelein13 found that surface tension forces emanateda heterogeneous spray and that the diminished intermolecularforces increased a diffusion process prior to atomization and jetvaporization. Another investigation of atomization andcombustion with a shear coaxial injector was presented byMayer et al.14 They observed that the spray, including variousdroplets, no longer existed at combustion conditions, and theynoted that the flame existed in the recirculation zone behindthe injector post. They also determined that length and OHradicals chemiluminescence had very crucial roles for flamestabilization, since the performance frequently changed to thevelocity ratio or momentum ratio. Generally, the liftoff lengthincreased linearly with gas jet velocities and was based on thebalance between the mean propagation speed and mean flowvelocity.15 There are many studies that probe the flamestructures of gas−gas phases with premixed and nonpremixedconditions.16−19 However, studies on specific flame structuresusing a liquid−gas phase are rather scanty even though thecombustion performance in terms of numerical data and

experimental results for liquid rockets includes the velocity,pressure, and frequency characteristics in a combustor. Also,there are no specific studies on the influence of the momentumflux ratio and different design parameters of injectors onatomization and combustion processes.In order to obtain a better understanding of the gas flow

structure of a tricoaxial injector that causes the atomization andcombustion performances, measurements of the velocityinjected into the inner- and outer-stages were conducted withprecision. The aims of the present work using a tricoaxialinjector were to investigate the velocity distribution, atom-ization characteristics, and flame structures with ethanol andnitrous oxide fuel at various momentum flux ratios. The focusesare liftoff length, flame structure, and OH radicals chem-iluminescence. Liftoff length and flame structures are observedand correlated with respect to their atomization characteristics.

2. EXPERIMENTAL APPARATUS AND PROCEDURESAtomized jets undergo liquid jet breakup, mixing, and a dropletcombustion process. As such, the atomization performance, whichdepends on droplet sizes and distributions, plays an important role inthe combustion process. In the case of a shear coaxial injector, thebreakup of the liquid column is affected by the shear force of thecoaxial gas jets injected at the annular gap. Thus, the fundamentalbreakup mechanisms of the shear coaxial and tricoaxial injector are thesame. Breakup length is governed by the momentum flux ratio, whichalso has an effect on the liftoff length of the flame and the blowoutcharacteristics. A tricoaxial injector can increase the mass flow rate ofthe gas phase and control the flame angle. This results in thedetermination of the spray angle. By obtaining experimental results,helpful information regarding the design of a combustion chamber andinjector is provided.

The schematic of the experimental setup for the spray test ispresented in Figure 1. The setup was composed of a tricoaxial injector,laser diffraction system, CCD camera, liquid reservoir with a capacityof 40 L, air reservoir with a capacity of 1000 L, needle valve, and plano-convex lens with a focal length of 500 mm. The tricoaxial injector wasthe most important part of this setup, since it injects propellants andproperly atomizes the liquid jets. Spray images of the breakupphenomenon were captured with a Photron SA 1.1 high speed CCDcamera with a 4 μs exposure time and 1024 × 1024 resolution. Foreach experiment, a large number of spray images were obtained andused to analyze the breakup of the spray jets. On average, 200 imageswere averaged for each test condition in order to reduce anyexperimental uncertainty due to the instability of the spray structure.Volume flow rates with gas jets were measured by multiple nozzles in a

Figure 1. Experimental setup for spray test with cold flow conditions.

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chamber that was designed to use the standard methods provided byANSI/AMCA 210-07, ANSI/ASHRAE 51-07, and KS B 0062. Theexperimental conditions of the ambient temperature were 293 ± 5 K,the humidity was 50%, and the atmospheric pressure was 105.2 ± 2.0kPa. The velocity of the gas was measured using hot-wire anemometrydue to the fact that the major effect of the breakup process is the shearforces induced by gases at the annular gap. To carry out this process, aDantec CTA module 90C10 and 55P11 probe with an accuracy of0.3% was used at measurement positions of 1, 3, 5, and 10 mm at theexit of the tricoaxial injector. To study atomization and gaseousvelocity around the flow fields, water and air were used. Theexperimental ranges of the water included an injection pressure of 5−100 kPa, injection velocity of 2.5−10.3, Reynolds number of 2686−10985, and Weber number of 93−1563.A summary of inner-stage and outer-stage operating conditions is

shown in Table 1. The experimental setup was put in place, and acombustion test (Figure 2) using a tricoaxial injector was conducted toanalyze the flame shape, temperature distribution, and OH radicalsaround the spray combustion fields. Temperature distributions weremeasured using an R-type thermocouple, and the OH radicalsdistributed in the high temperature region by the chemical reactionwere visualized with a band-pass filter (UG11) with a range of 240−395 nm and a short-pass filter (OD2) with a range of 0−425 nm.Ethanol and nitrous oxide were used as propellants, and the injectionpressures of both propellants were controlled with a microneedlevalve. The classifications of the gas injection modes are shown inFigure 3, namely the inner-stage injection mode, outer-stage injectionmode, and tricoaxial injection mode. The breakup mechanism of theinner-stage injection mode was the same as the that of the previousshear coaxial injector. In the case of the inner-stage injection mode, theinner gas was injected at the exit of din. The outer-stage gas, which didnot offer efficient atomization, was injected at the exit of dout. In thecase of the tricoaxial injection mode, the air-blast gases were injected atboth the din and dout annular gaps. Detailed specifications and a cross-sectional view of the tricoaxial injector are illustrated in Figure 4.Droplet sizes were measured using the laser diffraction apparatus, withthe Mie scattering technique considering the liquid sphere. Dropletsizes were measured with a refractive value (n) ratio of 1.33169, anextinction value (k) of 1.4680 × 10−8, and an absorption value (α) of2.9152 × 10−3 cm−1. This value was adapted to the experimentalresults by Hale et al.,20 as illustrated in Figure 5.The laser diffraction system was used to acquire droplet

information. The spray jets injected at the tricoaxial injector werelocated between the transmitter and the receiver. The distancebetween them was about 500 mm. The laser beam was moved alongthe center axis of the tricoaxial injector. Ligaments and nonuniformspherical droplets that were not able to be measured precisely withlaser diffraction anemometry could not be detected for the detailedspray jet determination. The laser diffraction system (Helos/Vario-KF) specifications included a 5 mW, 633 nm He−Ne laser with a 29mm beam diameter with a total accuracy of 1.15%, which includedrepeatability and comparability. The receiver had 31-channel multi-element detector rings and three radial centering elements that wereable to detect droplet diameters as small as 0.1 μm for fully sphericaldroplets. For the unsteady breakup process of atomized jets, the laserdiffraction instrument was set to sample the diffracted light signal at amaximum acquiring rate of 100 Hz, and it averaged all the data.Measured data were considered with ISO-13320-1, that is, the number

of particles in the working laser beam where the optical concentrationwas below 25%.

3. RESULTSWhen studying spray combustion, it is useful to analyze the gasflow and atomization characteristics. (For example, a blowoutof diffusion flames might emerge with high velocity gas jets.)The gas flow area formed by the tricoaxial injector was classifiedinto the main flow region, transition region, and fully developedregion. In the case of the tricoaxial injector, gas jets wereinjected into the inner and outer post gaps. Accordingly,turbulence strength and mixing characteristics could becontrolled by the momentum flux ratio. Downstream of theflow region, axial velocities decreased and the jet spread angleincreased to a greater width. The decreasing velocity near theinjector exit was caused by the pressure gradients and viscousmixing. The viscosity effects caused a faster decrease in thevelocity at the exit of the injector tip. Self-similarity character-istics, which explained the fully developed structure of the gasflow, appeared further downstream. Shear coaxial injectorscomposed of a center liquid nozzle and outer gas nozzle with anannular gap were usually used to break up the liquid jets. All ofthese experimental data were used to study spray combustioncharacteristics with liftoff lengths and flame structures.In order to investigate the flow structure of the tricoaxial

injector at various Reynolds numbers, hot-wire anemometrywas used. Gas velocity measurements were conducted underambient fields with injection Reynolds numbers in the 877−2687 range. The axial velocities for the gas injection of theinner-stage are illustrated in Figure 6. The gas velocities at R/d= 0 and z/d = 1 appeared due to the viscosity and pressureeffects. When the injection pressure of the inner-stageincreased, the gas velocity was measured at 138.0 m/s at z/d= 1, and the gas velocity was at its lowest at R/d = 0. The corevelocity from z/d = 1 to z/d = 3 had a greater effect on pressurerather than viscosity. The viscosity effect made the gas velocitiesof the center axis increase with the same conditions as theboundary velocity. However, the highest velocities weremaintained within z/d = 10. When the velocity profiles at z/d = 3 and z/d = 5 were viewed, the breakup of the liquidcolumn appeared at z/d = 5 due to the fact that the gasvelocities caused the shear forces in the liquid column. For themeasurement position of z/d = 10, the profiles of the gasvelocity at various injection Reynolds numbers show fullydeveloped shapes. To analyze the flow structures of thetricoaxial injector, the Reynolds numbers of the gas jets at theinner-stage and outer-stage were fixed to 877 and 1918,respectively. One of the critical parameters of the tricoaxial

Table 1. Inner-Stage and Outer-Stage Operating Conditions

parameters inner-stage and outer-stage expt ranges

injection pressure (kPa) 2−40injection velocity (m/s) 54.4−166.8mass flow rate (g/s) 0.7−2.1momentum flux ratio 0.29−2.72

Weber number 42.8−476.6Reynolds number 877−2687

Figure 2. Experimental test setup for combustion test.

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injector was the velocities of the gas jets at the inner- and outer-stages. The gas injections of the inner- and outer-stages createdimportant effects in terms of atomization and combustion.Figure 7 shows a comparison of the axial velocity distributionsof the tricoaxial injector at various injection Reynolds numbers.This was done to investigate the influence and interference ofthe inner and outer flows. The gas velocities at various injectionReynolds numbers at the inner- and outer-stages also showseparated profile peak structures. The gas velocity peakstructures are shown at z/d = 1 and z/d = 3. Within z/d =3, the gas injection of the inner- and outer-stages did not affectthe flow combination of each gas flow. When the inner and

outer gas jets were injected at each condition at 877 and 1918,the flow structures of the two peaks with maximum velocitieswere combined at z/d = 5 due to the viscosity effect. The outer-stage gas injection did not affect the breakup process due to thefact that the liquid column was only injected between R/d =−10 and R/d = 10. The shear forces only appeared for 100 m/sat the region of R/d = −35 and R/d = 35. As a result, the inner-stage gas injection significantly enhanced the breakup of theliquid column, and the outer-stage gas injection was coveredwith the spray jet. Also, the outer-stage gas flow decreased theoverspray phenomenon and the spray angle during the breakupprocess. The gas flow injected by the outer-stage appeared in a

Figure 3. Tricoaxial injector gas injection methods.

Figure 4. Tricoaxial injector cross-sectional view and specifications.

Figure 5. Refractive and extinction curves at various wavelengths.20

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partially developed shape with two peak structures at z/d = 10.However, further down the flow field stream, a fully developedgas flow structure appeared, and a boundary layer enclosed thecenter of the gas flows. The gas flow injected at the outer-stage

had less gas velocities than did the gas injection of the inner-stage. Further increasing the measurement length to greaterthan z/d = 50 significantly helped in the formation of fullydeveloped structures, which showed the highest velocity at R/d= 0.The pure liquid jets injected into the ambient field showed a

fluctuation in their column wave. The basic mechanism of aliquid jet breakup is the effects of hydrodynamic forces withturbulent energy. However, in the case of the tricoaxial injector,it uses shear forces to break the liquid column. Gas jets withshear forces are a more important parameter in influencing theliquid column. Acting on shear forces produces an instability inthe liquid jets that takes the form of irregular ligaments oncolumn surfaces. Spray images at various momentum flux ratiosof the inner- and outer-stages are shown in Figure 8. Min can bedefined as the inner-stage momentum flux ratio, and Mout is theouter-stage momentum flux ratio.The shear forces acted on the liquid column that was injected

at the center of the nozzle. As the momentum flux ratioincreased, the liquid column experienced an atomizationprocess. At the momentum ratio of 0.29, there were smalleffects in terms of the shear forces so that the liquid columnpartially broke the liquid column, and the deformation of theliquid column appeared within z/d = 10. The appropriate statewith no liquid jet ligaments to be burned showed a momentumflux ratio over 1.38. However, as the droplet diametersdecreased, the gas injection velocity (i.e., shear forces) alsoincreased, and a flame blowout occurred. The momentum fluxratio is very important in determining the liftoff lengths and toretain flame propagation. Figure 9 illustrates the tricoaxial sprayimages at various momentum flux ratios. At the momentum fluxratio fixed conditions of the inner-stage with 0.56, the liquidcolumn was deformed by the shear forces caused by the gas jetsof the outer-stage. However, there were no significant effects interms of the shear forces breaking the liquid column for themistlike droplets. Also, it should be noted that such effectscould affect the overspray characteristics, spray angle, and flameangle due to the outer gas jet being enclosed and covered bythe spray jet. Figure 9b shows the spray images at an inner-stage fixed momentum ratio of 1.38 and outer-stage fixedmomentum ratios of 0.29 and 2.45. The overall spray patternswere the same at the momentum flux ratio of the outer-stage at0.29 and 2.45. The primary effects of the breakup process weregoverned by the gas jet of the inner-stage. Under theseconditions, the gas jets of the outer-stage could not help theliquid column to produce a fine droplet. As the outer gas jetsincreased, the liftoff length increased. However, the additionalgas jet that exceeded the flame propagation speeds caused theblow-off characteristics. The liquid column breakup wasgoverned by the aerodynamic forces related to the shearforce at the inner- and outer-stages. The breakup length wasmeasured as the average of 100 photos taken using the shadowgraph method. In the experimental case of the inner gasinjection, from the measured data of the breakup length of theliquid column versus the momentum flux, the empiricalcorrelation equation that considers the linear interpolationand linear multiple regression methods was expressed with eq1. The calculated and measured liquid column breakup lengthsare shown in Figure 10. In the present study, the momentumflux ratio was used for the nondimensional numbers to definethe breakup length of the liquid column. In this equation, theR2 coefficient of determination was 94.1%. As the momentumflux ratio increased, the liquid column breakup length (lb/d)

Figure 6. Axial velocities at various Z/d and Reynolds numbers.

Figure 7. Comparison of axial velocity distributions at variousinjection Reynolds numbers.

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decreased, and the scattered data fit well with the R2 = 100%line. At the momentum flux ratio from 0.29 to 0.57 with theinner-stage gas injection, as the momentum flux ratio of theouter-stage gas injection increased, the breakup length (lb/d)continued to show the same data.

= − − +l d M M/ (9.57 0.5180 ) Mb out in

( 0.58 0.062 )out (1)

The Sauter mean diameter (SMD, D32), which can bedefined as the volume/surface area ratio as a particle of interest,was used to investigate the atomization performance. Dropletsizes were measured using a laser diffraction analyzer that hadmeasurement errors for an accuracy of 0.3%, repeatability of0.5%, and comparability of 1.0%. The combined standarduncertainty, which is the standard uncertainty and expandeduncertainty, was calculated at ±5.85 μm. The SMD, which

depended on the momentum flux ratio of the inner- and outer-stages, is shown in Figures 11 and 12. The SMD is presentedfor a wide range (0.29−2.72) of momentum flux ratios. Theoverall SMD decreased continuously from 360 to 35 μm as themomentum flux ratio increased. In the case of the inner-stagefixed momentum flux ratio of 0.29, the overall SMD value wasdistributed from 360 to 35 μm. At the inner-stage fixedmomentum flux ratio of 1.11, the SMD was not significantlychanged due to the fact that the gas jets of the outer-stage couldnot affect the shear force at the center of the liquid column, andthe gas jet injected at the inner-stage had greater shear forcesthan the outer-stage. As expected, the effects were clearlyobserved and significant. The inner gas injection caused theSMD to decrease, and the outer gas injection was able to createa boundary layer around the spray jets. The spray jets weresubjected to shear forces that affected their flow motion and

Figure 8. Spray images at various momentum flux ratios for the inner- and outer-stages.

Figure 9. Spray images at various momentum flux ratios.

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breakup processes in the flow field. To derive an empiricalequation at various tricoaxial injector momentum flux ratios, eq

2 was expressed by using the linear interpolation and linearmultiple regression methods. In this equation, the R2 coefficientof determination was 97.9%. The calculated and measuredSMD is shown in Figure 10. In the present study, themomentum flux ratio was used as the nondimensional numberto define the breakup length of the liquid column. In thisequation, the R2 coefficient of determination was 97.9%. Unlikethe breakup length trends, all of the tricoaxial injectormomentum flux ratios and SMD distributions fit well withthe R2 = 100% line.

= − − +M MSMD (109.83 18.34 ) Mout in

( 1.16 0.28 )out (2)

The flame structures were studied by using a tricoaxialinjector with ethanol and nitrous oxide. Ethanol was injected atthe center of the nozzle, and nitrous oxide was injected at theannular gaps of the inner- and outer-stages. In the case of theshear coaxial injector, as the shear forces increased, theatomization performances increased and the blowoff character-istics of the diffusion flame appeared. The liftoff length wasdefined as the height of the spray flame at the exit of thetricoaxial injector. Figure 13 shows the flame structure of theinner-stage gas injection with momentum flux ratios of 0.29 and1.38. The liftoff length appeared at 33 mm, and a large scalevortex flame structure also emerged at the momentum flux ratioof 0.29. Also, under a momentum flux ratio of 0.29, unburnedfuel droplets that the SMD distributed to 200 μm werecombusted further downstream from the flame. Under thiscondition, the shear forces of the inner gas jet were insufficientto make fine droplets for combustion. As can be seen in Figure13, the spray flame at a momentum flux ratio of 1.38 had atendency to create straightforward structures downstream.However, the spray flame with a momentum flux ratio of 0.29went toward the upward direction due to the fact that weakshear forces could not break the liquid jets properly and enclosethe spray flame. Figure 14 shows that the gas jet of the outer-stage was able to change the spray flame angle, liftoff length,and flame color at a fixed inner-stage momentum of 1.38 andouter-stage momentum flux ratios of 0.29 and 1.38. The reasonfor these results can be explained with the images of the sprayflame. From the analysis of the spray flame behavior, it wasunderstood that ethanol and nitrous oxide flames moved

Figure 10. Calculated and measured liquid column breakup lengths atvarious momentum flux ratios.

Figure 11. SMD distributions at various momentum flux ratios.

Figure 12. Comparison between the measured and calculated SMD.

Figure 13. Inner-stage gas injection flame structures at Min = 0.29 andMin = 1.38.

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downstream of the injector as the momentum flux ratio of theinner- and outer-stages increased. The additional gas jetsdecreased the spray flame angle and increased the liftoff length.The outer-stage momentum flux ratio of 1.38 was brighter thanthe momentum flux ratio of 0.29, which means that it addedmore oxidizer into the spray flame.A band-pass filter and short-pass filter were used to visualize

the OH radicals that the time averaged images calculated usingthe Abel transformation, and this was used to analyze the crosssection of the axis symmetry flame. Generally, OH radicalsemerged at the conditions of the initial combustion region inwhich stable molecules combined separately and very quickly.OH radicals, which existed at the chemical reaction region at ahigh temperature, appeared through the fuel and oxidizerboundary of the flame front. The characteristics of the OHradical distribution were analyzed and compared with the sprayflame images, as shown in Figures 15 and 16. In Figure 15,different OH radical structures are shown. This means that theshear coaxial jets strongly affected the spray flame structuresand OH radical distributions. In the case of the momentum fluxratio of 0.29, the OH radicals were concentrated in front of thespray flame region, where the atomization characteristics werepoor. The OH radicals with narrow band shapes appeared atthe momentum flux ratio of 1.38 along the outer boundary ofthe spray flame. The shear force effect caused the spray flameangle to decrease, and the OH radicals were distributed furtherdownstream. As can be seen in Figure 16, the strength of theOH radicals at the outer-stage momentum flux ratio of 1.38emerged to a greater extent than the momentum flux ratio of

0.29. This implies that the increase of gas jets at the outer-stagewas able to play a significant role in increasing the OH radicals.Subsequently, the OH radical distributions along the boundarylayer of the spray flame were governed by the outer gas jets.Liftoff lengths and normalized breakup lengths are shown in

Figure 17, where the momentum flux ratio of the inner-stage isfixed to 0.57, 1.38, and 2.19, and the outer-stage varies from0.29 to 2.72. As depicted in Figure 17, the effect of the outer-stage was further studied by analyzing the plotted data atvarious momentum flux ratios at fixed inner-stage momentumflux ratios. As can be seen in Figure 17, despite increasing themomentum flux ratio of the outer-stage, breakup length did notchange at the fixed inner-stage momentum flux ratio. When theinner-stage momentum flux ratio was 1.38 and 2.19, thebreakup length ranged from 5 to 7. However, the normalizedliftoff length appeared to have similar trends with 45 due to thefact that the outer-stage’s only role was to decrease the flameangle, which increased the flame temperature. In addition, thegas jet injected at the outer-stage induced the blowoutcharacteristics over the momentum flux ratio of 1.6. The liftofflengths and SMD versus momentum flux ratio of the tricoaxialinjector are plotted in Figure 18. In the case of the outer-stage,the flame blowout appeared in all experimental conditions. Thisis because the gas jets of the outer-stage were not able to breakthe liquid column for spray combustion that satisfied thedroplet sizes below 225 μm. Although the SMD was producedto 255 μm, spray combustion did not emerge, due to the factthat the shear coaxial jets injected at the outer-stage exceededthe flame propagation speed so that the spray flame could notachieve liftoff under those experimental conditions. As can beseen in the data for the inner-stage gas injection, SMD variedfrom 200 to 50 μm. Under these conditions, liftoff lengthincreased to 43 as the momentum flux ratio increased to 2.72.However, as the momentum flux ratio increased over 2.72, thepartial mixture ratio changed to an oxidizer rich condition, andat the high temperature region, the gas jet that had beeninjected at the inner-stage caused a blowout of the existingspray flame. This also explained why the flame propagationspeed exceeded the maintenance speed of the spray flame. Thegas velocity of the inner-stage gas jet with a momentum fluxratio of 0.29 was 54.4 m/s, and the liftoff length was 33 mm atthe tricoaxial injector tip. From the results of Figures 17 and 18,the gas jet of the outer-stage was not able to change thebreakup length. Rather, it only affected the liftoff length with amomentum flux ratio of 1.6. The flame temperature character-istics at various tricoaxial injector momentum flux ratios areshown in Figure 19. To measure the flame temperature, an R-type thermocouple was used, and flame temperature was

Figure 14. Tricoaxial injection flame structure at fixed Min = 1.38, Mout= 0.29, and Mout = 1.38.

Figure 15. OH radical images at various inner-stage gas injections using the Abel transformation at Min = 0.29 and Min = 1.38.

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measured 300 mm downstream from the tricoaxial injector,where the thermocouple probe moved 10 mm toward thecenter axis. Flame temperature generally decreased as themeasurement point moved toward the outer boundary. Whenthe inner-stage of the momentum flux ratio increased, at theradial distance of 0 mm, which was the center region, theoverall temperature of the spray flame increased from 925 °C to1055 °C. However, at a fixed inner-stage of the momentum fluxratio, as the gas jets of the outer-stage increased, the flametemperate also increased from 1100 °C to 1250 °C at thecenter of the spray flame. These results indicate that the

additional gas injection of the outer-stage helped the mixingprocess with the existing spray flame via the effect of the inner-stage.

4. CONCLUSIONA tricoaxial injector was used to experimentally study spraybreakup and flame characteristics through a comparison ofinner- and outer-stage gas injections. The liquid jet breakupprocess was widely influenced by the shear force as the dropletmean diameters decreased with an increase in the shear force ofthe gas jets at the exit of the annular gap. Moreover, the flamestructure, liftoff length, and flame angle also changed with shearforce variance. In this study, the overall characteristics of theaxial gas flow, macroscopic breakup, distributions of dropletmean diameters, and flame structures were investigated usinghot-wire anemometry, a spray and flame visualization system, aswell as a laser diffraction system. The variances on the flowstructures in the axial direction at various gas injection ratios forthe inner- and outer-stages were studied throughout theexperiments, and the following conclusions were reached.The gas jets injected at the outer-stage showed completemixing characteristics at Z/d = 10, where a similar phenomenonwas observed at Z/d = 5 for the gas injection made at the inner-stage. With the increase in Reynolds number for the outer-stageinjection of the tricoaxial injector, a phenomenon was observedin which an outer layer was formed by an annular gap alongwith an outer-stage gas jet around the inner gas jet.

Figure 16. OH radical images at various inner-stage gas injection momentum flux ratios using the Abel transformation at fixed Min = 1.38 and Mout =0.29, Mout = 1.38.

Figure 17. Normalized liftoff length and breakup lengths.

Figure 18. Normalized liftoff lengths at various momentum.

Figure 19. Flame stabilization characteristics according to SMD andmomentum flux.

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dx.doi.org/10.1021/ef402251s | Energy Fuels 2014, 28, 2770−27792778

Also, with the increase in the momentum flux ratio for theouter-stage injection, the spray angle decreased due to theannular gas jets. The correlation equation, using themomentum flux ratio, was used along with the linear regressionmethod for the calculation of breakup length and SMD. Spraybreakup increased as the momentum flux ratio of the inner-stage increased. However, the spray jets dispersed to the outerspray field.The flame angle was reduced with the injection of gas from

the outer-stage. Also, a fuel rich zone in the rear flame regiondecreased with the additional supply of gas near the flameboundary. Moreover, OH radicals extended toward the rearflame region with the increase in momentum flux ratio from theinner-stage. As a result of the increasing momentum flux of theouter-stage injection, the boundary of OH radicals evolved, andthe intensity of the OH radicals generated in the flame regionwas enhanced. Also, the flame temperature increased as gas wasinjected from the outer-stage. The distributions of the axial gasflow due to the different gas injection ratios of the tricoaxialinjector were measured, and the correlation between the flowdistribution and the liquid breakup was established. In addition,the experimental results of the flame structures and temper-ature distributions were plotted and presented.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: jykoo@kau.ac.kr. Tel.:+82 10 4732 1316.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was supported by a National Research Foundation ofKorea (NRF) grant funded by the Korean Government(MEST) (NRF-2012M1A3A3A02033146).

■ NOMENCLATURE

d = liquid nozzle diameter (mm)dinner = inner gas nozzle diameter (mm)douter = outer gas nozzle diameter (mm)tgas‑1 = inner gas nozzle thickness (mm)lo = liquid nozzle orifice length (mm)lb = liquid column breakup lengthRrecess = recess length (mm)θliquid = liquid nozzle inlet angle (deg)θinner = inner gas nozzle inlet angle (deg)θouter = outer gas nozzle inlet angle (deg)M = momentum flux ratio (ρgvg

2/ρ1v12)

Min = inner-stage momentum flux ratio (ρg-invg‑in2 /ρ1v1

2)Mout = outer-stage momentum flux ratio (ρg-outvg‑out

2 /ρ1v12)

Pg = air pressure (bar)Rein = inner-stage Reynolds number (ρνind/μ)Reout = outer-stage Reynolds number (ρνoutd/μ)SMD(D32) = average of particle size, Sauter mean diameter(μm)Vin = inner-stage gas velocityVout = outer-stage gas velocityVl = liquid velocity

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