nscom

G. Singla , P. Scouaire, C. Rolon, S. Candel

2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

investigation aims at providing fundamentalinformation on the combustion process involvingthis couple of propellants injected in a transcriticalstate. The experimental study is carried out on a

1540-7489/$ - see front matter 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.doi:10.1016/j.proci.2004.08.063

* Corresponding author. Fax: +33 1 47 02 80 35.E-mail address: ghislain.singla@em2c.ecp.fr (G.

Singla).

Proceedings of the Combustion Institu

Proceedingsof the

CombustionKeywords: Transcritical combustion; Cryogenic ames; High pressure

1. Introduction

Many recent studies [13] have provided a de-tailed understanding of cryogenic propellant com-bustion under subcritical and transcritical

conditions. Research has concerned liquid oxygenand gaseous hydrogen injected from a single ele-ment at various chamber pressures (0.17 MPa).There is current interest in the development ofreusable liquid rocket engines operating withmethane and oxygen as propellants. The presentEM2C Laboratory, CNRS, Ecole Centrale Paris, Grande Voie des Vignes 92295 Chatenay-Malabry, France

Abstract

Injection of liquid uid initially at subcritical temperature into an environment in which the temperatureand pressure exceed the thermodynamic critical conditions is an important phenomenon in many high per-formance devices like liquid propellant rocket engines. This is found, for example, in the Space Shuttlemain engines or in the Ariane 5 Vulcain engine both operating with liquid oxygen (LOx) and gaseoushydrogen (GH2). This article is concerned with the less standard situation where both reactants are in atranscritical state. One case of current interest in propulsion, that of combustion of cryogenic oxygenand methane injected at high pressure, is investigated experimentally. A coaxial injector delivers oxygenat a temperature of 85 K and methane at 120 or 288 K. The pressure in the chamber takes values between4.5 and 6 MPa. Emission images from excited state OH (A2R, denoted OH*) and CH (A2D, denoted CH*)are recorded and averaged. The Abel transform is used to determine the mean ame structure from theseaverage images. Data indicate that the ame is stabilized in the vicinity of the injector. When both propel-lants are transcritical, the ame features two conical regions of light emission, one spreading close to theliquid oxygen boundary and the other located further away from the axis near the liquid methane bound-ary. The outer ame boundary is also conical with a relatively large expansion angle. This ame structurenotably diers from that observed when one of the propellants is injected in a subcritical or transcriticalstate while the other is gaseous. An analysis of the relevant characteristic times suggests that under trans-critical conditions the rate of combustion is mainly controlled by turbulent energy transfer to the propel-lants. This determines the mass uxes from the dense regions to the lighter gaseous streams governing therate of conversion into products.Transcritical oxygen/tramethane

*critical or supercriticalbustion

te 30 (2005) 29212928

www.elsevier.com/locate/proci

Institute

through the ame and dene the regions where

has a square cross-section of 50 50 mm2. A visu-alization module, with 75 mm long windows on itsfour sides, can be placed at any point along thechamber. The windows are made of quartz, trans-parent to near UV radiation. In the present exper-iments, the visualization module was locatedagainst the injection plane, providing a full viewof the initial ame. A convergingdiverging nozzlemade of graphite denes the operating pressure.

Operating conditions are presented in Table 1for selected experiments at high pressure. In testsT1T2, the two reactants are transcritical. In testsG1 and G2, methane is injected as a gas while theoxygen jet is subcritical (G1) or transcritical (G2).The mass ow rate of oxygen remains constant ata value _mLOx 45 g s1. Consequently, the heatrelease is around 0.55 MW for all injection condi-tions if all the oxygen is consumed. The mixtureratio E _mLOx= _mCH4 is between 0.28 and 0.43,which is well below the mass stoichiometric values = 4 characterizing the oxygen/methane reaction.

3. Diagnostics and processing methods

Local light emission from free radicals repre-sents one interesting experimental observable inthe study of reacting ows. In an attempt to inter-

G1 43.9 101.2 85 288 0.93 1.02G2 44.4 143.1 85 288 1.11 1.22

2922 G. Singla et al. / Proceedings of the Combustion Institute 30 (2005) 29212928light emission takes place in the mean. These re-gions delimit the ame zone and give an idea onthe intensity of chemical conversion.

The experimental facility, optical diagnostics,and image processing methods are described rst.Doubly transcritical ames are analyzed next. It isshown that their structure diers from that ob-served in the case where only one of the propel-lants is transcritical. It is found in particularthat light emitted by both OH* and CH* radicalsoriginates from two conical regions. In the Abeltransform images, these conical regions form fourbeams of light. This structure is compared to thatfound when the central propellant (oxygen) istranscritical while the surrounding stream ofmethane is gaseous. The last section contains adiscussion of the possible factors controlling theobserved ame structures and a tentative interpre-tation of observations.

2. Experimental conguration

Experiments are carried out on a cryogenicmodel scale combustor designated as Mascotte.This facility is schematically shown in Fig. 1.The bench was adapted to study the LOx/CH4combustion (version V04). The most notablechanges with respect to the previous versions con-cern the fuel feed line, which was modied to al-low injection of either hydrogen or methane.The heat exchanger, placed on the feed system,is powerful enough to liquify the methane streamat a maximum mass ow rate _mCH4 250 g s1.This allows investigations of combustion condi-tions in which liquid methane is injected togetherwith liquid oxygen. The ow rate of oxygenranges from _mLOx 20 to 100 g s1. The amespreads in a combustion chamber capable of with-standing pressures up to 10 MPa. This elementsingle coaxial injector placed in a square cross sec-tion combustor allowing optical access. The testbench partially reproduces conditions prevailingin the preburner of the projected engine. Threeof the tests described in this article correspondto a pressure that exceeds the critical pressuresof both oxygen and methane (pc (O2) = 5.04 MPa,pc (CH4) = 4.6 MPa). One test is carried out at apressure that is below the critical value for oxygenbut above the critical value of methane.

The ame structure is investigated in three ba-sic situations, the rst corresponding to transcrit-ical injection of both propellants, the second andthird being such that one propellant (oxygen) issubcritical or transcritical while the other (meth-ane) is supercritical. The ame geometry is de-duced from emission imaging of two radicalsOH* and CH*. Average emission images formedby summing instantaneous distributions are trea-ted with an Abel transform to obtain a sliceFig. 1. (A) Experimental conguration for OH* andCH* simultaneous emission imaging. A beam splitter isemployed so that the two cameras visualize the region ofinterest. (B) Left: eld of view for the OH* camera.Right: eld of view for CH* camera (neareld close-up).

Table 1Selected operating conditions

Name _mLOx _mCH4 Tinj pr

(g s1) (g s1) O2 CH4 O2 CH4

T1 44.1 158.2 85 120 1.07 1.18T2 44.1 134.7 85 120 1 1.09

G. Singla et al. / Proceedings of the Combustion Institute 30 (2005) 29212928 2923pret CH* and OH* emission in diusion ames, aset of calculations has been carried out for coun-terow strained ame congurations of pureoxygen impinging on pure methane. The chemicalmechanism GRI Mech 3.11 was used in combi-nation with reactions describing CH* and OH*formation and destruction with appropriate rateconstants [4]. Production of OH* is described asa result of oxidation of CH via the reactionCH + O2 OH* + CO. In oxycombustionames, the oxidation of CH4 leads to formationof H2, which in turn can quickly diuse throughthe reaction zone. One must account for reactionsinvolving atomic hydrogen, oxygen, and OHradicals, species that are found only in the amefront and in burned gases for the last one:H + O +M OH* + M and H + OH + OHH2O + OH*. Production of CH* results from oxi-dation reactions C2H + O2 CH* + CO2 andC2H + O CH* + CO. Radiative and collisionalremovals were also added to the chemical scheme.Systematic calculations carried out at 0.1 MPa,for fuel and oxidizer temperatures of 300 K andfor a range of strain rates (103104 s1), indicatethat peaks of mass fractions YOH and YCH aresimilar and correspond well with the peak of tem-perature. The distributions of OH* and CH* arechemically and spatially correlated with theirrespective ground states and their peaks are notfar from the region of maximum heat release rate.While the range of parameters explored is for themoment limited, these calculations show thatchemiluminescence from OH* and CH* may beused to characterize the chemically active regionsin the ame.

Spontaneous emission of OH* is recorded ata rate of 15 Hz with an intensied (CCD) camerawith a useful resolution of 151 256 pixels. Thecamera is equipped with a Nikon 105 mmf = 4.5 UV objective. A UG5 glass lter may beused to block radiations above 400 nm and aWG305 glass lter to suppress radiations below283 nm, while passing 5070% of the light emit-ted between 306 and 320 nm where chemilumi-nescence is observed. Spontaneous emission ofCH* is detected at a rate of 2.5 Hz with anICCD camera with a useful resolution of464 428 pixels. The camera is equipped with avisible Nikon 80200 mm f = 2.8 objective pro-viding a close-up on the injector eld. ASWP604 glass lter blocks radiations above450 nm, and a FCG-059 glass lter suppressesradiations below 400 nm, while passing 90% ofthe light emitted between 420 and 440 nm whereCH* chemiluminescence is observed. For the twocameras, the exposure time was xed at 30 ls toobtain nearly instantaneous images. The twocameras are synchronized every six images. Theinstantaneous images are averaged, and the resultis Abel transformed to obtain a slice through theame. This type of numerical tomography is suit-able if (1) the ame is axisymmetric, (2) self-ab-sorption of the light radiated by the ame isnot too large, (3) light ray deection by refrac-tion index gradients remains limited, and (4) dis-tance between the camera and the ame is large,compared to the radial size of the combustionchamber.

The emission intensity distribution (the ameshape), recorded by the two cameras, varies inspace and time but is axisymmetric in the mean.The image is viewed from the side and emissionis accumulated along the line of sight. The aver-age radial intensity distribution can be calculatedfrom the average image via the Abel inversion,giving a slice of the mean volumetric emissiondistribution. It has been shown [5] that the aver-age ame position deduced from OH-PLIF mea-surements or from Abel inverted emission imagesnearly coincides if there is weak absorption. Athigh pressure, absorption of OH* emission byOH radicals could be appreciable [6]. Absorptionof CH* emission would be more limited because(1) the spatial distribution of CH is narrow, (2)its maximum molar fraction is low. As a conse-quence, the medium is optically thin. Neverthe-less, Juniper et al. [7] shows that the positionof maximum intensity in the Abel inverted aver-age image will be the same with or withoutabsorption.

Another problem, due to the injection of liquidpropellant, is that light can be deected by strongrefractive index gradients at the edge of the densejets of transcritical propellants. At the boundaryof supercritical oxygen or methane, in the caseof doubly transcritical injection, the densitychanges from 1100 to 100 and 420 to 50 kg m3 ,respectively, over short distances [8]. When bothpropellants are injected in a transcritical state,the ame is located between two dense areas,and some refraction may take place when the lightbeams cross the boundaries of the dense methanestream. It is important to estimate the light beamdeection. The diculty, in the present situation,is to account for the near critical behavior of therefractive index. Experimental observations indi-cate that the LorenzLorentz law R = (1/q){(n2 1)/(n2 + 2)} can be used to predict thechange of refractive index n knowing the changein density [9]. Accordingly, the refractivity con-stant depends on the chemical nature of the med-ium, and q is the density at the pressure andtemperature at which n is measured [10]. To ac-count for the imaging parameters (the ratio ofthe distance between camera and the zone of inter-est notably), a detailed analysis indicates that theshift in the light emitting regions is proportionalto the radial size of the ame. In the near-eldof the injector where the ame is less expandedthe error is minimal. It will not exceed 3% ofthe vertical position of the emission point at max-imum of deection, inducing a tolerable image

distortion. In summary, neither absorption norrefraction from jets will signicantly interfere withthe relevant part of the Abel inversion.

4. Structure of transcritical ames

Comprehensive reviews of the state of knowl-edge in supercritical mixing and combustion wererecently given by Yang [11] and Bellan [12].Depending on uid properties and ow charac-teristics, two cases of study may be found. Atsubcritical chamber pressures, injected liquid jetsundergo a classical cascade of processes associ-ated with atomization. For this situation, inertialand surface tension forces promote the formation

ture. Emission of CH* (Figs. 2AC) originatesfrom a cone featuring a half angle of about 24.The outer ame boundary in these four shortduration exposures is relatively stable and formsa nearly straight line. The light emitted withinthe cone is organized around four linear beams ar-ranged in a fan-like pattern. This structure is par-ticularly clear in Fig. 2C. Instantaneous images ofOH* emission taken with the wide eld cameraalso feature this special fan-like distribution (Figs.2DF). This is however, less visible because the

Fig. 2. (AC) Instantaneous CH* emission images,close-up on the neareld, exposure time 30 ls. (DF)

2924 G. Singla et al. / Proceedings of the Combustion Institute 30 (2005) 29212928Instantaneous OH* emission images, eld of view equalto the visualization window, exposure time 30 ls. Pointof operation T1.of a heterogeneous spray of ligaments, pockets,and droplets, which evolves continuously. Whenthe chamber pressure and temperature approachthe critical conditions, a jet of liquid undergoes atranscritical change of state. The uid jets exhibitmany characteristics distinct from their counter-parts at low pressure. Indeed, as surface tensionand enthalpy of vaporization (Dhv) become van-ishingly small near the critical point, the interfaceseparating the liquid and gas phases disappears.The uid properties and their spatial gradientschange continuously through the eld. Thereare thermodynamic and transport anomalies nearthe critical point, a phenomenon commonly re-ferred to as the near critical enhancement. As aresult, volumetric changes induced by the unu-sual behavior of the uid near the critical pointmay play an important role in the structure ofame. Considering this, it is interesting to studythe structure of the ame for several injectionconditions, notably when both uids are trans-critical or when one of them is transcritical whilethe other is supercritical.

4.1. Doubly transcritical injection

Typical distributions of light emission fromCH* and OH* are displayed in Fig. 2. Theseinstantaneous images reveal a characteristic struc-Fig. 3. (A) Average images of CH* emission, calculatedfrom 30 instantaneous images with exposure time 30 ls.(B) Abel transform of the time averaged CH* emissionimages. (C) Average images of OH* emission, calculatedfrom 150 instantaneous images with exposure time 30 ls.(D) Abel transform of the time averaged OH* emissionimages. Point of operation T2.

image dynamic range is mainly used to representthe most luminous region located downstreamand the vicinity of the injector is less distinguish-able. A close-up on the injector neareld (not in-cluded) reveals the fan-like structure, whichexactly coincides with that observed in the CH*images. The ame spreads from the injector andextends slightly beyond the end of the viewingwindow. The conical regions of light emissionare also visible in the average images obtainedby summing the short time exposures (Figs. 3Aand C). Examining Fig. 3C, one would think thatthere is no emission in the injector neareld. Thisis an eect of the color code used to represent thefull range of intensities. If one focuses on the near-eld, these intensities are made more apparentand feature the pattern observed in Fig. 3A(hence, the intensity slice shows a distribution ofvalues in this region).

The distribution of light intensity in the amemay be calculated from the average image viathe Abel inversion. The upper and lower sides of

Fig. 5. Combined emission and backlighting images,close-up on the neareld. The color scale corresponds toa slice of CH* emission and the light to dark blue scaleto the average jet position. (A) Point of operation G1.(B) Point of operation G2.

Fig. 6. Schematic representation of cryogenic amesinvestigated in this article. (A) Subcritical oxygen,gaseous methane. (B) Transcritical oxygen, gaseousmethane. (C) Transcritical oxygen, transcritical methane.

G. Singla et al. / Proceedings of the Combustion Institute 30 (2005) 29212928 2925Fig. 4. (A and C) Average images of OH* emission,calculated from 200 instantaneous images with exposuretime 30 ls. (B and D) Combined Abel transformedemission and backlighting images. The color scalecorresponds to a slice of OH* emission and the lightto dark blue scale to the average jet position. (A and B)Point of operation G1. (C and D) Point of operation G2.

resents the velocity dierence between thereactant streams. A typical vaporization time is gi-

2926 G. Singla et al. / Proceedings of the Combustion Institute 30 (2005) 29212928the average emission image are transformed inde-pendently, yielding distributions that are not quitesymmetrical with respect to the axis (Fig. 3B andD). The dierence may be used to assess the qual-ity of the image processing procedure. It also re-sults from a slight non-uniformity in the injectedow. The central region nearly cylindrical and de-void of luminosity corresponds to a dense oxygenjet. Two conical regions of light emission are re-vealed: the rst spreads near the liquid oxygenboundary with a relatively constant intensity level,and the second, closely follows the liquid methaneboundary, with an axially decreasing intensity le-vel. In the downstream region, the light intensityincreases, and the ame forms a cap with a slightlyreduced level near the centerline (Fig. 3D). Theouter boundary of the luminous cap interceptsthe wall of the combustion chamber and in theseregions, the assumptions used for the Abel trans-form are invalidated.

Another interesting observation is that an icecylinder is formed around the methane jet and par-tially blocks the light radiated from the neareld.This ice shell reaches a size of about 4 mm at theend of the run. This is due to accretion of recirculat-ing water freezing as it comes in contact with thecold methane stream. This eect occurs only whenboth propellants are injected at low temperature.

4.2. Injection of sub- and transcritical O2/supercrit-

ical CH4

It is interesting to compare the previous datawith results corresponding to supercriticalinjection of methane and subcritical (G1) or trans-critical (G2) injection of oxygen (Figs. 4 and 5).Figs. 4A and C show time averaged OH* emissionimages while the Abel transforms appear in Figs.4B and D. Neareld obtained from CH* emissionare displayed in Fig. 5. The ame position with re-spect to the LOx jet may be obtained by averagingimages recorded with backlighting. The central jetappears as a dark region while the ame is shownon a color scale around the jet. Flame structurescorresponding to LOx/GCH4 injection are similarto those found in earlier studies on LOx/H2 cryo-genic ames. Light emission originates from an ini-tially cylindrical envelope followed by an intenseexpanding zone. The second part of the ame iswell apparent in the averaged images (Figs. 4Aand C). The reactive region abruptly closes beforethe end of the viewing window. There is a centralzone corresponding to the oxygen jet devoid ofluminosity, bounded by two narrow layers of lightemission. The ame, stabilized on the lip of theinjector, is wrapped around the oxygen jet. At a re-duced pressure pr (O2) < 1 (Fig. 4B), the ame fol-lows the surface of the cylindrical liquid jet for56dO2 before blooming rapidly with an expansionangle a of about 20. When the reduced pressureexceeds one, pr (O2) > 1 (Fig. 4D), the ame ex-ven by sv d2qLOx=8DO2qg ln1 B where Bis the Spalding transfer number B = cp (Tg TLOx)/Dhv. One nds that sv > sm for most dropletsizes of interest. This still holds if convective andstripping eects are included in the vaporizationtime estimate. One also nds that sm > sc, wheresc is a typical reaction time. Under subcritical con-ditions, sv is the slowest time, and vaporizationcontrols the process. This produces ame shapesof the type shown in Fig. 4B with an expansion re-gion, which is characteristic of break-up andatomization. In this case, it is known that increas-ing the momentum ux ratio J qCH4v2CH4=qLOxv

2LOx enhances atomization leading to a short-

er ame with an augmented radial blooming.pands more progressively up to 7dO2 where itsblooming is less pronounced (a. 10). The amelength (Lf) is shorter at pr (O2) > 1 Lf 11dO2than at pr (O2) < 1 where Lf 14dO2 . The timeaveraged emission intensity in the near injector re-gion is more important when oxygen is injected ina transcritical state in the chamber (Fig. 5). Thereduction of the ame length is associated with ashorter oxygen jet. The length and thickness ofthe internal core diminish as the chamber pressureincreases (LO2 7dO2 at pr (O2) > 1 while LO2 10dO2 at pr (O2) < 1), but the observed changesmay be due in part to the augmented mass owrate of methane, which induces a 40% increase inmethane ow velocity.

5. Discussion

The three types of ames investigated in thisarticle are represented schematically in Fig. 6 toallow a better understanding of the following dis-cussion. In analyzing these various ames, oneshould account for phase equilibrium aspectsand consider the multi-components nature of thedierent streams. However, for simplicity, it ispossible to consider that the oxygen behaviormay be assimilated to that of pure liquid in isown vapor, i.e., that the partial pressure of oxygenin the gas near the dense oxygen is equal to thechamber pressure. One may think that part ofwater vapor and, to a lesser extent, CO2 will betransformed in a liquid phase in the low tempera-ture regions close to the jet. In cryogenic jet amesunder subcritical conditions (Fig. 6A), the liquidjet is atomized and oxygen has to evaporate andto be transferred to the ame front by turbulentuctuations, where it reacts with gaseous meth-ane. Turbulence is essentially determined by thelarge velocity dierence existing between the gas-eous oxygen and methane streams. The corre-sponding mixing time may be estimated assm d=vCH4 vLOx, where d is a typical trans-verse mixing layer thickness and vCH4 vLOx rep-

central conical region of reaction. This however,can be discarded on the basis of experiments carried

one of the reactants is injected in a subcritical or

G. Singla et al. / Proceedings of the Combustion Institute 30 (2005) 29212928 2927As the chamber pressure exceeds the criticalvalue of oxygen (Fig. 6B), surface tension and la-tent heat are reduced to zero. As indicated in [7],[13], and [14], droplet formation and evaporationare replaced by mixing and mass transfer fromthread like structures that evolve from the liquidcore and diuse rapidly within the shear layer.The transcritical uid behaves like a gas but witha highly nonuniform distribution of density. Inthe central core region where the temperature isbelow critical, the density is very high. In the outerlayer where the temperature has increased and isabove the critical value, the density is low. Underthese conditions, mass transfer processes betweenthe dense and light regions depend on the turbu-lent rate of energy transfer from the outer to theinner layers. Thus, at pressures above critical,mixing becomes the slowest and therefore mostinuential process.

The rate of mass transfer from the dense re-gions is determined by the surface area of the con-densed jet and by the local strain rates. Thisprocess is more eective than the atomization/va-porization and mixing process taking place at sub-critical pressures. This explains to some extent thereduction in the oxygen core length and radial ex-tent as observed in Fig. 4D. Increasing the veloc-ity of methane enhances the rate of mass transferfrom the oxygen stream. The ame, located in theshear layer between the oxygen and the methanejet, expands to a lesser extent in the radial direc-tion and becomes shorter as shown in Fig. 5.

The ame formed by a coaxial injector fed withtranscritical oxygen in the center surrounded by atranscritical stream of methane is more complex(Fig. 6C) with two outstanding features: (1) afairly stable outer ame boundary, (2) two conicalregions of light emission. The nearly conical outerboundary of the ame is adjacent to a recircula-tion zone of burned gases at a temperature of afew hundred degrees. Along this transcritical sur-face, methane injected in excess comes into con-tact with burned gases characterized by a lowdensity. The rate of growth ai of hydrodynamicinstabilities at this interface is proportional toqg=qCH41=2. The large density dierence betweenthe inner and outer uids reduces the rate ofgrowth of these instabilities, thus diminishing theamplitude of uctuation of this outer boundaryexplaining the weakly perturbed appearance.

The two conical regions of light emissionobserved in instantaneous images and Abel trans-formed average images dier from ame structurescharacteristic of a single transcritical reactant. Bothregions of light emission correspond to chemicallyactive layers. The region closest to the central jetseems to develop at a nite distance from the LOxpost-lip, and its initial diameter is markedly lessthan that of the LOx injector. This reactive layerinvolves oxygen from the core and gaseous meth-ane transferred from the annular stream of trans-transcritical state. This is conrmed by experi-ments carried out with methane injected in gas-eous form while the liquid oxygen parametersare kept constant. A change in behavior betweenthe subcritical and transcritical conditions is madeevident. It is found that the last case is mainlycontrolled by turbulent mass transfer processesfrom the central core, essentially governed bythe velocity dierence between the two streams.

Acknowledgments

The generous support of CNES and Snecma isgratefully acknowledged. Experiments were car-ried out on the Mascotte facility at Onera without on a cold ow simulation in which liquid waterand nitrogen were coaxially injected around an in-ner source of light. The scattered light was negligi-ble with respect to the source. In the secondinterpretation, the outer conical region is fed bygaseous methane and some oxygen, which is di-verted from the central core and entrained by thehigh speed transcritical methane. Fig. 6C gives atentative idealization of this conguration.

6. Conclusion

High pressure combustion of LOx and meth-ane is investigated in this article. The ame isformed by a coaxial injector fed with a low speedoxygen stream in the center surrounded by a high-er speed stream of methane. In the case whereboth reactants are initially transcritical, stabiliza-tion takes place in the neareld. The ame fea-tures two regions of emission where lightradiation originates from OH* and CH* radicals.This indicates the presence of two reaction layers.The rst develops near the LOx post lip and essen-tially follows the boundary of the central streamof oxygen. The second luminous region spreadsnear the outer boundary of the liquid methanejet. These two emission regions are nearly conical.The region where light is radiated is bounded by athird cone, which denes the border between thedense uid region and an outer region occupiedby lighter uid. The surface separating these tworegions is weakly turbulent due to the large den-sity dierence between the inner and outer uids.The ow and ame structures formed in the dou-bly transcritical injection situation studied herethus dier to a great extent from that where onlycritical methane. For the outer conical region, onemay envisage two interpretations. The rst wouldbe that its luminosity is due to scattering of lightby the inner surface of the transcritical methanestream. The light itself would originate from the

the assistance of L. Vingert and his team. The timeaveraged oxygen jet structure was calculated fromdata provided by P. Gicquel.

References

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2928 G. Singla et al. / Proceedings of the Combustion Institute 30 (2005) 29212928of the fuel jet. 1. What are the benets of that reverse the large spread angle observed experimentally.Comments

Suresh Aggarwal, University of Illinois at Chicago,

USA. Flame images show two reaction zones, which

may indicate the existence of a partially premixed ame.

The oxygen may leave through the inner (lean) reaction

zone and then burn in the outer (diusion) reaction zone.

Reply. This interpretation is close to that given in this

article and it might well be true.

d

Andrezj Sobiesiak, University of Windsor, Canada. In

your experimental set-up an oxidizer is delivered insidereactants delivery arrangement? 2. Why is the spread an-

gle of the liquid methane jet so much greater than that of

the gaseous methane?

Reply. 1. This arrangement is standard in many rock-

et engines. One wishes to prevent a possible contact of

hot oxygen with the chamber walls. For the outer injec-

tors it is preferable to have the fuel delivered around the

oxidizer.

2. In the transcritical injection of methane and oxy-

gen, the two reactants cross the critical point and their

specic volumes increase substantially. This explains

Transcritical oxygen/transcritical or supercritical methane combustionIntroductionExperimental configurationDiagnostics and processing methodsStructure of transcritical flamesDoubly transcritical injectionInjection of sub- and transcritical O2/supercritical CH4

DiscussionConclusionAcknowledgmentsReferencesComments