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Cherian A. Idicheria, Isaac G. Boxx, Noel T. Clemens

Center for Aeromechanics Research, Department of Aerospace Engineering and Engineering Mechanics,The University of Texas at Austin, Austin, TX 78712-1085, USA

Received 13 June 2003; received in revised form 19 June 2004; accepted 19 July 2004

Available online 12 August 2004

Abstract

An experimental study was performed with the aim of investigating the structure of transitional and turbulentnonpremixed jet flames under different gravity conditions. Experiments were conducted under three gravity levels,viz., 1 g, 20 mg, and 100 g. The milligravity and microgravity conditions were achieved by dropping a jet-flamerig in the University of Texas at Austin 1.25-s and NASA-Glenn Research Center 2.2-s drop towers, respectively.The flames studied were piloted nonpremixed propane, ethylene, and methane jet flames at source Reynolds num-bers ranging from 2000 to 10,500. The principal diagnostic employed was time-resolved cinematographic imagingof the visible soot luminosity. Mean and root-mean-square (RMS) images were computed, and volume renderingof the image sequences was used to investigate the large-scale structure evolution and flame tip dynamics. Therelative importance of buoyancy was quantified with the parameter, L, as defined by Becker and Yamazaki (Com-bust. Flame 33 (1978) 123149). The results showed, in contrast to some previous microgravity studies, that thehigh-Reynolds-number flames have the same flame length irrespective of the gravity level. The mean and RMSluminosity images and the volume renderings indicate that the large-scale structure and flame tip dynamics areessentially identical to those of purely momentum-driven flames provided L is less than approximately 23. Thevolume renderings show that the luminous structure velocities (i.e., celerities) normalized by the jet exit velocityare approximately constant for L < 6, but scale as

3/2L for L > 8. The flame length fluctuation measurements

and volume renderings also indicate that the luminous structures are more organized in low gravity than in normalgravity. Finally, taken as a whole, this study shows that L is a sufficient parameter for quantifying the effects ofbuoyancy on the fluctuating and mean characteristics of turbulent jet flames. 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Keywords: Turbulent nonpremixed flames; Microgravity; Buoyancy

1. Introduction

Becker and Yamazaki [1,2] and Becker and Liang[3] were among the first to systematically study the

* Corresponding author. Fax: (512)-471-3788.E-mail address: clemens@mail.utexas.edu

(N.T. Clemens).

effects of buoyancy on the characteristics of turbu-lent nonpremixed jet flames, such as soot formation,entrainment, and luminous flame length. They pro-posed that the effects of buoyancy could be quanti-fied by a nondimensional buoyancy parameter, L(defined below), which is a measure of the relativeimportance of the buoyancy force to source momen-tum over the entire flame length. They concluded that

0010-2180/$ see front matter 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.doi:10.1016/j.combustflame.2004.07.002Combustion and Flame 1

Characteristics of turbulentnormal- and low004) 384400www.elsevier.com/locate/jnlabr/cnf

npremixed jet flames underavity conditions

C.A. Idicheria et al. / Combustion and Flame 138 (2004) 384400 385the effects of buoyancy on the characteristics of theflame become negligible when this nondimensionalparameter is less than unity. In their experiments theylowered L by increasing the Reynolds number; how-ever, this raises the important issue of whether anyobserved differences in the flame characteristics aredue to the reduced importance of buoyancy or to thelarger Reynolds number.

In the past couple of decades, microgravity en-vironments have been used to investigate the effectsof buoyancy on a wide range of combustion sys-tems. The microgravity environment offers the ad-vantage that buoyancy effects can be isolated, be-cause gravity can be changed without having to mod-ify the Reynolds number. Bahadori et al. [4] andHegde et al. [57] were among the first to investigatenonpremixed jet flames in the laminar-to-turbulentregime in normal and microgravity conditions. Theirprimary diagnostic was video-rate (30 fps) luminosityimaging, which was used for flow visualization andto obtain flame length data over a range of Reynoldsnumbers. Their results showed that there are signifi-cant differences in the characteristics of normal andmicrogravity nonpremixed jet flames. For example, ata Reynolds number of about 5000 their microgravityflames were more than twice as long as their normal-gravity flames, the latter of which had L 6.5. Thisindicates a much stronger dependence of flame lengthon L than would be indicated by the results of Beckerand Yamazaki [1]. This raises the issue of whether Lis a sufficient parameter for quantifying the effects ofbuoyancy.

With particular focus on the underlying turbu-lent structure, studies of transitional nonpremixedjet flames have shown that disturbances originateat the base of the flame in microgravity and travelupwards as Reynolds number is increased, whereasin normal gravity, the disturbances originate nearthe flame tip and work their way down [4]. Fur-thermore, normal gravity studies of turbulent non-premixed flames have shown that flame tip burnoutdynamics are closely related to the large-scale orga-nization of the jet flame [8], and hence are stronglyaffected by buoyancy [9]. To date, there is no con-sensus as to the nature of the large-scale motionspresent in purely momentum-driven round jet flames,although there is evidence for both axisymmetricand helical structures [5,8,9]. It has also been sug-gested that buoyancy can substantially influence thelarge-scale structure of even nominally momentum-driven flames, since the low-velocity flow outside ofthe flame will be more susceptible to buoyancy ef-fects [10]. Even subtle buoyancy effects may be im-portant because changes in the large-scale structurehave implications for the fluctuating strain rate, whichinfluences the structure of the reaction zone.There are evident limitations in the range ofconditions that were achieved by Becker and Ya-mazaki [1,2], by Becker and Liang [3], and in pre-vious microgravity studies [47]. For example, inRefs. [13], they were not able to obtain values ofL less than 3 and so they could not study flamesthat were momentum-dominated (according to theirown criterion) over the full length of the flames,whereas Refs. [46] investigated only a limited rangeof Reynolds numbers (250 < ReD < 5800). Thisstudy aims to improve upon and add to our knowledgeof turbulent nonpremixed jet flames by investigatinga range of Reynolds number and L wider than inthese previous studies and to investigate specificallythe effect of buoyancy on the large-scale luminousstructures observed in the jet flames. We take ad-vantage of an ability to study flames at a range ofReynolds numbers and under three different gravitylevels, viz., 1 g, 20 mg, and 100 g. The three gravitylevels make it possible to alter the value of L throughtwo orders of magnitude, while maintaining the sameReynolds number. The reduced gravity levels areachieved by using the 1.25-s University of Texas droptower facility (UT-DTF) and the 2.2-s drop tower atNASA Glenn Research Center (GRC). The primarydiagnostic employed was cinematographic imagingof the flame luminosity. The cinematographic imag-ing improves upon the video-rate (30-Hz) imagingused in previous studies of microgravity nonpremixedjet flames [46] because it enables us to investigatethe evolution and dynamics of large-scale turbulentstructures. Furthermore, the flame length results inRefs. [46] seem to suggest that L is not sufficient toquantify the effects of buoyancy, and so a major ob-jective of this work is to address this specific issue andto determine at what value of L the turbulent struc-ture reaches its asymptotic momentum-dominatedstate.

2. Experimental program

2.1. Drop-rig

The experiments were conducted using a self-contained combustion drop-rig in the UT and GRCdrop towers. A schematic of the drop-rig is shownin Fig. 1. The drop-rig consists of a turbulent jetflame facility and an onboard image and data ac-quisition system assembled in the NASA-GRC 2.2-sdrop tower frame. The fuel jet issues from a 1.75-mm(inner diameter) stainless steel tube, surrounded bya 25.4-mm-diameter concentric, premixed, methaneair flat-flame pilot (operated near stoichiometric con-ditions). The pilot flame was used to ignite the mainjet during the drop and also to keep the jet flame

386 C.A. Idicheria et al. / Combustion and Flame 138 (2004) 384400Fig. 1. Schematic diagram of the drop-rig.

attached. Flame luminosity was imaged using a Pul-nix TM-6710 progressive scan CCD camera, capa-ble of operating at 235 or 350 fps, at resolutions of512 230 and 512 146 pixels, respectively. Thecamera was electronically shuttered, with the expo-sure time depending on flame luminosity (1/235 to1/2000 s), and was fitted with a 6-mm focal length,f/16 CCTV lens, chosen to maximize the field of view(typically 405 mm). The drop-rig was fully automatedthrough a custom-configured passive back-plane typeonboard computer (CyberResearch Inc). The onboardcomputer had no monitor or keyboard due to spaceconstraints in the rig and was controlled remotelyfrom a notebook computer. A program developed inLabVIEW was used for timing and control of the ex-periment. A more detailed description of the drop-rigis given in Idicheria et al. [11].

2.2. 1.25-s drop tower

The 1.25-s UT-DTF is 10.7 m tall and has a 2.5-msquare cross-sectional area. The tower is equippedwith a 2-ton capacity electric hoist and a cargo hookat the end of the hoists chain that acts as the quick-release mechanism. At the base of the drop tower isa deceleration mechanism consisting of a container1.7 m long by 1.1 m wide by 1.8 m deep, filledwith flame-retardant, HR-24 polyurethane foam. Thefloor of the container is lined with two 150-mm-thick sheets of foam, and the rest of the container isfilled with 150-mm foam cubes. After allowing forthe space taken up by the electric hoist and the de-celeration mechanism, the drop tower has a 7.6-mfree-fall section. This allows approximately 1.25 s oflow-gravity time per drop. In order to characterizethe milligravity conditions, data were acquired us-ing a Kistler Model 8304-B2 K-Beam capacitiveaccelerometer. These measurements acquired in theUT-DTF indicate that the gravity levels range from0 mg at the beginning of the drop to 20 mg by theend (the latter value is due to aerodynamic drag be-cause no drag shield is used). The g-jitter (definedas peak to peak variation) from these measurementswas typically 3 mg. A Kistler Model 8303-A50 K-Beam capacitive accelerometer was used to measurethe deceleration of the drop-rig on impact at the endof each drop. Impact loading thus measured rangedfrom 25 to 30 g. To reduce the effects of outside dis-turbances while performing the experiments in theUT-DTF, the sides of the drop-rig were closed withaluminum sheets.

2.3. 2.2-s drop tower

The 2.2-s drop tower at NASA-GRC is approxi-mately 24 m tall. The drop-rig is enclosed in a dragshield to minimize the aerodynamic drag on the ex-periment. The assembly consisting of the drop-rig anddrag shield is attached to a pneumatic release systemat the top of the tower prior to the drop. At this point,the drop-rig stands 191 mm from the base of the dragshield. After the release, the drop-rig falls through the191 mm inside the drag shield while the whole assem-bly of drag-shield and drop-rig falls through 24 m.At the end of the drop the assembly impacts an airbag and comes to rest. During the 2.2-s drop time mi-crogravity levels of 100 g is attained. Impact levelsduring the deceleration are in the range of 1530 g.

3. Experimental conditions

Three different jet fuels were studied (propane,ethylene, and methane) and experiments were con-ducted for a range of Reynolds numbers (2000 3were taken in normal gravity. Fig. 5a shows ethyl-ene flames at the highest Reynolds number considered(ReD = 10,500) that develop under normal-gravity(left) and milligravity (right) conditions. It is seenthat at this highest Reynolds number, the structureof the flame seems to be very similar regardless ofgravity level. This is perhaps not a surprising re-sult since the L values in normal gravity (L = 3.7)and milligravity (L = 1.0) are close to the criterionfor momentum-dominated flames of L = 1 proposedby Becker and Yamazaki [1]. Fig. 5b shows ethyl-

C.A. Idicheria et al. / Combustion and Flame 138 (2004) 384400 389Fig. 4. Sample time-sequenced luminosity images: (a) normal gravity, ethylene ReD = 2500, t = 0.011 s; (b) milligravity,ethylene ReD = 2500, t = 0.011 s; (c) normal gravity, propane ReD = 8500, t = 0.017 s; and (d) microgravity, propaneReD = 8500, t = 0.017 s.

ene flames at a Reynolds number of 5000, and itappears that there are greater differences in the tur-bulent structure than at the higher Reynolds number,which is consistent with the larger difference in L.Figs. 5c and 5d each show the instantaneous structureof propane flames under three different gravity con-ditions. The Reynolds numbers are 8500 and 5000for Figs. 5c and 5d, respectively, and the normal-,milli-, and microgravity flames are shown at left, cen-

ter, and right, respectively. It is clear from these fig-ures that the structure of the milli- and microgravityflames is very similar, which suggests that both theflames are approximately nonbuoyant. Since the Lvalues of these milligravity flames range from 23,this seems to suggest that the criterion for the devel-opment of momentum-dominated turbulent structuremay be larger than unity. This issue will be discussedfurther below.

390 C.A. Idicheria et al. / Combustion and Flame 138 (2004) 384400

e ReDand m

and (

toons of these differences in the luminous structure of

the normal- and low-gravity transitional flames. Thecartoons are meant to show an exaggerated view ofthe differences, but in reality, either type of flame canexhibit characteristics of the other; i.e., the buoyantflames can exhibit more axisymmetric structures orthe low-gravity flames can exhibit a sinuous struc-ture. Nevertheless, the differences discussed aboveare readily evident upon viewing the time sequencesand approximately describe the gross features of thetwo types of flames. Specific examples of these trendsin the instantaneous images can be seen by comparinghigh and low L flames in Figs. 4a and 4b, Figs. 5cand 5d, and the startup sequences in Figs. 2a and 2b.These differences in the structure also seem to have abearing on the flame tip dynamics as will be discussedbelow.

Mean and RMS luminosity images were computedfrom the time sequences, excluding the startup andshutdown transient frames. Sample mean luminos-ity images corresponding to the same conditions as

Fig. 6. Cartoon of the luminous flame structure of transi-tional flames in (a) normal gravity and (b) low gravity.

shown in Fig. 5 are presented in Fig. 7. Each setof images (i.e., those separated by vertical lines inFig. 7) is for the same fuel type and Reynolds number.These mean images are superior in showing some ofthe gross differences that characterize the flames un-der different conditions. For example, Fig. 7a showsthat the flame with L = 3.7 is very similar in itsmean structure to the one with L = 1. A carefulviewing of all of the images in Fig. 7 supports thisgeneral view that flames with L of order unity andbelow are essentially identical in their mean luminos-ity (flame height and width). As the L of the flamesFig. 5. Sample instantaneous luminosity images: (a) ethylen(right); (b) ethylene ReD = 5000, x/D = 43279 normal (left)normal (left), milligravity (center), and microgravity (right);ligravity (center), and microgravity (right).

Upon careful viewing of the instantaneous im-ages and movie sequences, a few generalizations canbe made about the luminous turbulent structures thatcharacterize the low-Reynolds-number flames whenthere is a large difference in the magnitude of L.For example, the most obvious trend seen in the tur-bulent structures of the transitional flames is that inlow gravity they are more axisymmetric, extend overa relatively small scale (e.g., about 12 luminous jetwidths), and exhibit a relatively regular spacing. Innormal gravity, the structure is similar to that of thelow-gravity case in the lower portion of the flame, butfarther downstream the flames tend to exhibit a large-scale sinuous structure whose wavelength is severaljet widths long. Fig. 6 shows highly simplified car-= 10,500, x/D = 43279, normal (left) and milligravityilligravity (right); (c) propane ReD = 8500, x/D = 76308,

d) propane ReD = 5000, x/D = 76308, normal (left), mil-

C.A. Idicheria et al. / Combustion and Flame 138 (2004) 384400 391

10,50igravipropa

Fig. 8. Variation of normalized flame length with Reynolds

number at different gravity levels.

becomes larger than about 3, the flames become pro-gressively thinner than their momentum-dominatedcounterparts.

Fig. 8 shows the variation of the mean visibleflame length (obtained from mean images) normal-ized by the tube exit diameter for all the cases studied.Precision uncertainty levels (95% confidence) com-puted from repeated runs are also shown. The confi-dence intervals are in the range of 4D to 35D forall three flames, with higher differences in the lower-Reynolds-number cases. It is evident from Fig. 8 thatthe ethylene and propane flame lengths exhibit vir-tually no difference with gravity level at the high-est Reynolds-numbers considered. Fig. 8 shows thatthe mean flame lengths of the ethylene jet flamesdiffer by at most 15% across the different grav-ity levels and over the full Reynolds-number range.

Reynolds number. Substantial differences in the meanluminous flame length between gravity conditionsare seen only in the low-Reynolds-number propaneflames (e.g., the low-gravity flame is longer than thenormal-gravity flame by 45D at ReD = 2500). How-ever, a majority of the low-gravity propane flamesappear to be longer than their normal-gravity coun-terparts by about 10%. The normal-gravity ethyleneflame length is seen to increase from L/D of ap-proximately 180 to 200 as the Reynolds number in-creases from 2500 to 10,500. Differences in flamelength between gravity conditions for ethylene areseen for Reynolds numbers less than 6000, as the ma-jority of the normal-gravity flames are longer than thelow-gravity flames. However, the flame lengths arevery similar for Reynolds number higher than 6000.As mentioned previously, the methane flames weretested under milligravity and normal-gravity condi-tions only. The methane flames at the two Reynoldsnumbers investigated are longer in milligravity thanFig. 7. Sample mean luminosity images: (a) ethylene ReD =ethylene ReD = 5000, x/D = 43279 normal (left), and mill(left), milligravity (center), and microgravity (right); and (d)(center), and microgravity (right).0, x/D = 43279, normal (left) and milligravity (right); (b)ty (right); (c) propane ReD = 8500, x/D = 76308, normalne ReD = 5000, x/D = 76308, normal (left), milligravity

For propane, the variation in length with gravity levelis 20% at the lowest Reynolds-number, whereas athigher Reynolds numbers the difference is less than10%. Only low-Reynolds-number methane flameswere tested because at higher Reynolds numbers theflames were lifted, which was not desirable for thepurposes of this study.

There are some trends in flame-length behavior fora particular fuel and across gravity conditions that areevident in Fig. 8. The normal-gravity propane flamesare seen to increase in their normalized mean lumi-nous flame length (L/D) from a value of approxi-mately 200 to 270 as the Reynolds number increasesfrom 2500 to 8500. The milligravity and micrograv-ity propane flames also show the same trends, in-creasing mean luminous flame length with increasing

392 C.A. Idicheria et al. / Combustion and Flame 138 (2004) 384400in normal gravity by approximately 12 and 5% atReynolds numbers of 2000 and 2500, respectively.

Fig. 8 also shows the data of Hegde et al. [6]for propane flames under normal and microgravityconditions. Their microgravity data and the currentlow-gravity data differ substantially over the entireReynolds-number range. The normal-gravity data ofboth studies, however, show better agreement, but stillare significantly different in magnitude and trend withReynolds number. The computation of visible flamelengths will depend on the definition of the length,and therefore absolute differences are not surprising;however, in contrast to the current findings, the dif-ferences in trend between normal and microgravityflames are seen to be very large even for Reynoldsnumbers greater than 4000. Note that similar differ-ences in the flame lengths between normal and mi-crogravity conditions were also observed in methaneand propylene jet flames [4].

The reason for the difference between the mea-surements of Hegde et al. [6] and the current studyis not known, but one other microgravity study [12]shows agreement with the current measurements.Page et al. [12] studied pulsed, turbulent nonpremixedethylene/oxygen-enriched-air jet flames in micro-gravity, but they also included flame length measure-ments for steady, unpulsed jet flames at a Reynoldsnumber of 5000. Their normal and microgravity flamelengths did not exhibit the large difference observedin Refs. [46], but the normal-gravity flame was ac-tually slightly longer than the microgravity one. Thisfinding is consistent with the current study where thenormal-gravity ethylene flame at ReD = 5000 wasalso observed to be slightly longer than the flame inthe milligravity case.

Despite the agreement of the current results withthose of Ref. [12], the difference from Bahadoriet al. [4] and Hegde et al. [5,6] could mean that thecurrent setup is generating anomalous results, evenat normal gravity. Therefore, to serve as validationof the current normal-gravity results, Fig. 9 showsnormal-gravity flame length data taken in the currentstudy plotted together with the data of Becker and Ya-mazaki [1] and Mungal et al. [9]. It can be seen thatthe present data agree quite well both in trend and invalue with previously published work for the samefuel and the same range of Reynolds numbers.

To give further confidence in the reliability ofthe normal-gravity results, a series of tests were per-formed to see if the current normal-gravity flameswere sensitive to the particular setup used. First of all,tests were conducted at normal gravity to see if non-piloted lifted propane flames differed substantially intheir length from the piloted attached flames. The dif-ferences seen in the flame heights for these conditionswere small. Second, tests were conducted with theFig. 9. Comparison of current normal-gravity flame lengthdata with other published data.

burner in various configurations; specifically, normal-gravity tests were conducted with the burner insideand outside the drop rig and with and without the pi-lot flame housing. In all of these tests, the differencein observed flames lengths was small. This series oftests showed that the current normal-gravity flameswere not highly sensitive to how they were generated.

It seems likely, therefore, that the reason for theobserved differences among the various microgravitystudies is that transitional low-gravity flames are par-ticularly sensitive to the boundary conditions underwhich they develop. The reason for this proposed in-creased sensitivity is that under normal gravity condi-tions, buoyancy-induced fluctuations are the primarymechanism that triggers the transition to turbulence.In microgravity, this source of disturbances is re-moved and therefore it leaves the flame sensitive toother, possibly much weaker, disturbances. In otherwords, a microgravity flame can be sensitive to theexact nature of the boundary conditionseven whenthe same flame under normal gravity would not bebecause the disturbances under normal gravity wouldbe dominated by buoyancy. Under this argument, thereason for the increased flame lengths of Refs. [46]is that they exhibit an extended laminar or transitionalregion as compared to the current study and Ref. [12].The effect of buoyancy on the transition to turbu-lence is well known in laminar flames. For example,flames that are completely laminar in microgravity(e.g., [13]) can be highly wrinkled and turbulent innormal gravity owing to buoyancy-induced vorticity.In fact, a major advantage of the microgravity envi-ronment is that it enables one to study low-strain-ratelaminar flames that would be dominated by buoyantinstabilities in normal gravity.

If this argument is correct, then it would be ex-pected that flame length data obtained in microgravity

C.A. Idicheria et al. / Combustion and Flame 138 (2004) 384400 393

= 10,d miland (

30.24 m ). In general, the enclosure around a flamecan have an effect because it allows recirculation ofproducts into the oxidizer stream, and therefore canchange the overall stoichiometry and density ratio.However, Refs. [47] state that the flame lengths werethe same irrespective of the run time of the flamethey investigated, and so it appears that confinementwas not an issue. Another important configuration dif-ference across the experiments is geometry near thejet exit. In the current study, the flames were pilotedwith a 25-mm concentric laminar premixed flame,but otherwise the jet-exit region was unobstructed.In Refs. [47], the flame was unpiloted, and a baseplate was used that was located 5 to 10 mm below thenozzle exit. According to the authors of Refs. [47],the presence of this plate could have impeded the en-trained air near the nozzle exit, and this could have ledto lift-off and blowout at moderately low Reynoldsnumber. With regard to the experimental configura-tion used in Ref. [12], the flames were enclosed and

different, it could be due to differences in the sootproperties, temperature or the underlying fluid me-chanics. Nevertheless, the RMS fluctuations can pro-vide useful information because we are interested indetecting differences in the normal- and low-gravityflames, regardless of the underlying mechanism. TheRMS luminosity is useful toward this end becauseit provides a more sensitive measure of the poten-tial differences (as do all higher order statistics) thanthe mean luminosity. Fig. 10a shows RMS images forthe ethylene flames at ReD = 10,500 in normal andmilligravity. In these images black corresponds to amaximum and white corresponds to a minimum RMSvalue. The flames have noticeable similarities, butclear differences are also apparent, such as the lowerpeak RMS values on the centerline of the L = 1.0flame. Furthermore, more drastic differences can beseen when flames with a larger difference in L are(Fig. 10b). Figs. 10c and 10d compare the RMS lu-minosity for the propane flames (ReD = 8500 andFig. 10. Sample RMS luminosity images: (a) ethylene ReD(b) ethylene ReD = 5000, x/D = 43279 normal (left), annormal (left), milligravity (center), and microgravity (right);ligravity (center), and microgravity (right).

would exhibit much more scatter than equivalent dataobtained in normal gravity. For example, whether ajet flame is piloted or not, or enclosed or free, mayhave a greater impact on the flow development underlow-gravity conditions; therefore, slight differencesin the flow configuration between the current work,Hegde et al. [5,6], and Page et al. [12] may lead tolarge differences in the flame heights observed underlow-gravity conditions. Given this possibility, the dif-ferences between the experimental configuration ofthe current study and those of Refs. [47] and [12]are documented below. The jet flames in Refs. [47]were unpiloted and enclosed in a cylindrical chamber(with a volume of 0.087 m3), whereas in the cur-rent study the jet flame issued into the quiescent airinside the drop-rig (with an unoccupied volume of500, x/D = 43279, normal (left), and milligravity (right);ligravity (right); (c) propane ReD = 8500, x/D = 76308,d) propane ReD = 5000, x/D = 76308, normal (left), mil-

stabilized by an igniter (which was always present)and issued into a weak co-flow. It is not known if thesedifferences are those that are most responsible for thedifferences in the flame lengths, but the fact that suchdifferences in geometry are present give future re-searchers specific issues to consider when designingexperiments that will be used to study transitional mi-crogravity jet flames.

The RMS fluctuations of the flame luminosity timesequences were computed to determine if the trendsthat are observed in the mean images are also seen influctuating quantities. It is well known that soot lu-minosity depends on many factors and so cannot berelated in a simple manner to a particular property ofthe soot, such as the soot volume fraction. As a con-sequence, if the RMS fluctuations for two cases are

394 C.A. Idicheria et al. / Combustion and Flame 138 (2004) 3844005000). In Fig. 10c it is seen that the fluctuations arenearly identical for the L = 2.1 and L = 0.38 cases;however, both differ substantially from the L = 7.9case. A similar trend is observed for the image setof Fig. 10d. Interestingly, regardless of fuel type orReynolds number, the low-L flames all have qual-itatively similar RMS contours; i.e., the fluctuationspeak near the periphery of the flame and remain loweven at the flame tip. This observation is consis-tent with expectations of a momentum-dominated jetwhere the largest scalar fluctuations occur at the outeredges of the jet where the intermittency is largest [14].

4.2. Flame tip dynamics

The time-resolved data, such as shown in Fig. 4,enable us to investigate fluctuations in the instanta-neous flame length and the dynamics of the flame tipburnout process. Careful examination of the imagesreveals that in normal gravity (or more correctly, highL), the flame structure elongates near the flame tipand often tears away from the main body and thenburns out. Examples of this feature can be seen in thethird and ninth images from the left in Fig. 4a and thefourth and tenth images from the left in Fig. 4c. Thelow-gravity image sequences (Figs. 4b and 4d) indi-cate different flame tip behavior because the struc-tures in low-gravity conditions are more compact andthicker near the flame tip and the tearing of flamestructures from the flame body is not as common. Inlow gravity, the luminous structures more typicallyconvect downstream and burn out as a whole.

The characteristics of the flame tip fluctuationscan also be seen by considering the time histories ofthe instantaneous luminous flame length as shownin Fig. 11. These data are for propane flames atvarying Reynolds number and were generated bycomputing the instantaneous luminous flame length.It is expected that the flame tip fluctuation fre-

Fig. 11. Instantaneous flame tip location for propane flamesat various L.quency will scale with the local large-scale time-scale /Uc (with the local width and Uc thecenterline velocity) [9,15], but in the current studythe local velocity is not known for all conditions.Since /Uc x2/(U0D) (D/U0)(x/D)2, then/Uc (D/U0)(L/D)2 for a turbulent momentum-dominated flame of length L. For the same fuel (andhence stoichiometry), then L/D will be nearly con-stant and the large-scale time (/Uc) will scale asD/U0; therefore, the time axis has been scaled by thecharacteristic time scale D/U0. This scaling shouldbe sufficient for removing the effect of differences inthe local convection velocity on the flame tip fluctua-tions for flames that are momentum-dominated and ofthe same fuel type. These plots show that the flame tipfluctuations are very similar for L values of 2.8 andbelow, which indicates that the fluctuations are asso-ciated with the same type of large-scale motions inall of the momentum-dominated cases. The L = 7.9case seems to exhibit higher frequency fluctuations,and this is clearly the case at L = 10.1 also. Sincethe time-scale normalization used does not accountfor buoyant acceleration, these higher-frequency fluc-tuations are clear evidence of the effect of buoy-ancy on the flame tip dynamics. Careful inspectionof Fig. 11 reveals some interesting trends in the na-ture of the flame tip fluctuations. For example, at thelower values of L the flame-tip time histories exhibitramp-like characteristic, whereby the flame lengthgradually increases and then abruptly decreases. Sim-ilar ramp-like oscillations in the flame length wereobserved in Ref. [9] and in the liquid-phase, acid-base flames in Ref. [15]. The liquid-phase flameswere purely momentum-driven, and they exhibiteda particularly high degree of quasi-periodicity [15].The movie sequences acquired in the current studyshow that the ramp-like behavior is associated withthe flame tip burnout characteristics. In particular, themovies show that for momentum-dominated flames,a large-scale luminous structure will form near theflame tip, travel downstream, and then the entirestructure will burn out in a relatively uniform man-ner. It is the burnout of the entire structure that causesthe flame length to abruptly decrease. In Ref. [15] itis argued that the rapid burnout of the flame tip struc-ture indicates that the entire structure is mixed to arelatively uniform composition. In some cases, theflame tip seems to burnout starting from its upstreamedge, which was also observed in liquid flames [15].In Ref. [15], this upstream-to-downstream mode ofburnout was attributed to the entrainment motions,which sweep ambient fluid into the structure from theupstream side and so it is this side that reaches stoi-chiometric proportions first.

Although ramp-like structures can at times be seenin the high L traces of Fig. 11, they are not as

C.A. Idicheria et al. / Combustion and Flame 138 (2004) 384400 395dominant as at lower L. At L = 10.1, the struc-tures in the time traces are jagged, but more sym-metric than at lower L. This apparent difference inthe ramped structures seems to suggest that the highL flames also deviate from the mode of burnout de-scribed above. In particular, the large-scale structuresnear the flame tip are stretched out by the buoyancyforces into the sinuous structures described above,and apparently the entrainment motions create lessuniformly mixed structures that burnout more gradu-ally. In addition to the difference in the ramp-like timetraces, careful observation of the movie sequences in-dicates that the luminous structures at the flame tip inthe momentum-dominated flames seem to be more or-ganized, or coherent, than the ones that exhibit strongbuoyancy effects. The more regular flame length fluc-tuations in low-gravity seem to be related to the moreregularly spaced structures as illustrated in Fig. 6. Theflame tip fluctuations shown in Fig. 11 also suggesta lower degree of organization with increasing buoy-ancy, since the fluctuations seem to be more random athigh L. The observation that the liquid-phase flames,which are momentum-dominated, exhibit a high de-gree of periodicity, even at higher Reynolds numbers,seems to add support to this hypothesis. This issuewill be discussed further below, but it should be notedthat in Ref. [9] it was remarked that the flame tipfluctuations seemed to be organized across the samerange of L as considered here. In fact, it seems thattheir low and high L cases all exhibit the ramp-likeburnout characteristics and arguably exhibit the samedegree of organization. Since their data were taken athigher Reynolds numbers than in the current study itis possible that this is the reason for the apparent dis-crepancy.

4.3. Volume rendering

Volume rendering of jet flame image sequenceswas used to investigate further the characteristics ofthe large-scale luminous structures. In this image-processing technique, discussed in Ref. [9], the two-dimensional (x, y) images are stacked along the timeaxis (t ) as shown in Fig. 12. A three-dimensionalvolume (x, y, t ) of the jet flame edge is then gener-ated using image processing. This rendered volumeenables qualitative and quantitative comparisons offeatures such as large-scale structure evolution andcelerity. The celerity is the absolute velocity of a lu-minous structure measured in the laboratory frame ofreference and is not necessarily a convection velocity,because a luminous structure can theoretically havea different speed than the local flow velocity. A sim-ulated light source, usually to the left of the stackedimages, provides illumination of the rendered surfaceand shadowing for depth perception. The advantageFig. 12. Illustration of volume-rendering technique.

of the volume rendering technique is that the large-scale structuresvisualized as wrinkles or bands inthe renderingscan be readily tracked over their en-tire lifetimes. The slope of each band in the volumerendering is equal to the celerity of the luminousstructure. In these renderings, higher celerity struc-tures will exhibit bands that have larger slopes. In thecurrent study, the renderings were computed using aPentium III machine equipped with 1 GB of RAM anda commercial software package called SlicerDicer.

Using this technique, Mungal et al. [9] found thecelerity of luminous structures to be about 12% ofthe jet exit velocity irrespective of the buoyancy para-meter (up to L = 9) and fuel type. This observationthat the celerity is constant is intriguing because thefluid velocities decay with downstream distance, andit might be expected that the luminous structures ve-locities should decrease also. Mungal et al. [9] sug-gest the reason for the constant celerity is that thestoichiometric mixture fraction surface, on which theflame resides, is similar in shape to a constant velocitysurface, and so the luminous structures remain asso-ciated with nearly constant velocity fluid.

Sample renderings for ethylene and propane areshown in Fig. 13. The renderings (Figs. 13a13d) areshown from the side view and so the y-direction isinto the page. The wrinkles represent luminous struc-tures that travel up the flame with increasing time. Thefaster the structures move downstream, the larger willbe the slope of the wrinkles. The flame length varia-tions are seen by the spiky top surface of the render-ings. Figs. 13a and 13b show the rendering of ethyl-ene flames at ReD = 2500 for L values of 8.5 and 2.5.Fig. 13b shows the entire duration of the 1.25-s drop,including startup (t = 0) and impact. The impact ofthe drop rig into the deceleration system is marked bythe time when the flame length becomes very large.The movie sequences show this large flame length isassociated with the creation of a large super-buoyant,mushroom-like flame that is generated by the 1530 gdeceleration.

A comparison of Figs. 13a and 13b shows thatthere are significant differences between the two

396 C.A. Idicheria et al. / Combustion and Flame 138 (2004) 384400Fig. 13. Sample volume renderings: (a) ethylene, ReD = 2500, normal gravity (L = 8.5); (b) ethylene, ReD = 2500, milligravity(L = 2.5); (c) ethylene, ReD = 7500, normal gravity (L = 4.6); (d) ethylene, ReD = 7500, milligravity (L = 1.2); (e) propane,ReD = 5000, normal gravity (L = 10.1); (f) propane, ReD = 5000, milligravity (L = 2.8); and (g) propane, ReD = 5000,microgravity (L = 0.49).

cases. It can be clearly seen that the flame tip fluc-tuates at a higher frequency in normal-gravity than inmilligravity. Also, the wrinkles in the normal-gravitycase have higher slopes than those for the milligrav-ity case implying higher celerities in normal-gravitythan in milligravity. Renderings for a higher Reynoldsnumber of 7500 are presented in Figs. 13c (L = 4.6)and 13d (L = 1.2). The large differences seen at thelower Reynolds number are not readily apparent inthese renderings, and the superbuoyant flame is lessprominent in the milligravity case; however, subtledifferences in the flame tip oscillation frequencies arestill visible on careful viewing.

Figs. 13e13g show renderings for propane at aReynolds number of 5000 at three different gravitylevels, rotated by 25 about the y-axis. Owing to thehigh density of propane, at this Reynolds number, thejet exit velocity is relatively low and so these flamestake longer to reach a steady state in low-gravity con-ditions. Fig. 13f shows that this relatively long startuptransient is seen to take up about one-third of thedrop time. Comparing the slopes of the bands be-tween Figs. 13e13g, it is apparent that the buoy-ancy parameter has a dominant effect on the lumi-nous structure celerities for the propane flames also.The normal-gravity case (Fig. 13e) exhibits wrinkles

C.A. Idicheria et al. / Combustion and Flame 138 (2004) 384400 397that seem to have a finer spacing and which exhibitlarger slopes than the milligravity and microgravitycases (Figs. 13f and 13g). The similarity in slopes be-tween Figs. 13f and 13g indicate the negligible effectof buoyancy when L changes from 2.8 to 0.49, butthe time at which the flow is stationary is so short thatthe L = 2.8 (milligravity) case is not very convincingin this regard.

The nearly constant slope of the wrinkles inall of the renderings indicates that the structuresmove downstream at approximately a constant ve-locity, in agreement with previous observations in jetflames [9]. Occasional pairing of the structures canalso be seen as a coalescence of the wrinkles in therenderings. Although this might not be readily appar-ent to the reader, after looking at many such render-ings, and after watching the movies, we can concludethat the pairing of the structures is more dominant inthe strongly buoyant cases. In other words, the lumi-nous structures in the momentum-dominated flamesseem to have longer lifetimes, or to maintain theiridentity longer, than in the buoyant flames. Perhapsa related observation is that the difference in the na-ture of the flame tip fluctuations, as discussed above,can also be seen in these renderings. For example, acomparison of Figs. 13e and 13g shows that the vari-ations in the flame length at normal gravity appear tobe much larger than in microgravity.

4.4. Celerity measurements

Fig. 14a shows a plot of the ratio of the luminousstructure celerity to jet exit velocity as a percentage,Us/U0 (%), versus the buoyancy parameter, L. Thenormal-gravity flames (high L values) are associ-ated with higher celerity, which can be attributed tothe buoyant acceleration. This suggests that luminousstructure celerity is in fact buoyancy dependent, con-trary to the findings of Mungal et al. [9]. It shouldbe noted, however, that since Mungal et al. [9] stud-ied higher Reynolds number jet flames, it is possiblethat the disagreement is due to a Reynolds numbereffect. For L values less than about 6, the celerityis independent of the gravity level and fuel type. Inthis regime, there is reasonable agreement with thefindings of Mungal et al. [9]. The bars shown oneach data point represent the standard deviation ofthe celerities and therefore quantify the variation ofmeasured values. It is interesting to note that high-L cases have higher deviations, which imply thatthe structures have a wider distribution of celerity.However, the deviations become smaller with de-creasing L which suggests greater organization (orrepeatability) of the structure celerity. This conclu-sion of greater organization is consistent with thelack of merging of the luminous structures described(a)

(b)

Fig. 14. Celerity of large-scale structures as a percentage ofthe jet exit velocity: (a) linear plot; (b) loglog plot.

above, and the more regular fluctuations of the flametip that were observed under low-gravity conditions.This observation of a higher degree of organizationfor momentum-dominated flames is a new one, be-cause it is usually assumed that buoyancy increasesthe large-scale organization of turbulent flames (e.g.,the large billowing structures observed in oil-well orpool fires) [16]. Although the pure buoyancy-drivenlimit may indeed exhibit strong organization, it ap-pears that the first effect of buoyancy is to reduce theorganization by disrupting the hydrodynamic instabil-ity of the momentum-dominated jet.

The loglog plot (Fig. 14b) shows that for L > 8,the celerity values are consistent with a 3/2L scalinglaw. We can derive this result from a simplified mo-mentum equation analysis. It is assumed that if thestructure follows the local velocity at the stoichio-metric contour then the celerity should be equal to

398 C.A. Idicheria et al. / Combustion and Flame 138 (2004) 384400Fig. 15. Schematic diagram of the control volume used inthe celerity scaling analysis.

the local centerline fluid velocity at the stoichiometricflame length. Becker and Yamazaki [1] use a quasi-1-D momentum analysis to show that in the buoyancy-dominated limit the entrainment rate scales as 3/2x .We use this same procedure to show how the localvelocity scales at the flame tip under these same con-ditions. Consider the simplified geometry and controlvolume of a jet flame issuing into quiescent ambientfluid as shown in Fig. 15. Let the jet fuel of density0 exit the nozzle into the ambient of density from a tube of diameter D with a velocity U0 anda mass flow rate of m0. Assume the jet flame to bean inverted cone of width and height x, and thatthe density at each x-location can be approximated asan appropriate average density of f (i.e., a mixing-cup density [1]). Furthermore, the jet entrains ambi-ent fluid with a mass flow rate me, but assume thatthis entrained fluid has no initial momentum in the x-direction and so it does not contribute to the momen-tum balance. Owing to the presence of heat release thejet will experience a buoyancy force, FB, as shown inthe schematic. At a particular downstream location x,let the mass flow rate and velocity be given by m(x)and Uc(x), respectively. Also, to further simplify theproblem, assume that the downstream velocity profileis a constant and denote it as Uc as shown in Fig. 15.Applying the momentum principle in the x directiongives

(1)m0U0 + FB m(x)Uc = 0.Now consider the case where the flame is buoyancy-dominated, in which case the buoyancy-induced mo-mentum is much larger than the initial source momen-tum. Following these assumptions, Eq. (1) reduces to

(2)FB = m(x)Uc.The buoyancy force that is exerted on the flame, mod-eled as an inverted cone as discussed above, is

(3)FB 1122xg

( f

).

Owing to the reduced density in a flame, we have( f) and (3) simplifies to

(4)FB 1122xg.

The momentum at the downstream location, x, is ap-proximated as follows:

(5)m(x)Uc fU2c 2

4and substituting Eqs. (4) and (5) in Eq. (2) gives

(6)112

2xg fU2c 2

4.

Note that Eq. (6) is not a function of the source con-ditions (U0 or Ds) because the source momentumwas assumed to be negligible. However, because thecelerity is normalized by U0, we introduce the sourceparameters into Eq. (6) to obtain the relation for thenormalized centerline velocity,

(7)(

Uc

U0

)2(

gDs

U20

)(x

Ds

)(f

).

Now, Ris gDs/U20 and x (Ris)1/3(x/Ds) and,hence, Ris = 3x (x/Ds)3. Using these relations inEq. (7) yields

(8)UcU0

3/2x(

x

Ds

)1(f

)1/2.

Following the nomenclature of Tacina and Dahm [17],we define a modified source diameter, D+, which likeDs in nonreacting jets, is able to collapse velocityand mixture fraction decay data in turbulent flames.For our purposes, we define D+ Ds(/f)1/2,which differs somewhat from that of [17] and wasused because it was found to work better for scaling-mixture-fraction data measured in the current facil-ity [18]. With this definition of D+ we can write

(9)UcU0

3/2x(

x

D+)1

.

To obtain a scaling in terms of the flame length para-meters, x is replaced with L in Eq. (9). Furthermore, itis assumed that the celerity (Us) will scale with the lo-cal centerline velocity (Uc) and therefore at the flame

C.A. Idicheria et al. / Combustion and Flame 138 (2004) 384400 399tip we have

(10)UsU0

3/2L(

L

D+)1

3/2L .

Equation (10) shows that the normalized celeritynear the flame tip will approximately scale as 3/2Lprovided the flame is buoyancy-dominated. Fig. 14shows the celerity data plotted with a line that followsthe 3/2L scaling. It is seen that this scaling seems tobe appropriate for L > 8 or so. Note that Eq. (10)suggests that the celerity will depend on L/D+ andL but the effect of the former term will be small inFig. 14 if L/D+ is approximately constant. Specif-ically, the L/D+ value for the flames in the currentstudy were measured to be approximately 90 [18].This suggests that the normalized celerity will be afunction of L only.

A similar analysis can be used to explore thescaling of celerity at the momentum-dominated limit(L 0). The normalized centerline velocity of amomentum dominated jet flame is found to scaleas [17]

(11)UcU0

(

x

D+)1

.

At the flame tip it is again assumed that the celer-ity scales with the centerline velocity and hence fora momentum-dominated flame

(12)UsU0

(

L

D+)1

.

Equation (12) shows that the normalized celerity is(obviously) independent of L and will have a con-stant value if L/D+ is constant. Fig. 14 shows rela-tively good agreement with this scaling law becausethe celerities are independent of L for L < 6, andseem to exhibit similar values over this same rangeof L.

The analysis above shows that the celerity seemsto scale with the local mean velocity, but whether ithas the same value as the local mean velocity is an-other issue. To explore this further consider the mea-sured centerline velocity decay in a turbulent nonre-acting jet [14], which is given by

(13)UcU0

= 6.2(

x

Ds

)1.

Assuming that the velocity decay in a momentum-dominated reacting jet can be obtained by substitutingDs with D+ [17] in (13) gives

(14)UcU0

= 6.2(

x

D+)1

.Furthermore, assuming the celerity is the same as thecenterline velocity at the flame tip and that the nor-malized flame length L/D+ is approximately 90 [18],Eq. (14) will predict a constant normalized celerity(Us/U0) of approximately 7%. Fig. 14 shows thatthe mean celerities measured in this study range fromabout 8 to 18% at the low-L limit, and those ofRef. [9] were measured to be 12%. Both of these stud-ies, therefore, suggest that the luminous structurestravel faster than the local mean fluid velocity. Thereason why the celerity is different from the local fluidvelocity is not known but it is possible that the lumi-nous structures exhibit a wave-like behavior, with awave speed that differs from the local fluid velocity.For example, consider an essentially steady laminarflame surface that is located in a region of low-speedflow, but which surrounds a column of fast-moving jetfluid. If a velocity perturbation were to be introducedinto the high-speed jet fluid, then this disturbancewould travel downstream at the local jet fluid velocity.As the disturbance moves downstream it would causea bulge in the laminar flame surface, which wouldhave the same velocity as the disturbance. The bulgein the flame surface would have a larger velocity thanthe local fluid velocity. We do not know if this discus-sion correctly describes the physics of the flow, but atleast it emphasizes the point that although the celeritymay scale with the local fluid velocity, there is reallyno obvious reason why it should be equal to it.

5. Conclusions

The characteristics of turbulent nonpremixed jetflames were studied at Reynolds numbers rangingfrom 2000 to 10,500 and at three levels of gravity,viz., 1 g, 20 mg, and 100 g. The flames were pi-loted with a small concentric premixed methaneairflame to keep them attached to the flame base for allReynolds numbers considered. Time-resolved (cine-matographic) imaging of the natural soot luminositywas used to investigate the mean and RMS luminos-ity, flame tip dynamics, and evolution of large-scalestructures. The relative importance of buoyancy overthe entire length of the flame was quantified with theBecker and Yamazaki [1] buoyancy parameter, L.

The mean flame luminosity data show that the nor-mal and low-gravity flames exhibited approximatelythe same flame lengths for all Reynolds numberstested. This result is different from some previousstudies in the literature that have shown large dif-ferences in flame lengths between normal and mi-crogravity flames. It is conjectured that the reasonfor this difference is that the microgravity flames inthe previous studies may have exhibited an extendedlaminar/transitional region owing to the absence of

400 C.A. Idicheria et al. / Combustion and Flame 138 (2004) 384400

turbulence-induced vortical perturbations. This em-phasizes the importance of documenting the boundaryconditions under which the flames develop when con-ducting microgravity studies. Furthermore, the meanand RMS luminosity, and flame tip fluctuations sug-gest that the structure of the large-scale turbulencereaches its momentum-driven asymptotic state forvalues of L less than about 23. Volume render-ings of image time-sequences show that the large-

sions with Dr. Uday Hegde regarding the effects ofboundary conditions on microgravity flames.

References

[1] H.A. Becker, S. Yamazaki, Combust. Flame 33 (1978)123149.

[2] H.A. Becker, S. Yamazaki, Proc. Combust. Inst. 16

scale luminous structure celerity depends on the valueof L. In particular, the celerity was found to be nearlyconstant for momentum-dominated flames (L < 6),but to scale as 3/2L in the buoyancy-dominated limit(L > 8). It is argued that the celerity should scalewith the local fluid velocity, although not necessar-ily be equal to it, and a simple momentum-equationanalysis supports this view. Taken as a whole, theresults of this study indicate that L is sufficient toquantify the effects of buoyancy on both the mean lu-minosity and different measures of the fluctuations,provided the flame is turbulent.

Another interesting finding of this work is thatthe visible flame tip time histories, volume render-ings, and movie sequences, support the view that theluminous structures of the jet flames are better orga-nized, or coherent, when the flames are momentum-dominated than when they are influenced by buoy-ancy. This result contradicts the view that buoyantinstabilities should cause the flame-structures to be-come more coherent. Although this latter view may betrue at the buoyancy-dominated limit, it appears thatas buoyancy effects first become nonnegligible, thebuoyant acceleration disrupts the KelvinHelmholtzinstability of the jet, and this causes reduced coher-ence of the turbulent structures.

Acknowledgments

This research was supported under cooperativeagreement NCC3-667 from the NASA MicrogravitySciences Division. We thank our technical monitor,Dr. Zeng-Guang Yuan of NCMR, for his hard work infacilitating the NASA GRC 2.2-s drop tower experi-ments. Furthermore, we acknowledge useful discus-(1977) 681.[3] H.A. Becker, D. Liang, Combust. Flame 32 (1978)

115137.[4] M.Y. Bahadori, D.P. Stocker, D.F. Vaughan, L. Zhou,

R.B. Edelman, Modern Developments in Energy, Com-bustion and Spectroscopy, Pergamon, Oxford, 1995,p. 49.

[5] U. Hegde, L. Zhou, M.Y. Bahadori, Combust. Sci.Technol. 102 (1994) 95100.

[6] U. Hegde, Z.G. Yuan, D.P. Stocker, M.Y. Bahadori, in:Proceedings of Fifth International Microgravity Com-bustion Workshop, 1999, p. 259.

[7] U. Hegde, Z.G. Yuan, D.P. Stocker, M.Y. Bahadori,AIAA Paper 2000-0697, 2000.

[8] M.G. Mungal, J.M. ONeil, Combust. Flame 78 (1989)377389.

[9] M.G. Mungal, P.S. Karasso, A. Lozano, Combust. Sci.Technol. 76 (1991) 165185.

[10] W.M. Roquemore, L.D. Chen, L.P. Goss, W.F. Lynn,Lecture Notes in Engineering, vol. 40, Springer-Verlag,Berlin/New York, 1989, p. 49.

[11] C.A. Idicheria, I.G. Boxx, N.T. Clemens, AIAA Paper2001-0628, 2001.

[12] K.L. Page, D.P. Stocker, U.G. Hegde, J.C. Hermanson,H. Johari, in: Proceedings of Third Joint Meeting of USSections of the Combustion Institute, 2003.

[13] S.-J. Chen, W.J.A. Dahm, Proc. Combust. Inst. 27(1998) 25792586.

[14] C.J. Chen, W. Rodi, in: C.J. Chen (Ed.), Vertical Tur-bulent Buoyant JetsA Review of Experimental Data,Pergamon, London, 1980.

[15] W.J.A. Dahm, P.E. Dimotakis, AIAA J. 25 (1987)12161223.

[16] E.E. Zukoski, B. Cetegen, T. Kubota, Proc. Combust.Inst. 20 (1984) 361366.

[17] K.M. Tacina, W.J.A. Dahm, J. Fluid Mech. 415 (2000)2344.

[18] C.A. Idicheria, I.G. Boxx, N.T. Clemens, in: Proceed-ings of the Spring 2004 Technical Meeting of the Cen-tral States Section of The Combustion Institute, 2004.

Characteristics of turbulent nonpremixed jet flames under normal- and low-gravity conditionsIntroductionExperimental programDrop-rig1.25-s drop tower2.2-s drop tower

Experimental conditionsResults and discussionInstantaneous, mean, and RMS luminosityFlame tip dynamicsVolume renderingCelerity measurements

ConclusionsAcknowledgmentsReferences