coupling between fluid dynamics and combustion in a laminar vortex ring by (shin-juh chen and...

7/28/2019 Coupling Between Fluid Dynamics and Combustion in a Laminar Vortex Ring by (Shin-Juh Chen and Werner J.A. D

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AIAA 2000-0433Coupling Between Fluid Dynamics and Combustionin a Laminar Vortex RingShin-Juh Chen and Werner J.A. DahmLaboratory for Turbulence and Combustion (LTC)Department of Aerospace EngineeringThe University of MichiganAnn Arbor, MIGretar TryggvasonLaboratory for Turbulence and Combustion (LTC)Department of Mechanical Engineering & Applied MechanicsThe University of MichiganAnn Arbor, Ml

38th Aerospace SciencesMeeting & Exhibit .1O-13 January 2000 / Reno, NVFor permission to copy or republish, contact t he American institute of Aeronautics and Astronautics1801 Alexander Bell Drive, Suite 500, Reston, VA 20191


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(c)2000 Americ an Insti tute of Aeronautics & Astronautics or published with permiss ion of author(s) and/or author(s) sponsoring organization.AIAA Paper No. 2000-0433 presented at the 38th AIAA Aerospace Sciences Meeting and Exhibit, January lo-13,2000, Reno, NV

Coupling Between Fluid Dynamics and Combustionin a Laminar Vortex RingShin-Juh Chenl and Werner J.A. DahmZLaboratory for lkrbulence and Combustion (LX)Department of Aerospace EngineeringThe University of MichiganAnn Arbor, Ml 48109-2140

Laboratory for Turbulence and Combusti on (LTC)Department of Mechanical Engineering & Applied MechanicsThe University of MichiganAnn Arbor, MI 48109-2121

Flame-vortex interactions are critical to the understanding of turbulent reacting flows. The impact of exothermicity onreacting vortex rings is investigated both numerically and experimentally to assess he dominant effect of heat release.Experimental observations of ring trajectories show an initial i ncrease in ring speed in the early stage, followed by alarge reduction in speed. It is found numerically that dilatation due to combustion heat release is the dominant effectover enhanc ed diffusivities in a reacting vortex ring. Increasing fuel volume in the ring beyond a critical limit,obtained from a simple model, actually dec reases he amount of heat release during the early stage of the interaction.In addition, the increase in ring circulation led to a decrease in ring speed in the early stage of formation. Nitrogendilution of the propane fuel r educes the flame luminosity and burnout time, as well as changes in details of theformation and dissipation of the luminous cap, with little change in the primary structure or dynamics of theinteraction. The numerical simulations were successful in explaining most of the experimental observations,however, differences in flame structure and ring dynamics attributed to radiative heat loss were inconclusive whenradiative heat loss was modeled as an overall decrease n flame temperature.

,

1. INTRODUCTIONFlame-vortex interactions are considered a central component ofturbulent combustion, and it is for this reason that various flame-vortex c onfigurations play a key role in combustion sc ience sincethe initial study of Marble ( 1985). Exothermicity due tocombustion heat release can dramatically alter some of the mostfundamental aspects found in turbulent flows. Studies of suchflame-vortex configurations are providing important newinsights into elementary transport a nd reaction processes, sincethese interactions contain many of the fundamental elements offlow, transport and combustion that are present in turbulentflames. Moreover, these elements can be investigated undermore controllable conditions than is possible in direct studies ofturbulent reacting flows.The study of diffusion flame-vortex ring interactions can beconducted experimentally in several ways, as depicted in Fig. 1.Strawa and Cantwell (1985), Vandsburger et al (1988), Chen etal (1991), Hsu et al (1993), and Takahashiet al (1996) havestudied these interactions by examining vortices generated in

1. Graduate Student; AIAA Student Member.2. Professor ; AK4 Senior Member; Corresponding author.3. Professor.Copyright 0 2000 by Shin-Juh Chen

pulsed or turbulentjet diffusion flames. In Fig. la, some of thefundamental aspects of these nteractions are not easily accessiblefor close study. Recent studies of Rolon et al (1995), andThCvenin et al (1996), and Renard et al (1999) have used theconfiguration of a countefflow diffusion flame with a vortex ringcoming from either the fuel or the oxidizer side as shown in Fig.lb. In this configuration, the vortex ring is already fully formedas it approaches the diffusion flame, and consists entirely of fuelor oxidizer.In this paper, the configurationshown in Fig. lc was used. Thisconfiguration has been studied, experimentally undermicrogravity conditions by Chen & Dahm (1997, 1998,1999a,b), and under normal gravity conditions by Park & Shin(1997) and You et al (1998). The present interactionis similar tothat of Karagozian & Manda (1986) and Manda & Karagozian(1988), where the ring consists of fuel and oxidizer, andcombustion occurs from the start of the ring formation process.This configuration can provide a strong coupling bet ween thevortex ring and diffusion flame. A diffusion flame initiallyexists at the oxidizer-fuel interfaces near the nozzle exit plane.Fuel is issued impulsively from the exit of the round nozzle androlls up with oxidizer and the diffusion flame to form a laminarreacting vortex ring. At the same time, heat release due tocombustion affects the vortex ring and the transport propertieswithin it. The interactionproceeds until the fuel in the ring hasbeen consumed.Here we present experimentalresults for the reacting vortex ri ng


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FUEL

OXIDIZER

FUEL OXIDIZER FUEL(4 (b) (cl

Figore 1. Contigurations for studying flame-vor tex nteractions. (a) Direct investigation of vortices generated n a turbulent diffusion flame. (b)Counterflow diffosion flame with a vortex ring filled only with fuel and nteracting with the diffusion flame. (c) Configuration used n this study with avortex ring composedof both fuel and oxidizer and wrapping the already ormed ditision flame at the nozzle exit.

in microgravity that reveal certai n aspects of heat release effects,as well as numerical simulations that clarify the dominantmechanisms fr om which these effects originate. Some ofexperimental results were presented in Chen & Dahm (1998).The numerical simulations were conducted with the assumptionsof unity Lewis and Schmidt numbers, and the Navier-Stokes andmixture fraction equations were solved to reveal the impact ofheat release in a vortex ring. The effects of heat release, fuelvolume, ri ng circulation, stoichiometry, and radiative heat losson t he resulting flame structure and dynamics are discussed toshed light on the coupling between flow field and combustion ina reacting laminar vortex ring.

2. METHODSIn this study, experiments were conducted under microgravity

Figure 2. Experiment drop package.

conditions at the NASA 2.2 seconds drop tower facility. Asshown in Fig. 2, all hardware must be contained inside a dropframe with dimensions of 40.6 cm x 96.5 cm x 83.8 cm andweight not exceeding 158 kg. Numerical simulations were alsoconducted to explain some of the experimentally observedphenomena and to assess the dominant effects of combustionheat release n a reacting vortex ring.2.1 Microgravity experimentsThe apparatus of the current study was used in the previousstudies of Chen & Dahm (1997,1998,1999a,b) and is shown inFig. 2. The top shelf of the drop package contains the 20 cm x20 cm x 25 cm test section made of clear Lucite, a 640 x 480CCD array which records the resulting luminosity of a reactingvortex ring with all frames shown in inverse grayscale in thispaper, and a spark igniter box for the generation of a diffusionflame. An axisymmetric circular nozzle and plenum are attachedbelow the top shelf for the generation of vortex rings. Thebottom shelf houses the controlling electronics, on-boardcomputer, batteries, pressurized fuel tank, a vacuum tank,flowmetering valves, a fast-acting solenoid valve, and thenecessary fittings and tubings.A vortex ring is generated by impulsively issuing fuel throughthe circular nozzle of diameter D = 2 cm i nto an air environmentwhich is held at atmospheric conditions inside the test section. Aschematic of the apparatus is shown in Chen & Dahm (1998).The experiment is conducted as follows. Initially, there is aclosed iris that s eparates the fuel and air interfaces as shown inFig. 3. The iris opens prior to the drop and exposes the fuel and

Figure 3. (Left) Cl osed iris. (Right) Open iris which exposes thecircular nozzle and the igniter and allows the formation of a diffusionlayer between he fuel and oxidizer nterfaces.2


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Drop No. 16 Drop No. 17 Drop No. 9

:

rDrop No. 22 DropNo.19 Drop No. 14

(4 (b) Cc)Figure 4. Symmetricand repeatable nteractions are accessibl e nder microgravi ty conditions.Circulations in (b) and (c) am respectively six times and eleven times that of (a).

air interfaces to form a combustible diffusion layer. Through theopening of a vacuum reservoir, the layer is drawn toward a sparkigniter to form a diffusion flame which flattened after a 30-40ms of drop time. Immediately after the drop, a solenoid valveopens and releases pressurized fuel into the plenum, and exitsthrough the circular nozzle to generate a vortex ring that wrapsthe already formed diffusion flame around itself (Karagoziau etal 1988; Southerland et al 1991). The resulting reacting vortexring moves away from the nozzle and bums until the fuelenclosed in the ring has depleted.The ring circulation I and ejected fuel volumeVF are determinedfrom hot-wire anemometry measurements of velocity at thecenter of the nozzle exit plane with the assumption of a top-hatvelocity profile. A typical fuel injection velocity programmeexhibits a sharp rise when the solenoid valve open s and a slowdecay when the valve closes. By v arying the fuel tank pressure,solenoid valve timing s equence, and flowmetering valve settings,different ring circulations and fuel volumes can be achieved toobtain a wide range of flame-vortex interactions.Fuels considered in this study include methane, ethane, propane,and nitrogen diluted fuels reacting with air, and are investigatedunder various hydrodynamic conditions.2.2 Numerical SimulationsThe axisymmetric, viscous, and compressible form of theNavier-Stokes equations were solved for zero gravity on astaggered 66 x 13 0 computational grid that spans the 8 cm x 16cm axisymmetric half-domain. The left boundary is theaxisymmetry, and outflow boundaries are on the right and top.The bottom boundary consists of a no-slip wall with a uniformfuel inflow through a l-cm radius nozzle over a specifiedduration.A one-step nfinitely fast reaction with unity Lewis and Schmidtnumbers was used. This allows the replacement of the speciesand energy equations with a single equation for the mixturefraction field, 5. Kinematic viscosity and scalar diffusivity weretaken to vary with temperature as T&)1.7, and molecular weight

was assumed to be a constant so that density varied as T(c)-].The advection terms were discretized using the third-orderaccurate QUICK scheme, and the diffusion terms with a second-order centered scheme. The time integration was performed byusing a first-order explicit scheme. A projection method wasused to solve the Navier-Stokes equations, with the pressurefield obtained from a Poisson equation by a multigrid method.Dilatation due to combustion heat release was represented by asource term in the Poisson equation, and was expres sed in termsof mixture fraction by rearranging the continuity and scalartransport equations along with the ideal gas equation of state(e.g., Baum et al 1990).The reaction of methane in air was considered in the numericalstudies. The stoichiometric mixture fraction and maximumtemperature were fixed at the methane-air values of 0.055 and2226 K, with fuel and ambient temperatures at 300 K.

3. RESULTS AND DISCUSSIONThe experimental study spans a wide range of hydrodynamicand chemical parameters. For a given nozzle diameter (2-cm inthis study), there is a finite amount of fuel that can be injectedinto the vortex ring before reaching the overfilled limit. Thislimit c orresponds to length-to-diameter ratio of 3.6-4.4 (Gharibet al 1998) which is based on an equivalent cylindrical volumeof fluid emanating from the nozzle, and corresponds to limitingfuel volume of 23-28 cm3. Even t hough the results from Gharibet al (1998) were for nonreacting vortex rings, the agreementwith the current experiment is quite good (Chen & Dahm,1999a,b). This study will focus only on non-overfilled cases.Under microgravity conditions where buoyancy is effectivelyeliminated, very repeatable and symmetric results c an beobtained. Figure 4 shows results from three different casestaken from six separate drops, and the images are time-matchedas closely as allowable. It is evident in these figures that highlysymmetric and repeatable flame-vortex interactions are

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Figure5. Pr opane-air ombustionor various uel volumeand i ng circulation.(a) l- = 125cm%, v, = 4 cc, 2, = 0 ms, tz = 33 ins, tj = 100ms, t4 = 133ms, r, = 167ms.(b) I- = 150cm%, V,= 9 cc, t, = 0 ms, tz = 67 ms, tj = 167 ms, t4 = 267 ms, ts = 367 ms.(c) r = 150 cm%, VT = 22 cc, t, = 0 ms, tz = 100 ms, tj = 533 ms, t4 = 1000 ms, ts = 1133ms.(d) r = 930 cm%, V,= 30 cc, tI = 0 ms, tr = 33 ms, tj = 67 ms, t, = 330 ms, tti= 430 ms.

accessible n the experimental studies.Figures 5 and 6 suggest that exothermicity due to combustionheat release has a pronounced impact on both the structure anddynamics of reacting vortex rings. The impact on the ringdynamics is clearly seen in Figs. 7 ,and 8 showing the timehistory of non-dimensional flame heights for several cases.Moreover, Fig. 7 suggests that fuel volume may also play a roleon the resultingri ng dynamics. In order to discern the dominanteffect of combustion heat release, numerical simulations wereconducted, this al low numerical studies that are not accessi ble inexperimental settings to be conducted and provide access to awider range of parameters to be investigated than is permissiblein experiments, but requires some modeling of certain physicalprocesses.3.1 Effect of Heat ReleaseTypical flame heights obtained from experiments are shown inFigs. 7 and 8 with the ring height and time hydrodynamicallyscaled as H* = H/D, and t* = tlYD2, where H is the visibleflame hei ghts for several different fuels, circulations, and fuel

volumes. In the case of nonreacting vortex rings, if the corestructure are similar, then one would expect the ring t rajectoriesto collapse to a single curve when plotted in thesehydrodynamically scaled variables, however, they do not for thetrajectories of reacting vortex rings. This suggests that heatrelease appears to have an impact on the reacting ring trajectories.For an inviscidring the resulting cons tant speed would produce astraight trajectory having a slope that depends weakly on thevortex core size and internal s tructure. As for a viscous ri ng,viscous diffusion increases the core size with time andprogressively reduces the ring speed similar to the trend seen inFigs. 7 and 8. However, no reasonable core size for a constantviscosity ranging from cold to hot gas values will replicate theexperimental observations. Even spatial and temporal variati onsin viscosity are insufficient to explain the trend in thesetrajectories. This strongly suggests hat a heat release effect otherthan the simple increase in viscosity appears to dominate thereacting vortex ring trajectories.Numerical simulations were conducted for methane-ai r reactionto examine the origins of these effects. Methane is considered

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,.~.. , ,, Z,,.. . . : :.. :: fb). . :. :!?+. .. ;,,.,: ;,,., :.;.:: ., .: I:.::.I$:. ., .,.~ .: ..y. I._: ...: ,.: .:;..

: . ..: ,.,t1 t t3 4 k

Figure. . Nitrogendiluted propane&combustion. a) Propane iluted o 40% with nitrogen, = 196 cm*/s,V, = 24 CC, , = 0 ms, tz = 67 ms, t3= 233 ms, tq= 3 67 ms, t5= 500ms. (b) Propane il uted o 20% withnitrogen, = 217 cm2/s,VF= 26 cc, t, = 0 ms, t2= 67 ms, tj = 133ms, td= 200 ms, tj = 300 ms.

here in order to remove the effect of radiative, heat oss du e to thepresence of soot; this subject is discussed below. The relativeimportance of heat release effects on diffusion and dilatation dueto reaction and heat conduction in the fluid are compared in Fig.9. The mixture fraction fields indicate that there are differencesbetween the nonreacting ring and the reacting ring with dilatationturned off. These differences are mainly enhanced moleculardiffusion which is responsible for the more diffused scalar field,and th.edecrease n ring speed n the later s tage of interaction du eto enhanced viscous diffusion in the reacting case with dilatationturned off. However, the largest differences are seen whencomparing to the reacting case with dilatation t urned on, wherethere is a large reduction in ring speed, and the scalar field hasconsiderably s pread radially apart. These sharp differences aretherefore attributable to volumetric expansion due to combustionheat release.

5 0.6z 1 -0.0 -#e0 5 10 15

+ (Fl, 260, 6.5)e (Fl, 160,4.4)- (F2,240,24.4)- (F3, 276, 10.4)

20 25 30 35Non-Dimensional Time, t*

Figure 7. Typical trajectories f reacting ortex rings ?omexperimentsunder microgravity. Legendgives fuel (F, = 100% C3Hs, F2 =4O%C,Hs+ 60%N2, and F, = 20%C,Hs + 80%N2), circulation r(cm*/s),and uel volumeJ$(cd).

By examining the trajectories of these three cases, Fig. 10 clearlyshows that dilatation is the dominant effect over enhanceddiffusivities when combustion heat release s present in methane-air reaction. There are no differences in the trajectories ofnonreacting case and reacting case with dilatation turned offduring the early stage of interaction, that is for t* < 8. Dilatationis responsible for the increase in ri ng speed in the early stage,and the sharp reduction in ring speed in the later stage for thereacting case with dilatation turned on. This is consistent withthe experimentally observed reduction in ring speed over time inFigs. 7 and 8, and with the numerical results in Fig. 17b ofHewett& Madnia (1998) from their Runs 1 and 5. While theirsimulations are of a somewhat different problem, comparingtheirRuns 3 and 5 in their Fig. 12~ further supports the presentobservation that dilatation is the dominant effect of heat release na reacting vortex ring.

-7cc--o-- 12.5 ccA22CC--o- 36 cc

0.0 1 I I I I0 10 20 30 40Non-Dimensional Time, t*

3

Figure 8. Methane-air xperimentalesults or the effect f fuel volumeV, on vortex ing rajectoryH*(t*). r = 50 0 cm2/s or all cases.

5


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t* = 2.5 t* = 6.25 t* = 12.5 t* = 25.0

(b)

;.

0 /D 3 :.:. e,,,;;.I ^. ..,., i .,_*,. ,,i ;.::.;;, ,r, :. gz-g-,, i:&&0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Figure 9. Simulation results showing mixture fraction fields for V,= 10.5 cc and I? = 100 cm% for(a) nonreac ting ring, (6) reacting ring without dilatation, and (c) r eacting ring with dilatation.6


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6.0 3.5,//

/fqzz- - - - Reacting (Dilatation Off)

//-/

/ ,*--*-

*-/ I-

,-/ _--/ ,-

- - - - - Reacting (Dilatation Off)- Reaction (Dilatation On)IO.O~

Non-Dimensional Time, t .

j, 3.0gf 2.5520ii i3.$ 1.5*sg 1.0Ez 0.5

0.030

Figure 10. Comparis on ofmethane-air simulati on results or combusti on Figure 11. Methane-air simulation esults showing effectof fuelvolumereleaseeffects on vortex ring trajectory H*(t*). V, = 10.5 cc, I? = 500 V, on vortex ring trajectoryH*(f *). r = 100 cm%, and $,,= 2226 Kcm%, and T,,,,= 2226 K in all cases. in all cases.

Besides the reduction in ring speed with time for any gi ven casein Fig. 8, there are differences in the trajectories that are notaccounted for by the hydrodynamic scalingH*(t*) alone. Theeffect of fuel volume can also have an impact on both thestructure and dynamics of reacting vortex rings. This effect isalso found to be associated with dilatation and is explained next.3.2 Effect of Fuel VolumeThe structural and dynamical change in a reacting vortex ring dueto fuel volume changes is seen in Fig. 5. In these experiments(Figs. 5a-c), the ring circulation is essentially kept constant, andby just varying the fuel volume the flame structure and dynamicsare affected considerably. In Fig. 5a, a distinctive vortex ringshape is observed only in the early time. The later stage s moresuggestive of a diffusion-limited burnout of the rich section onthe centerline of the vortex ring. The increase of fuel volume inFigs. 5b and 5c leads to the formation of a luminous bubble, anda ringlike structure can be seen in the burnout stage of Fig. 5c.During the early stage of Figs. 5b and 5c, there are no majordifferences in the ring speed, however in the later stage, a largedifference in ring speed can be observ ed because of a larger fuelvolume in Fig. 5c.An increase in fuel volume beyond the overfilled limit asmentioned above can lead to the formation of overfilled vortex3 ring, which displays azimuthal instability and formation ofsecondary and tertiary rings as seen in Fig. 5d. For the case ofnon-overfilled rings, the volume of fluid that rolled up during the$gE3rmation from the 2-cm diameter nozzle is approximately

From the experimental results of methane-air reaction shown inFig. 8, it is evident that fuel v olume also plays an important rolein the ring dynamics. With the ring circulation and all otherparameters essentially identical except for the fuel vol ume, onewould expect the trajectories to look similar, yet there is a cleartrend with fuel volume that suggests a further heat release effect.The combustion in a reacting vortex ring can be approximated as

-r

I-t0 I 1 , 1 1 I5 10 15 20 25 30

Non-Dimensional Time, t*

two phases. This has been suggested by experimental studies ofSoutherlandet al (1991). The first is where the fuel and oxidizer,which initially roll up together during the ring formation phase,bum rapidly to consume the lean reactant. The second phase iswhere any remaining fuel is c onsumed by the diffusion of therequired oxidizer into the ring. An increase in fuel volumeduring the initial phase will decrease the amount of oxidizer thatrolled up into the vortex ring. Depending on the stoichiometricrequirements, there is a critical ratio of V,/V, for which theoxidizer becomes the lean reactant. Beyond such limit, anincrease in fuel volume will lead to less heat been released,which in turn, will lead to a decrease n ring speed in the earlystage. For methane-air reaction the amount of fuel beyondwhich the oxidizer becomes the lean reactant is when V, = 5cm3. This is based on the 50 cm3 of fluid that is rolled upduring the ring formation for a 2-cm diameter nozzle. Thiseffect is clearly demonstrated in the experimental results ofmethane-air combustion in Fig. 8 where all cases exceed thestated critical fuel vol ume.A numerical simulation was also conducted to address he effectof fuel volume on the reactingring dynamics. Figure 11 showsthe effect of fuel volume on the ring trajectories of methane-airreactions for fuel volumes rangi ng from 5 to 21 cc. Consistentwith the experimental results of Fig. 8, the numerical results doshow that increasing fuel volume beyond the critical limit willlead to a reduction in ring speed in the early stage. On the otherhand, in the later stage the increase in fuel volume contributed toan increase in ring heights, and this is expected since there ismore fuel to be consumed in the larger fuel volume case.3.3 Effect of Ring CirculationIn a nonreacting vortex ri ng, an increase in ring circulation leadsto an increase in ring speed, but when plotted inhydrodynamically scaled variables all the trajectories shouldcollapse to a single curve, assuming similar core structures, sincethe ring circulation merely scales the characteristic time. Whilethe scaled trajectories will be independent of circulation, anincrease in ri ng circulation can also increase mixedness of fuel


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/-.------

- 100 cm%

0.0 , I0 10 20 30 40 50 INon-Dimensional Time, t*

60

Figure 12. Methane-air simulation results showing effect of ringcirculation l? on vortex ring tmjectorykP(r*). V, = 10.5 cc, and T,,,=2226 Kin all cases.

and oxidizer inside a vortex ring (Verzicco & Aguirre, 1995),and result in the thinning of the diffusion layer on the forwardstagnation point of t he vortex ring due to an increase in strainrate. However, as noted above, in a reacting vortex ringdilatation due to combustion heat release affects the ringtrajectory. With the increase in mixedness, one should expect tohave increased dilatation for larger ring circulations.A numerical study was conducted to determine the effect of ring

0.0 1 I 1 I I I0 5 10 15 20 25 :Non-Dimensional Time, t*

Figure 13. Methane-air simulati on results showing effectof T,, onvortex ring traj ectory H*(t*). V, = 10.5 cc and r = 100 cm% in allcases.

stoichiometry on the reacting vortex ri ngs to be examined byvarying the nitrogen dilution of the fuel. In the three cases, thering cir culation and fuel vol ume are kept essentially constant.Nitrogen dilution of the fuel has the effect /of moving thestoichiometric surface closer to the fuel side with little change inadiabatic flame temperature. For a given strained laminar flamewith thickness AD the separation between the stoichiometricsurface and the nominal air-fuel interface is given by n, / & =ef(1 -@)/(l+$ ).circulation on the reactirm vortex rines. Figure 12 shows thenumerical results of thregri ng circul&on &es with time andheight hydrodynamically scaled, and to a first orderapproximati on, the trajectori es do collapse to a single curve.However, the small differences i n ring speed seen in t* < 20indicate that an increase in ring circulation contributes to adecrease in ring speed. This is unexpected since moremixedness from an increase in ring circulation should i ncreasedilatation. For a fixed fuel volume, the increase in ringcirculati on is achieved by reducing the total fuel injecti on time.The higher rate of fuel injection in the higher ring circulation caseis similar to the effect of f uel volume as discussed above wherehigher fuel volume leads to a reduction in dilatation. As figure12 indicates, the lower ring circulationdoes show the result ofhigher dilatation as compared to the higher circulation cases,during the early phase of the interaction. A numerical study wasalso conducted to determine whether fuel volume plays a role. Itwas found that even for fuel volumes below t he critical value (5cc), the same trend was seen and this further suggests that therate of fuel injection is also of importance.From these numerical studies, it is found that enhancedmixedness due to an increase in ring circulation as observed innonreacting vortex ring appears not to contribute t o an increase ndilatation for the reacting vortex rings. Moreover, the smalldifferences seen in Fig. 12 are attributable to the change in therate of fuel injection resulting from a change n ring circulation.3.4 Effect of StoichiometryThe comparison of Figs. 5c and 6 allows the effect of

8

The flame structure in all three cases looked somewhat similar,and this is consistent with the results in Fig. 12 of Manda andKaragozian (1988) for their range off* between 0.1 and 0.35.The dominant effects of nitr ogen dilution of the fuel appear o bea reducti on in flame luminosity and flame burnout time. Figure4c of t he pure propane case shows a luminous region thatenvelops the whole ring and burns out from both the leading andtrailing ends. In the case of 60% nitrogen with 40% propane inFig. 6a, a bright luminous cap is present only very near theleading edge of the flame and dissipates toward the leading edge.As for the case of 20% propane with 80% nit rogen in Fig 6b, nobright luminous cap is formed, however, t he brightest region isstill located at the leading edge. The reduction in flame burnouttime as nitrogen dilution increases is most likely connected toless soot been available for the soot burnout phase which cannotbe differenti ated from t he recorded luminosity.3.5 Effect of Radiative Heat LossNumerical results of Fig. 13 and the experimental resul ts ofFigs. 14 and 15 were used to determine the effect of radiativeheat loss on reacting vortex rings. Comparison of Figs. 14 and15 suggests marked differences in both the burnout time andflame structure even though both cases have similar ringcirculation and fuel volume, except the fuel type. Oneexplanation is t he increased in radiative heat loss in Fig. 15 dueto the higher sooting propensity of propane which may beresponsible for the difference in flame shapes. The formation ofthe luminous cap proceeds in a similar way, but the capdissipates in a very differently in the late stage of interacti on.


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Figure 14. Flame-vortex nteraction for ethaneburning in air, r = 282 cm% and V, = 26 cc. Flame burnout time is 667 ms.

Figure 15. Flame-vortex nteraction for propane burning in air, r = 273 cm2/s and V, = 26 cc. Flame burnout time is 1233 ms.

Since dilatation is the dominant heat release effect in thisconfiguration, numerical results of Fig. 1 3 determine whether thedifference in flame shapes in Figs. 14 and 15 may be due toreduced dilatation from a difference in the radiative heat loss. Inthe numerical study, radiative heat loss is simply modeled as areduction in r(c), however, Fig. 13 shows that for a reasonablereduction in Tmax the effect on the ring trajectory is minimal.This suggests that while dilatation suffices to explain most of theheat release effects discussed here, other mechanisms may beresponsible for the differences seen n Figs. 14 and 15.

4. CONCLUSIONSFlame-vortex interactions provide a mean for s tudying many ofthe fundamental aspects on the coupling between flow andreaction found in turbulentreacting flows. The combined efforts

of experimental and numerical studiek have demonstrated thatdilatationis the dominant effect of combustion heat release; thisis also consistent with the numerical simulations of Hewett &Madnia (1998) of a somewhat different configuration.Fuel volumes play a role in determining the amount of heatrelease during the initial stage, and it was found that increasingfuel volume beyond a critical limit as determined bystoichiometric requirements, while k eeping the circulationconstant, can result in a reduction in heat release which in tarnleads to a decrease n ring speed. Experimental results of purepropane fuel showed a clear effect of the fuel volume on theflame structure and burnout time for rings that do not exceed theoverfilled limit.The increase in ring circulation with fuel volume k ept constantgives a reduction in ring speed during the early stage of

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interaction. Results from nonreacti ng vortex rings hinted to anincrease in dilatation from an enhanced mixedness dueincreasing circulation. However, the numerical results forreacting vortex rings showed that enhanced mixedness has aminimal effect on dilatation for the range of ring circulati onstudied here.The effect of ni trogen dilution of propane cases was a majorreduction in flame luminosity and burnout time. However theflame shape was not considerably affected by, the nitrogendilution, and this was in agreement with the analytical results ofKaragozian & Manda (1986) and Manda & Karagozian (1988)for their vortex pair case. The reduction in burnout time is mostlikely due to the presence of less soot as nitrogen dilution isincreased.Experimental results revealedthat radiative heat loss can affectthe flame structure and dynamics in a reacting vortex ri ng.Numerical studies that mimic radiative heat loss bv lowerinp: theadiabatic flame temperature did not reproduce the differencesseen between the experimental results of propane and ethanecases. This suggests that a mechanism other than the simplereduction in dilatation due to radiative heat oss is responsible forthe marked differences.

AcknowledgementsThis work is supported by the NASA Microgravity CombustionScience Program under ContractNos. NASA-G-NCC3-663 andNASA-G-NAG3-1639.

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