Transient local extinction and reignition behavior of diffusion flames affected by flame curvature and preferential diffusion

685

Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute, 1998/pp. 685–692

TRANSIENT LOCAL EXTINCTION AND REIGNITION BEHAVIOR OFDIFFUSION FLAMES AFFECTED BY FLAME CURVATURE AND

PREFERENTIAL DIFFUSION

KENJI YOSHIDA and TOSHIMI TAKAGIDepartment of Mechanophysics Engineering

Faculty of Engineering

Osaka University

Suita, Osaka, 565-0871, Japan

Experimental and numerical studies are made of transient H2/N2-air counterflow diffusion flames un-steadily strained by an impinging micro jet. Two-dimensional temperature measurements by the laserRayleigh scattering method and numerical computations taking into account detailed chemical kinetics areconducted, paying attention to transient local extinction and reignition in relation to the unsteadiness,flame curvature, and preferential diffusion effects. The results are as follows: (1) Transient local flameextinction is observed where the micro jet impinges. However, the transient flame can survive instanta-neously in spite of quite high stretch rate where the steady flame cannot exist. (2) Reignition is observedafter the local extinction due to the micro air jet impingement. The temperature after reignition becomessignificantly higher than that of the original flame. This high temperature is induced by the concentrationof H2 species due to the preferential diffusion in relation to the concave curvature. The predicted behaviorsof the local transient extinction and reignition are well confirmed by the experiments. (3) The reignitionis induced after the formation of combustible premixed gas mixture and the consequent flame propagation.(4) The reignition is hardly observed after the extinction by micro fuel jet impingement. This is due to thedilution of H2 species induced by the preferential diffusion in relation to the convex curvature. (5) Themaximum flame temperature cannot be rationalized by the stretch rate but changes widely, depending onthe unsteadiness and the flame curvature in relation with preferential diffusion.

Introduction

The turbulent diffusion flame is supposed to bean ensemble of the laminar flamelets [1] that shouldbe strained unsteadily by lateral velocity componentfrom the air side or fuel side. The strained laminardiffusion flame has attracted research attention be-cause the investigation of the fundamental processesin the flame is useful for understanding the micro-scopic structure of turbulent diffusion flames. Theflamelets in turbulent diffusion flame might bestrained unsteadily and have positive or negative cur-vature and be affected by preferential diffusion, es-pecially when the fuel includes species of signifi-cantly different diffusivity. The transient behavior ofthe strained flames affected by the flow field, flamecurvature, and preferential diffusion are the subjectsof great interest.

In recent years, interaction between a counterflowdiffusion flame and counter-rotating vortices fromthe air side was experimentally and numerically stud-ied, and the enhancement of the reactions or localquenching due to the interaction was pointed out[2]. Quenching limit and the unsteady behavior ofthe counterflow planar diffusion flame strained un-steadily by sinusoidal velocity oscillations were ex-

perimentally and numerically studied, and the ef-fects of the unsteadiness on the strain rate atextinction were investigated [3]. Interaction be-tween a laminar jet diffusion flame and a packet offluid impinged from outside or inside of the flamewas numerically studied and there have been dis-cussions about unsteady extinction mechanisms inrelation to diffusion, convection, and chemical ki-netics [4]. In the premixed flame, interaction of theline-vortex pair and flame was studied using thePLIF imaging of CH and OH to observe the timeevolution of a vortex–flame interaction [5], and theeffects of unsteady stretch on the premixed flame bya counter-propagating vortex were investigated usingthe PIV technique, and the response of the flame tothe positive or negative stretch was discussed [6]. Inturbulent jet diffusion flames, vortex and flame in-teractions were experimentally studied, and the pro-cess of the flame extinction and flame structure wasobserved [7]. Simultaneous two-dimensional tem-perature and velocity profiles in a turbulent diffusionflame were measured by the Rayleigh scatteringmethod and RIV (Rayleigh image velocimetry) toobserve the local flame extinction in relation to in-stantaneous stretch rate [8].

686 LAMINAR DIFFUSION FLAMES

Fig. 1. Schematic drawing of thecounterflow burner and optical mea-suring equipment.

In this paper, transient H2/N2-air laminar coun-terflow diffusion flames strained by an unsteadily im-pinging micro air or fuel jet from the air or fuel sideare studied. These flames have concave or convexcurvature, following the previous studies [9] of thesteadily strained diffusion flames. Two-dimensionaltime-dependent temperature measurements by thelaser Rayleigh scattering method show the localflame extinction and reignition process. Time-de-pendent numerical computations taking into accountdetailed chemical kinetics and multicomponent dif-fusion predict well the characteristics of the experi-mental evidence and reveal the processes dominat-ing the transient behavior of the flame with respectto the flame curvature and preferential diffusion.

Experimental Apparatus and Procedure

The schematic drawing of the counterflow burnerand optical measuring equipment is shown in Fig. 1[9]. The details are described in Ref. [9]. The burnerconsists of two opposed, vertically arranged circularducts of 43 mm i.d. The separation distance betweenthe duct exits is 30 mm. Fuel (15% H2 and 85% N2volume fraction) and air are introduced from eachduct separately. A micro jet nozzle of 0.26 mm i.d.and 0.51 mm o.d. is installed coaxially upward at thecenter of the lower duct. The tip of the micro nozzleis adjusted at 5.0 mm above the lower duct exit. Themicro air jet is driven from the pressurized air tankby opening the solenoid valve. The air impinges ver-tically to the flame, which gives the additional strainand curvature to the flame.

Two-dimensional temperature measurementswere made by the Rayleigh scattering method, asdescribed in detail in Ref. [9]. The Rayleigh scatter-ing from the gas molecules in the measuring volumewas collected by a lens and introduced to a CCDcamera with an image intensifier (II). The measuringarea is 34 mm wide in the laser light direction and11 mm high in the vertical direction.

To detect the Rayleigh scattering image at an ap-propriate time, the Nd:YAG pulse laser, the CCD

camera with the image intensifier, and the solenoidvalve are synchronously operated by using a functiongenerator, a counter circuit, and a digital delay. Thegood repeatability of the unsteady flame behaviorenables us to get the time sequence of the transientflame temperature profiles by changing the delaytime of detecting the Rayleigh scattering images af-ter opening the solenoid valve.

Experimental Results and Discussion

Figure 2 shows measured two-dimensional tem-perature profiles in time series for the flame withunsteadily impinging micro air jet to the flame. Tem-perature is indicated by color. The term t in Fig. 2is the time lapse after the solenoid valve opens. Thevertical (axial) position is denoted by z. The horizon-tal position of the figures corresponds to the radialposition of the flame.

The average micro air jet velocity ua,inj is selectedto be 11.1 m/s, which is the average velocity at theexit of the micro nozzle. The proceeding observa-tions are noted from Fig. 2.

At t 4 13.5 ms, the tip of the micro air jet reachesthe high-temperature region of the flame, but theflame still maintains high temperature near the im-pinging region of the micro air jet. This indicates thelocal flame extinction has not yet occurred. At t 414.0 ms, the micro air jet penetrates the flame, andlocal flame extinction is observed at the central partof the flame. With lapse of time, the high-tempera-ture region grows from the planar flame region alongthe side of the micro air jet. At t 4 17.0 ms, thetemperature increases at the tip of the micro air jet,which indicates the reignition. The temperature atthe reignition part is about 300 K higher than thatof the original flat flame.

Formulation and Numerical Procedure

Formulations are made for the computations ofthe transient axisymmetric laminar counterflow dif-fusion flames. The main part is the same with that

Fig. 2. Measured sequential temperature profiles for theflames with impinging micro air jet.

Fig. 3. Configuration of strained counterflow diffusionflame by impinging micro jet and computation domain.

of Ref. [9]. The fundamental equations consist ofconservation equations of mass, axial momentum, ra-dial momentum, species, and energy, as describedin Ref. [10]. The thermodynamic and transportproperties are obtained from the CHEMKIN data-base [11–13]. We take into consideration eight spe-cies of H2, O2, N2, H2O, O, H, OH, and HO2. Six-teen pairs of elementary reactions are used, whichare the same as those used in Ref. [14], except thatthe carbon-containing reactions are excluded fromthe reactions. They were successfully used for pre-dicting H2–N2 diffusion flames [15].

The conservation equations are discretized usingupwind difference of third-order precision for con-vection terms and central difference of second-orderprecision for diffusion terms. 160 2 80 meshes arelocated in the axial and radial direction of the flame,respectively. The mesh size is 0.125 mm in the axialand radial direction. The size of the computation do-main is z 4 0–20 mm in the axial direction and r 40–10 mm in the radial direction, as shown in Fig. 3.The computation algorithm is based on SIMPLERmethods [16].

The computation conditions of the present flames

are as follows. The velocities at the opposed ductexits of the air and fuel are both selected to be ini-tially 25 and 125 cm/s, which indicates the coun-terflow velocity of the air and fuel. The separationdistance of the opposed burner duct is the same withthose of experimental conditions. The inlet tem-perature of the fuel and air is 298 K under atmo-spheric pressure. Two typical cases are computed.They are the counterflow diffusion flames strainedunsteadily by an impinging micro air jet or fuel jetfrom the air side or fuel side. The micro air or fueljet nozzle exit of 0.25 mm inner diameter is locatedon the central axis 5 mm downstream from the airor fuel burner duct exit. The micro air or fuel jetvelocity, ua,inj or uf,inj is selected to be 11.1 m/s, re-spectively.

The boundary conditions of the computations areas follows. In the flame with the impinging micro airjet emanating from the air side, the position z 4 0mm in the computation domain corresponds to thefuel duct exit. The location z 4 20 mm correspondsto 10 mm downstream of the air duct exit and 5 mmdownstream from the tip of the micro air jet nozzle.At the boundary of z 4 0 mm, the axial velocitycomponent is set to be uniformly 25 cm/s, and theradial velocity component is set to be zero. The axialand radial velocity profiles at the boundary of z 420 mm were given as follows. The velocity profile 10mm downstream from the air duct exit was sepa-rately obtained by preliminary computations whenthe micro air jet of 11.1 m/s was surrounded by thecoflowing air of 25 cm/s from the air duct.

TRANSIENT BEHAVIOR OF STRAINED DIFFUSION FLAMES 687


Fig. 4. Predicted sequential tem-perature profiles for the flames withimpinging micro air jet.

In the flame with the impinging micro fuel jetfrom the fuel side, the boundary of z 4 0 mm inthe computation domain corresponds to 5 mmdownstream from the tip of the micro fuel jet nozzleexit and 10 mm downstream from the fuel duct exit.The boundary of z 4 20 mm corresponds to the airduct exit. The velocity profile at z 4 0 mm was givenas that of 5 mm downstream from the micro fuel jetnozzle exit, where the micro fuel jet flow is sur-rounded by the coflowing fuel of 25 cm/s. At theboundary of z 4 20 mm, the axial velocity compo-nent is set to be uniformly 125 cm/s, and the radialvelocity component is 0.

At the symmetric axis of r 4 0 mm, the radialvelocity component is set to be zero, and all of theother variables have zero gradient. At r 4 10 mm,the boundary condition of the velocity component isdetermined to fulfill the mass continuity equation,and all of the other variables have zero gradient. Weuse the values of all variables of the flat counterflow

diffusion flame without the impinging micro jet asthe initial condition and begin to introduce the microair or fuel jet impinging to the flat flame at time t 40 ms. The micro air or fuel jet velocity is increasedfrom zero to 11.1 m/s in a short time of 0.125 msand then kept constant at 11.1 m/s. We obtained theconverged solution in each time step by iteration toresolve the unsteady strained flame behavior.

Numerical Results and Discussion

Figures 4–7 show the computed results for theflame with the impinging micro air jet from the airside. The fuel and air come from the upper-side andlower-side boundary at z 4 0 and 20 mm, respec-tively. The time t denoted in the figures indicates thelapse time after starting the micro air jet injection.

Figure 4 shows temperature profiles at varioustime lapse. At t 4 2.0 ms, the width of the high-temperature region becomes narrower where the air


Fig. 5. Profiles of unreactednessfor the flames with impinging microair jet.

Fig. 6. H2 ratio contours for the flame with impingingmicro air jet at t 4 11.0 ms.

jet impinges. But the flame still maintains high tem-perature instantaneously at the region near the mi-cro air jet impingement. At t 4 2.5 ms, the tem-perature decreases to reach local flame extinction.With the lapse of time, at t 4 8.5 ms, the high-temperature region travels from the planar flame re-gion toward the tip of the micro air jet along the sidepart of the micro air jet. At t 4 9.5 and 11.0 ms, thetemperature at the tip of the micro air jet increasessignificantly, which indicates the reignition. Thetemperature at the tip of the jet becomes about 270K higher than that of the steady flat flame. This tran-sient behavior corresponds well to the experimentalresults.

Figure 5 shows the profiles of unreactedness at thetime before and after reignition for the same flame

of Fig. 4. The unreactedness is defined as the fol-lowing equation:

eqQ 1 Qunreactedness 4 (1)

eqQst

where Qeq is the heat release during the reactionsfrom the original fuel and oxidizer at local point ac-cording to the chemical equilibrium compositionand Q is the heat release during the reactions fromthe original fuel and oxidizer according to the pres-ent gas composition. is the heat release duringeqQst

the reactions from the stoichiometric mixture of theoriginal fuel and oxidizer according to the equilib-rium gas composition. The above-defined unreact-edness denotes the degree of the nonequilibrium orthe premixedness of the unburned fuel and oxidizer.It is noted that at t 4 8.5 ms just prior to reignition,a high level of unreactedness is observed near thetip of the micro air jet that indicates the premixedgas is formed during the local flame extinction, andat t 4 11.0 ms, the unreactedness tends to decreaseduring the process of reignition.

Figure 6 shows the H2 ratio contours at t 4 11.0ms for the same flame of Figs. 4 and 5. The termsz0 and r0 are the reference dimensions and they areboth selected as 10 mm. The H2 ratio is defined bythe mole fraction of H2 in the fuel prior to combus-tion [9,14,15]. If there were no preferential diffu-sion, the H2 ratio would be 0.15 in the foregoingflame, which corresponds to the mole fraction of H2in the original fuel. It is the measure indicating howmuch H2 in the original fuel is concentrated or di-luted by the preferential diffusion among species inthe flame. As shown in Fig. 6, the H2 ratio near thetip of the flame is 0.29 at the stoichiometric condi-tion, which is shown by the broken line. The valueis extremely high as compared with 0.15 of the origi-nal fuel. This means that the fuel component of H2is concentrated near the stoichiometric regionaround the central axis due to the preferential


Fig. 7. Temperature profiles forthe flame with impinging micro fueljet.

Fig. 8. Profiles of unreactednessfor the flame with impinging microfuel jet.

Fig. 9. H2 ratio contours for the flame with impingingmicro fuel jet at t 4 11.0 ms.

diffusion. H2 of high diffusivity tends to be concen-trated due to the concave curvature of the equi-con-centration contours. This is one of the main reasonswhy the maximum temperature becomes signifi-cantly higher than that of the flat flame.

Figures 7 and 8 show the typical computed resultsfor the flame with the impinging micro fuel jet. Theair and fuel flow from upper-side and lower-sideboundary of z 4 20 and 0 mm, respectively.

Figure 7 shows the temperature profiles. At t 41.5 ms, the flame still maintains high temperature atthe region where micro fuel jet impinges, eventhough the width of the flame becomes narrow andthe local stretch rate is at high level near the im-pinging region. This phenomenon is the same as thatof the flame with the impinging micro air jet. At t4 11.0 ms, in contrast to the flame with the im-pinging micro air jet, the temperature near the cen-tral axis is low, and reignition after the local extinc-tion is hardly observed.

Figure 8 shows the unreactedness near the centralaxis at t 4 11.0 ms for the same flame of Fig. 7. Itis noted that the unreactedness has lower value even


Fig. 10. Local maximum temperature Tmax versus localflame stretch rate St.

after the local extinction. This indicates that the pre-mixed gas formed after the flame extinction has onlya low potential for heat release.

Figure 9 shows the H2 ratio profile at t 4 11.0 msfor the same flame of Figs. 7 and 8. The value isabout 0.08 near the central axis at stoichiometriccondition. It is extremely low as compared with 0.15of the original fuel. This means that the fuel com-ponent of H2 is diluted with N2 due to the prefer-ential diffusion. H2 with its high diffusivity tends todiffuse divergently due to the convex curvature ofthe equi-concentration contours. This is the reasonwhy only low unreactedness is observed and the reig-nition is hardly observed in this flame.

In Fig. 10, local maximum temperature Tmax ver-sus local flame stretch rate St are plotted based onthe previous computations. The stretch rate is eval-uated along the stoichiometric line at the centralaxis. The maximum temperature is determined atthe cross section at the central axis. The solid lineindicates the case for steady flat flames changing theopposing fuel and air velocity, keeping fuel compo-sition the same. For the steady flat flames, the max-imum temperature decreases monotonically with in-creasing stretch rate to reach flame extinctionbeyond the right end of the line. The left end of thesolid line corresponds to the initial condition of theflames above. The broken line or chain line is thatof the flame strained by the impinging micro air orfuel jet, respectively. It is noted that in both flames,flames can maintain high temperature instantane-ously in spite of the quite high stretch rate, which isabout 5–10 times the critical strain rate for thequenching of a steady flat flame. In the flamestrained by the impinging micro air jet, the flametemperature increases from about 600 to 1500 K dueto the reignition at the tip of the flame at a stretchrate of about 2000 s11 that is about 3 times ofquenching limit of the steady flat flame. In the flamestrained by the impinging micro fuel jet, once the

flame temperature decreases due to the local extinc-tion near the central axis, temperature cannot riseagain even though the stretch rate decreases to thelow regime for which the steady flat flame can exist.It is noted that the maximum temperature in theflame cannot be rationalized by the local stretch ratebut changes widely depending on the flame config-uration such as the effect of unsteadiness and thepreferential diffusion effect in relation to the flamecurvature.

Summary

1. Transient local flame extinction is observed wherethe micro jet impinges. During the process of theextinction, the transient flame can surviveinstantaneously under the quite high stretch ratewhere the steady flame cannot exist.

2. Reignition is observed after the local extinctionby the micro air jet impingement. Thetemperature after reignition becomessignificantly higher than that of the original flame.This high temperature is induced by the H2concentration due to the preferential diffusion inrelation to the concave curvature. The predictedbehaviors of the local transient extinction andreignition are well confirmed by the experiment.

3. The reignition is induced after the formation ofcombustible premixed gas mixture of high levelof unreactedness and the consequent flamepropagation.

4. The reignition is hardly observed after theextinction by micro fuel jet impingement. This isdue to the H2 dilution due to the preferentialdiffusion in relation to the convex curvature andthe consequent low level of unreactedness.

5. The maximum flame temperature cannot berationalized by the stretch rate but changeswidely depending on the unsteadiness and theflame curvature in relation to preferentialdiffusion.

Acknowledgments

The authors thank Dr. M. Komiyama, Mr. A. Miyafuji,Mr. R. Nakata, and Mr. T. Imai for their assistance inexperiments.

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COMMENTS

Norbert Peters, RWTH Aachen, Germany. This is a veryimportant contribution to the understanding of curvatureeffects in diffusion flames. These effects are not taken intoaccount in the flamelet theory and should be.

Author’s Reply. Yes, it can be said that the local flametemperature, flame extinction and reignition behavior aremuch affected by the flame curvature with respect to pref-erential diffusion and unsteadiness. These need to be ad-equately considered to understand the flame phenomenaand to formulate the flamelet theory.

Documents

Transient local extinction and reignition behavior of diffusion flames affected by flame curvature and preferential diffusion