Experimental Thermal and Fluid Science 34 (2010) 1290–1294

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Experimental Thermal and Fluid Science

journal homepage: www.elsevier .com/locate /et fs

Phenomena in oscillating downward propagating flames induced by externallaser irradiation method

June Sung Park, Osamu Fujita *, Teruaki Honko, Yuichiro Yamada, Hiroyuki Ito, Yuji NakamuraDivision of Mechanical and Space Engineering, Hokkaido University, Kita13 Nishi8, Kita-ku, Sapporo, Hokkaido, Japan

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 August 2009Received in revised form 22 February 2010Accepted 22 May 2010

Keywords:Acoustic instabilityExternal laser irradiation methodFlame curvatureDiffusive-thermal instabilityLewis number

0894-1777/$ - see front matter � 2010 Elsevier Inc. Adoi:10.1016/j.expthermflusci.2010.05.010

* Corresponding author. Tel./fax: +81 11 706 6385.E-mail address: ofujita@eng.hokudai.ac.jp (O. Fujit

Experiments in C2H4/CO2–O2 premixed flames (Le < 1) propagating downwardly in a tube have been con-ducted to observe transition phenomena from laminar flame front to turbulent flame propagation trig-gered by external laser irradiation method. To investigate the exact motions of flame tip fluctuation atthe initial moment of irradiating CO2 laser, the completely flat flame front is selected as a default flame,which is corresponding to the primary acoustic instability as reported by Searby (1992) [1].

According to the time-resolved observation, the flame front exposed to CO2 laser beam shows extre-mely unstable flame motions in which highly curved flame front towards unburned mixture is subjectto diffusive-thermal instability. Then, the sudden enhanced burning state (increased flame surface)caused by flame instability induces the secondary acoustic instability which is akin to the observationin Ref. [1]. In the present study, we report the detailed descriptions of flame fronts on the transientbehaviors leading the primary acoustic instability to turbulent motions actively induced by the absorp-tion of externally irradiated CO2 laser beam.

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1. Introduction

It has been well known that the inherent instabilities of pre-mixed flame always appear e.g. hydrodynamic, body-force and dif-fusive-thermal instabilities. The hydrodynamic effects are causedby density jump through the reaction zone and flow redirectionsin the vicinity of flame surface satisfying continuity. Its linear anal-ysis was firstly conducted by Darrieus and Landau as fundamentalsof hydrodynamic instability [2,3]. In diffusive-thermal instability,the disparity between reactant and thermal field could induceunstable behaviors. When the Lewis number (hereafter, Le), whichis defined as the ratio of thermal diffusivity (a) to mass diffusivity(D) of deficient reactant, is less than unity, this preferential diffu-sion of the deficient reactant versus heat into the reaction zonehas a destabilizing influence on resulting flame behaviors [4,5].The buoyancy-driven instability, well known as Rayleigh–Taylorinstability, is also an important phenomenon that a heavy fluid isaccelerated into a light one, rendering negative density profilealong the direction of gravity. Thus, flame front propagating up-wardly is buoyantly unstable [4,5]. The occurrence of flame insta-bilities has been a hot issue, because it is a basic process inturbulent flames involving wrinkled flame surfaces, flame bulges,and local flame extinction (flame holes) [6,7].

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a).

The propagation of premixed flames in tubes is widely used toinvestigate the genesis of flame front behaviors, and there aremany reports of propagating flames in tubes, showing a varietyof flame shapes along the direction of propagation (curved, flat,wrinkled, tulip, and cellular shapes). It has been proposed thatthe formation of these shapes could be caused by the combinedeffects of a number of factors such as non-slip conditions at thewall surface, reverse flow in the vicinity of central parts of a flame,Darrieus–Landau instability, and acoustic waves [8–10].

Recently, Tsuchimoto et al. have conducted experiments toinvestigate oscillation phenomena in upward propagation flames[11,12]. They formed flame surfaces with a convex structure to-wards the unburned mixture on freely propagating flames usingCO2 laser irradiation. The intentionally-formed flame surface couldmodify the subsequent flame propagation behaviors, and it waspossible to induce unstable motions in the flames. The external la-ser irradiation preheats the unburned mixture locally in front ofthe reaction zone, and the flame propagation velocity increases lo-cally, here the flame front acquires a strongly curved shape whichis sensitively subject to Lewis number effects.

Searby has studied experimentally the classic flame propaga-tion in a tube, conducted with various equivalence ratios and tubesizes. According to the characteristics of flame propagation, thefour distinct regimes were categorized into; (1) a curved shapewith large cell and no sound just after ignition, (2) primary acous-tic instability with a flat flame surface, (3) a violent secondaryacoustic instability, (4) turbulent motion [1]. Following these

Table 1Tested gas composition.

C2H4 O2 CO2 Le U SL

9% 21% 70% 0.79 1.29 25.1

U: equivalence ratio, Le: Lewis no., SL: 1-D laminar burning velocity (cm/s, CHEM-KIN3.7, Premix Code, GRI-Mech3.0).

J.S. Park et al. / Experimental Thermal and Fluid Science 34 (2010) 1290–1294 1291

observations, Searby and Rochwerger presented the theoreticaland experimental verification of the primary thermo-acousticinstability based on the stability diagram, predicting the behaviorsof downwardly propagating flames in a tube [13]. It should, how-ever, be noted that these studies were on the flame front behaviorsexamined by varying the mixture ratio to change its own burningintensity, and there were not enough descriptions on transitionphenomena between each regime. Although there have been manyresearches on acoustic influences experimentally and theoretically[14], there is no attempt to induce the transition process of inten-sified burning. To understand transition phenomena might be thekey factor to master their sequential procedures within acousticinfluences.

In this paper, we report transient phenomena from a flat flameto turbulent motion in downward propagating flames triggered byactive controls of flame curvatures using CO2 laser irradiation. Thepreheated mixture induces flame instability, and then the in-creased flame surface enhances the burning rates over that of sta-ble flames. Due to the procedures, the burned gas flow behind thereaction zone could show sudden pronounced changes. Once thehigher flow rate of burned gas in a tube is induced, the flame prop-agation is susceptible to acoustic influences [8,9]. We identify thepresence of links between the primary acoustic instability andthe secondary acoustic instability, in which there exists the peri-odic fluctuation of flame surface area and flame tip position dueto the diffusive-thermal effects. Thus, it can be one of controlmechanisms to set the ground for the future studies.

The purpose of this paper is to present the detailed descriptionsof transition phenomena on flame front shapes and its characteris-tics induced by external laser heating, which can be responsible forthe transition to turbulent motion.

2. Experimental configuration

The experimental apparatus used in the present study is sche-matically outlined in Fig. 1. The propagation tube (transparent ac-rylic tube, inner diameter 50 mm and length 450 mm) is placedvertically and is filled with the tested gas (Table 1) at atmosphericpressure. Ethylene gas is the main absorption media of CO2 laseraccording to the NIST chemical database [15]. Carbon dioxide is se-lected as a diluent to reduce the burning velocity. The Le is esti-mated by using the definition, Le = a/D (a: thermal diffusivity ofmixture, D: mass diffusivity of O2 to the mixture) [16]. Since theLe of rich mixture is less than unity, the flame front might be unsta-ble with some disturbances on the flame surface. There is an auto-

CO2 Laser

High speed camera

MirrorShutter

Lens

IgniterFlame

Propagationtube

(450mm)

Laser beam(3.3mm)

Fig. 1. Schematic outline of the experimental setup.

matic opening system powered by an electro-magnet and amechanical spring at the upper end of the tube. When the sparkigniter is activated, the exhaust part is simultaneously opened.The premixed gas inside the tube is continuously exposed to the la-ser beam (beam diameter 3.3 mm, SYNRAD Firester v20, wave-length 10.6 lm) in the time 0.6 s after ignition, thus CO2 laserbeam preheats the unburned mixture just in front of flame surfaceat the center. The time-dependent behaviors of oscillating flamefronts are captured by a high speed camera (nac HSV-500C2,500 fps, exposure time 2 ms) and analyzed by PC. To determinethe position of flame front, the luminosity of all pixels for all indi-vidual pictures is quantified by checking the grey scale of JPG file.Then, the position of the flame front is defined as where the lumi-nosity exceeds a given criterion. The onset of the laser exposure isdefined as t = 0.0 s in the present study.

3. Results and discussion

Fig. 2 shows examples of flame fronts during flame propagationinside a transparent tube without CO2 laser irradiation. Upon igni-tion, large soft flame front propagates downwardly due to Darrius–Landau instability, and it quickly becomes flat within 0.4 s at theupper part of the tube. The downward travel of a flat flame isshown in Fig. 2, similar to that of primary acoustic instability inRef. [1], attained under the condition (Table 1) mainly dealt within this paper. This flame is selected as the default flame to observetransient phenomena from extremely flat flame to turbulent mo-tion through the external laser irradiation method. In experiments,we irradiate CO2 laser beam into the unburned gas mixture in frontof flame surface. As the applied laser power increases, the acceler-ation of flame velocity is pronounced considerably. In the cases ofrelatively low laser power (�4 W), the slightly deformed flamefront propagates with weak flame oscillation, and the flame frontwith low flame curvature continues to propagate without anyother acceleration, even though we do not present the data inthe paper. This is because the diffusive-thermal instability is notbeing amplified any longer due to the low flame curvature evenwith CO2 laser irradiation. The relatively low flame curvature isnot sufficient to enhance burning state. Thus, the increase in flamesurface area and burned gas velocity is not ample to induce thesecondary acoustic instability.

Fig. 3 shows (a) sequential images of the downward propagat-ing flames observed at the arrows on the green curve in Fig. 4and (b) a different camera view taken with shorter exposure time(1/2500 s) at time instant 146 ms with a laser power of 12 W. Thedeformed flame front propagates with periodic oscillation. Theflame front is strongly curved and the convex flame front towardsunburned mixture is gradually augmented, accompanied by aremarkable acceleration of the flame until 130 ms as shown inFig. 4. In the present study, only the mixture of Le < 1 is treated,where diffusive-thermal instability appears in the curved flamesurface. When the apex of the flame is highly deformed and hasa high curvature (convex towards the unburned mixture), theflame surface could focus deficient reactants (by mass diffusion,O2) in the vicinity of the flame tip, resulting in high temperaturelocally. Also, the presence of defocusing heat (heat loss) could re-duce the temperature at the flame tip, but the influence of heat loss

Fig. 2. Still images of temporal evolution of flame fronts without laser power.

Fig. 3. (a) Still images of flame shapes at the time instants 66; 82; 98; 114; 130; 146 ms after irradiating CO2 laser, (b) different camera view with short exposure time(1/2500 s) corresponding to the image in 146 ms of Fig. 3a with a laser power of 12 W.

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is relatively small in comparison with the influence of mass diffu-sion (non-equidiffusion effect, Le < 1). This non-equidiffusion pro-cess is the background to the ‘‘oscillatory-propagation-behaviors[12]”.

Turning to Fig. 3, special attention should be paid to trans-versely propagating motion of the wrinkled surface around thecenter that was generated by flame tip oscillation. At 114–146 ms after the irradiation, the evolution of second bulges onthe flame front becomes more active than that at flame tip, andthere is a gradual formation of concave structures between theindividual cells. These behaviors are corresponding to the processof the secondary acoustic instability [1]. After these moments,the corrugated flame front is finally established, and there is a sud-den decrease in flame velocity (146 ms in Fig. 3a and b). The in-crease in the area of the flame surface could make it susceptible

to inducing acceleration of burned gas. The following stage of theflame behaviors, secondary acceleration (turbulent motion) willbe discussed latter in this paper.

Fig. 4 depicts temporal variation of the flame tip positions withthe laser powers 0–18 W. The slope of flame tip positions after60 ms is increased with the flame oscillation as the laser power in-creases. During the first acceleration in Fig. 4, periodic changes inthe flame position also appear due to the diffusive-thermal effects[4,5]. The solid arrows in 12 W are corresponding to pictures inFig. 3. After the first acceleration, sudden decreases in flame veloc-ity are observed (e.g. 130 and 146 ms in Fig. 3) in the cases of 12and 18 W. Acoustic influences might be prominent like secondaryacoustic instability.

Fig. 5 displays sequential images of the temporal evolution of aflame front with a laser power of 6 W after the first acceleration in-

0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.350

2

4

6

8

10

12

14

16

Flam

e tip

pos

ition

aft

er ir

radi

atin

g C

O2 la

ser,

cm

Time,s

Laser power 0 W Laser power 6 W Laser power 12 W Laser power 18 W

Fig. 4. Flame tip position vs. time with various laser power inputs.

J.S. Park et al. / Experimental Thermal and Fluid Science 34 (2010) 1290–1294 1293

duced by CO2 laser, which can be referred to in Fig. 4 respectively(solid arrows on red line). During the propagation, the laser powerinput increases the local flame velocity, and then the projectedflame is observed by 202 ms in Fig. 5. At 290 ms, the locally pre-heated mixture located ahead of the flame is subject to the buoy-ancy influence and relatively strong upward flow is generated,overcoming the local flame velocity at the center. The flat flamefront is observed between 246 and 290 ms. Finally, the concaveflame shape towards fresh mixture is clearly attained at 290 ms.It can be proposed that for Le < 1, this negative (concave) flamecurvature reduces the local velocity at the flame center becauseof the diffusive-thermal effect, and that other portions of the flamesurface around the flame bulge propagates fast forward because ofits positive curvature as described in Fig. 6. The transverse propa-gation in each cellular structure leads to narrower cell size (seemsto be smooth surface). Once these procedures are repeated, the

Fig. 5. Still images of flame shapes from 15

Fig. 6. The hypothesis of unstable flame front at

resulting flame surface would be wrinkled with smaller cell sizesthan those induced by the convex. When the flame surface is per-turbed by the unstable behaviors, it is more than likely that this in-creased flame surface expedites the burning rate in the reactionzone. For the enhanced states of burning in Fig. 5, there may be aconsiderable influence on the flame propagation by acousticwaves. As shown in Fig. 5, after the saturation of the wrinkled sur-face induced by concave surface (334 ms), a secondary acceleration(378 ms) with turbulent motion is triggered suddenly as shown inFigs. 4 and 5, akin to the phenomena observed in self-turbulentflame [1,8,9].

The above discussed unstable flame behaviors induced by activecontrols of the flame shape without the external flow disturbance.To investigate the transient phenomena of premixed flames, theposition of the flame tip, its velocity (a), and still images of tran-sient flame motion (b) with the laser power 16 W are shown inFig. 7. During the first acceleration, the flame velocity oscillateswith the increase in oscillation amplitude, in which these flamebehaviors are corresponding to 66–114 ms in Fig. 3. In Fig. 7a,the increase in flame velocity at advancing waves exceeds the de-crease in that at retreating waves considerably. It is implied thatthese sudden acceleration of the flame velocity is closely relevantto diffusive-thermal effects (Le < 1) [17,18]. After the enhancedburning states in the first acceleration, the flame tip has almostpaused in regime (A). In this regime (A), the flame front becomesflat roughly with the development of concave structures. Then, itis suddenly accelerated in regime (B), accompanied with strongnoise. It must be noted that the concavity facilitates the small scalestructures of wrinkled surface in Fig. 5. These appearances couldalso be observed with laser powers of 6, 12, and 18 W.

The previous study [1] identified four types of flame frontbehaviors among the wide range of equivalence ratios; Darrieus–Landau, the primary (flat flame), secondary instability and turbu-lent flame. However, for each regime, there must exist the transi-tion on a case-by-case basis. In the present study, only one gas

8 ms to 378 ms with laser power 6 W.

the concave surface towards unburned gas.

0.00 0.05 0.10 0.15 0.20-600

-400

-200

0

200

400

600

800

1000 Regime(B)

Regime(A)

0

2

4

6

8

10

12

14

Flam

e tip

pos

ition

, cm

Flame tip position

Flam

e ve

loci

ty, c

m/s

Time, s

Flame velocity

(a)

(b)

Fig. 7. Temporal variations of flame position and velocity (a) and still images oftransient flame motion and (b) with laser power 16 W.

1294 J.S. Park et al. / Experimental Thermal and Fluid Science 34 (2010) 1290–1294

mixture (Le < 1) was used to form the primary acoustic instabilityas a default, and we could observe transition phenomena leadingthe flat flame to turbulent motion through the diffusive-thermaleffects. The experimental results with the unstable behaviors ob-tained in the paper make it possible to suggest that the transition(the primary acoustic instability to turbulent motion) of propagat-ing flames in tubes could be initiated by artificial controls on flameshapes (e.g., CO2 laser irradiation in this study).

4. Concluding remarks

Experimental studies of premixed flame propagating down-wardly have been conducted to elucidate transient motions fromlaminar premixed flames to highly accelerated turbulent flames in-duced by artificial controls of flame shapes, in which the diffusive-thermal instability is the main cause of this transient phenomenon.Without CO2 irradiation, the flat flame front propagates until theend of the tube. With the highly curved flame surface induced bythe active control of flame front, however, highly accelerated flametip is verified with the fluctuation of its position.

We observed different unique phenomena depending on laserpower inputs. When the applied laser power is low (�4 W), theslightly deformed flame surface kept propagating without anyexplosive acceleration. Meanwhile, for the higher laser powers,

the extremely transient behaviors of flame front shapes and veloc-ity appear due to the diffusive-thermal effects and acoustic waves.In the first acceleration of flame tip, the rapid increase in flamevelocity at the advancing wave rather than the decrease at theretreating wave is observed due to the diffusive-thermal effects(Le < 1, in Fig. 7a). The increased flame surface by flame oscillationmay facilitate acoustic influences on flame propagation, causingthe secondary instability (134–158 ms in Fig. 7). Then, highly cor-rugated flame surface (convex and concave structure) becomesroughly flat with sudden decrease in flame tip velocity as seen inFig. 6b. In the final stage of flame propagation, the flame frontshows highly accelerated turbulent motion with the breaking ofwrinkled flame structure (184 ms in Fig. 7). In the previous studies,conducted with various mixture conditions [1,8–10], the self-ex-cited flame accelerations have been reported depending on equiv-alence ratio, tube length and diameter. The experimental resultsobserved so far in this study, however, present the resulting turbu-lent motions attributed by active controls of flame front shapesand surface area using external laser irradiation method. Theimportance of flame shapes could suggest that the corrugatedflame structure has a strong effect on dynamic behaviors of prop-agating flames in tubes. Although possible mechanisms are pro-posed in the present study, further investigations would benecessary to validate our concerns.

Acknowledgement

This study was supported by Grants-in-Aid for Scientific Re-search (KIBAN(B) #18360095 and KIBAN(B) #21360090) fromMEXT Japan.

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