the blowout mechanism of turbulent jet diffusion flames

14
Combustion and Flame 145 (2006) 481–494 www.elsevier.com/locate/combustflame The blowout mechanism of turbulent jet diffusion flames Chih-Yung Wu a , Yei-Chin Chao a,, Tsarng-Sheng Cheng b , Yueh-Heng Li a , Kuo-Yuan Lee a , Tony Yuan a a Institute of Aeronautics and Astronautics, National Cheng Kung University,Tainan, 701, Taiwan, ROC b Department of Mechanical Engineering, Chung Hua University, Hsinchu, 300, Taiwan, ROC Received 10 June 2005; received in revised form 25 November 2005; accepted 9 January 2006 Available online 10 March 2006 Abstract The complicated flame stabilization mechanisms and flame/flow interactions in the blowout of turbulent non- premixed jet flames are experimentally studied using phenomenological observation, 2D Rayleigh scattering, 2D laser-induced predissociative fluorescence (LIPF) images of OH, and particle image velocimetry (PIV) techniques. The blowout process may be categorized into four characteristic regions: pulsating, onset of receding, receding, and extinction. Based on experimental findings, a blowout mechanism is proposed. The maximum “waistline” point of the stoichiometric contour, defined as the point where the radial distance between the elliptic stoichio- metric contour and the jet axis reaches a maximum value, can be regarded as the dividing point separating the unstable and stable regions for the lifted flame in the blowout process. If the flame base is pushed beyond the max- imum “waistline” point, the flame will step into the pulsating region and become unstable, triggering the blowout process. The triple flame structure is identified and found to play an important role in flame stabilization within the stable liftoff and pulsating regions. In the pulsating region, the stabilization point of the triple flame moves along the stoichiometric contour, stabilizing the flame where the flame base is bounded by the contours of lean and rich limits. If the flame is pushed beyond the tip of the stoichiometric contour, the stabilization point and triple flame structure vanish and the flame becomes lean. The flame then recedes downstream continuously and finally extinguishes. © 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Blowout; Blowout process; Turbulent diffusion flames; Jet flames; Pulsation region; Receding; Triple flame 1. Introduction For several decades [1], the blowout phenomenon was regarded as a special limiting point of the liftoff stability of laminar or turbulent jet flames. In the past, interest was mainly focused on predicting the * Corresponding author. Fax: +886 6 2389940. E-mail address: [email protected] (Y.-C. Chao). blowout limit and revealing the stabilization mecha- nism of the liftoff flames. Various models and phys- ical mechanisms have been proposed to delineate the liftoff behavior and blowout limits, including the early premixed combustion model [1], the flamelet extinction model [2], the large-scale mixing model [3,4], the combined premixed flame propagation and flamelet extinction model [5], and the recent triple- flame model [6–13]. Theories and models used to predict the blowout limits are similar to those de- scribing the stabilization mechanism of the liftoff 0010-2180/$ – see front matter © 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved. doi:10.1016/j.combustflame.2006.01.004

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The complicated flame stabilization mechanisms and flame/flow interactions in the blowout of turbulent nonpremixedjet flames are experimentally studied using phenomenological observation, 2D Rayleigh scattering, 2Dlaser-induced predissociative fluorescence (LIPF) images of OH, and particle image velocimetry (PIV) techniques.The blowout process may be categorized into four characteristic regions: pulsating, onset of receding, receding,and extinction. Based on experimental findings, a blowout mechanism is proposed.

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Page 1: The Blowout Mechanism of Turbulent Jet Diffusion Flames

Combustion and Flame 145 (2006) 481–494www.elsevier.com/locate/combustflame

The blowout mechanism of turbulent jet diffusion flames

Chih-Yung Wu a, Yei-Chin Chao a,∗, Tsarng-Sheng Cheng b, Yueh-Heng Li a,Kuo-Yuan Lee a, Tony Yuan a

a Institute of Aeronautics and Astronautics, National Cheng Kung University, Tainan, 701, Taiwan, ROCb Department of Mechanical Engineering, Chung Hua University, Hsinchu, 300, Taiwan, ROC

Received 10 June 2005; received in revised form 25 November 2005; accepted 9 January 2006

Available online 10 March 2006

Abstract

The complicated flame stabilization mechanisms and flame/flow interactions in the blowout of turbulent non-premixed jet flames are experimentally studied using phenomenological observation, 2D Rayleigh scattering, 2Dlaser-induced predissociative fluorescence (LIPF) images of OH, and particle image velocimetry (PIV) techniques.The blowout process may be categorized into four characteristic regions: pulsating, onset of receding, receding,and extinction. Based on experimental findings, a blowout mechanism is proposed. The maximum “waistline”point of the stoichiometric contour, defined as the point where the radial distance between the elliptic stoichio-metric contour and the jet axis reaches a maximum value, can be regarded as the dividing point separating theunstable and stable regions for the lifted flame in the blowout process. If the flame base is pushed beyond the max-imum “waistline” point, the flame will step into the pulsating region and become unstable, triggering the blowoutprocess. The triple flame structure is identified and found to play an important role in flame stabilization withinthe stable liftoff and pulsating regions. In the pulsating region, the stabilization point of the triple flame movesalong the stoichiometric contour, stabilizing the flame where the flame base is bounded by the contours of leanand rich limits. If the flame is pushed beyond the tip of the stoichiometric contour, the stabilization point and tripleflame structure vanish and the flame becomes lean. The flame then recedes downstream continuously and finallyextinguishes.© 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Keywords: Blowout; Blowout process; Turbulent diffusion flames; Jet flames; Pulsation region; Receding; Triple flame

1. Introduction

For several decades [1], the blowout phenomenonwas regarded as a special limiting point of the liftoffstability of laminar or turbulent jet flames. In thepast, interest was mainly focused on predicting the

* Corresponding author. Fax: +886 6 2389940.E-mail address: [email protected]

(Y.-C. Chao).

0010-2180/$ – see front matter © 2006 The Combustion Institute.doi:10.1016/j.combustflame.2006.01.004

blowout limit and revealing the stabilization mecha-nism of the liftoff flames. Various models and phys-ical mechanisms have been proposed to delineatethe liftoff behavior and blowout limits, including theearly premixed combustion model [1], the flameletextinction model [2], the large-scale mixing model[3,4], the combined premixed flame propagation andflamelet extinction model [5], and the recent triple-flame model [6–13]. Theories and models used topredict the blowout limits are similar to those de-scribing the stabilization mechanism of the liftoff

Published by Elsevier Inc. All rights reserved.

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Nomenclature

d jet diameterdε effective diameterfe acoustic excitation frequenciesHL theoretically predicted axial distance

from jet exit to lean flammability levelcontour

Hs theoretically predicted axial distancefrom jet exit to stoichiometric contour

Hw theoretically predicted maximum“waistline” distance, the axial distancefrom jet exit to the location where theradius of the elliptic stoichiometric con-tour reaches a maximum value

hs measured distance from jet exit to stoi-chiometric contour

hw measured maximum “waistline” dis-tance

Su laminar burning velocityUb blowout velocity of pure fuelYs stoichiometric mass fractionz axial distance from the jet exitηex location where the lifted flame extin-

guishes completelyηph upper boundary of the flame pulsating

rangeηpl lower boundary of the flame pulsating

rangeθ̄c axial profile of mean fuel mass fractionθ̄e fuel mass fraction at the jet exitθ̄s fuel mass fraction at stoichiometric con-

tourρe gas density at jet exitρ∞ ambient gas density

flames. Therefore, several existing blowout theoriesand models stem from the stabilization mechanism ofthe lifted flame. The concept of the premixed flamemodel [1] was adopted by Kalghatgi [14] to delin-eate the blowout velocity. Based on the experimen-tal data, Kalghatgi [14] was able to empirically scalethe nondimensionalized blowout velocities with theReynolds number based on the mean stoichiometricenvelope and a universal formula was proposed. In themeantime, Broadwell et al. [3] proposed that flamestabilization occurs when hot gases, which have beenexpelled to the edge of the jet by earlier large-scaleturbulent structures, are re-entrained and ignite non-combusting eddies of the jet. In both theories, theblowout velocity of mixture gases or diluted fuel alsocan be estimated based on the initial conditions ofthe fuel at the burner exit. Recently, the theories pro-posed by Kalghatgi [14] and Broadwell et al. [3] wereexamined using an extended database of methane,propane, and hydrogen jet flames with various inertdilutions [15]. The results showed that most of themeasured blowout velocities agree with the predic-tions using a universal formula proposed by Kalghatgi[14] and with those calculated using the large-scalemodel [3] by including a Reynolds number effect.A more complete database about blowout limits ofa jet flame has been constructed with those inert-diluted fuels. In addition, the analysis of diffusivityeffects on blowout limits shows that diffusive proper-ties in terms of mass and thermal diffusivities are notthe dominant parameters in the blowout of turbulentjet flames. In other words, the Schmidt number doesnot play a major role in the turbulent blowout process.Moreover, in a recent experiment [16], it is shown

that the blowout of a turbulent diffusion jet flame isa transient process with a series of events occurringconsecutively, though it usually happens rapidly andunpredictably. It is noticeable that the phenomena offlame flickering and flame base pulsation can usuallybe found before the flame blows out and it is generallybelieved that flamefront instabilities play importantroles in the blowout process [17,18]. However, the de-tailed mechanism of the blowout process is still notclear.

To delineate the flow/flame interaction character-istics responsible for blowout, proper tools and instru-ments for observations and measurements are needed.Flame instabilities in the blowout process are verysensitive to external disturbance. Hence, nonintrusiveand simultaneously two-dimensional flame/flow mea-surements, such as particle image velocimetry (PIV)[19], are desirable to provide required informationfor the blowout process. With the advancement ofhigh-power laser technology, it becomes feasible toapply nonintrusive laser techniques in hostile com-bustion environments. Laser-based diagnostic tech-niques have been successfully developed to providenonintrusive spatially and temporally resolved mea-surements of flow characteristics and chemical prop-erties [20]. In the present study, Rayleigh scatteringimaging and laser-induced fluorescence (LIF) imag-ing are applied to measure local mixture fractionahead of the flame base and to identify the flamebase location, respectively. Rayleigh scattering imag-ing is one of the good methods for binary-mixtureflow visualization of the flow field under suitable as-sumptions [21,22]. On the other hand, laser-inducedfluorescence (LIF) provides the ability to detect flame

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radicals and pollutant species at ppm or even sub-ppmlevel and has therefore received considerable attentionin flame measurements [23,24]. To resolve some im-portant issues in combustion processes, simultaneousRayleigh imaging combined with LIF imaging [25]can be utilized to delineate the spatial structure of re-action zones and the existence of either thin flameletzones or broad distributed ones is of primary impor-tance in modeling turbulent flames.

In a previous study [16], the blowout of a partiallypremixed methane jet flame was delineated and clas-sified. To simplify the initial conditions of the fuelstream at the jet exit and to verify the effects due tomultiplex parameters when the jet flame blows out,jet flames with inert-diluted fuel were used for a se-ries of experiments. Blowout limits can be estimatedsimply based on initial conditions of fuel at the jetexit [15], although in the blowout process a reced-ing liftoff flame is strongly affected by the upstreamflow. Local mixture fraction and thermal/flow charac-teristics ahead of the flame base may vary with liftoffheight due to the turbulent cascade and mixing. Tolook further into the intrinsic physical and chemicalproperties of the flame base under blowout conditions,and to probe into the relationship between local prop-

erties of flow ahead of the flame base and initial con-ditions at the jet exit, the present study describes two-dimensional laser diagnostics, including qualitativetracing of the flame base, PIV, and simultaneous LIPFand Rayleigh imaging of inert-diluted methane andpropane turbulent jet flames in the blowout processand the results are quantitatively compared with thetheoretical prediction based on a model extendedfrom the universal formula of blowout velocity pro-posed by Kalghatgi [14]. Furthermore, a new blowoutmechanism of a turbulent jet diffusion flame is pro-posed based on the measurement and theoretical re-sults.

2. Experimental setup

The experimental setup is shown schematicallyin Fig. 1a. The jet flame burner consists of a well-contoured circular nozzle 5 mm in diameter, fromwhich the fuel/diluent mixture emerges. The noz-zle wall is contoured with a fifth-order polynomialprofile, and the area contraction ratio is 400. Fu-els and diluents are metered by rotameters and elec-tronic flowmeters. The accuracies of the rotameters

Fig. 1. Experimental setup: (a) essentials of the experimental arrangements; (b) arrangements of lasers and optics of PIV device;(c) schematic diagram of 2D Rayleigh and 2D LIPF.

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for fuel metering and electric flowmeter for air me-tering are 0.5% of full scale and ±1.5% of full scalefrom 10 to 100% of full scale respectively. We haveprovided these data in the revision. Readings of ro-tameters and electronic flowmeters are recorded tocalculate the experimental blowout velocity, definedas the bulk fuel stream velocity when the flame blowsout. Compressed air from the tank and diluents andfuel from the cylinders are filtered, metered, and pre-mixed in the mixing chamber. In this study, 30%nitrogen-diluted and argon-diluted methane and 50%nitrogen-diluted and argon-diluted propane are inves-tigated, and the flame conditions accompanied by var-ious fundamental parameters tested in this study arealso listed in Table 1. Furthermore, noise reductionand settling chambers are used to improve the flowquality. The nozzle exit velocity shows a top-hat pro-file, and the turbulence intensity at the jet centerlineis about 0.5%. The whole system is placed insidean anechoic room. Characteristic frequency of thelifted jet flame in the cold flow region is examinedby a probe microphone (B&K 4182). A non-catalytic-coated R-type (Pt/Pt-13Rh) thermocouple of diameter50 µm is used to measure the temperature and thecharacteristic frequency at the flame base. Images offlame structures and flame-base tracing are obtainedby a high-sensitivity three-chip color CCD camera(Sony DXC-9000) with external triggering functions.The frame rate can reach 60 frames per second, andthe serial images are digitized by the frame grabberfor further analysis.

Arrangement of the PIV devices, including twoNd:YAG lasers and optics, is shown in Fig. 1b. Thelaser beams are aligned with optics through two po-larizers and a wave plate. The resulting beam is thenexpanded by three cylindrical lenses into a laser sheetapproximately 0.7 mm in thickness, which is actually

measured on the projection screen. The time intervalof the PIV system is controlled by a pulse signal/delaygenerator. The fuel and air streams are seeded withsieved fine Al2O3 particles of sizes less than 10 µm.A high-resolution, high-sensitivity, and low-dark-current camera (SharpVision 1300DE) is used forimage recording. This CCD which is equipped witha progressive scan interline CCD sensor is especiallysuitable for PIV. The image array has 1300 × 1030pixels, limited to 1280 × 1024 in practice, and thepixel size is 6.7 × 6.7 µm. All images are captured,digitized through a 16-bit digitizer, and recorded ona hard disk for further analysis.

For identification of the reaction zone, the laser-induced predissociative fluorescence (LIPF) of OHmolecules from v′′ = 0 to v′ = 3 in the A2Σ ← X2Π

system is employed [26]. The laser beam spreads intoa thin sheet of height 34 mm and thickness 0.2 mmby a single cylindrical lens (f = 1000 mm) and inter-sects vertically through the flame axis. Only the 25-mm central portion of the laser sheet, where the laserintensity is high and uniform, is used for imaging. TheOH fluorescence signal is imaged onto an intensifiedCCD camera (576 × 384 pixels) with a UV cameralens (Nikkor, f = 105 mm, f/4.5). A 10-mm-thickbutyl acetate liquid filter is placed in front of the cam-era to remove the Rayleigh scattering. The OH fluo-rescence signal is collected at 297 nm, correspondingto the fluorescence produced from the 3 → 2 tran-sition. On the other hand, the 2D Rayleigh scatter-ing system with KrF laser is employed to visualizethe upstream fuel–air mixing characteristics and toobtain qualitative and quantitative concentration pro-files. 2D Rayleigh imaging can be performed using anexperimental setup identical to the laser-induced pre-dissociative fluorescence (LIPF) system, except thata narrow-band filter of 248 nm is used instead of the

Table 1Measured and theoretical estimated parameters of each condition for blowout process observation

30% N2-diluted CH4 30% Ar-diluted CH4 50% N2-diluted C3H8 50% Ar-diluted C3H8

Ub (m/s)a 42.6 34.0 52.6 47.6fe (kHz) 2.2 2.0 4.7 4.5Su (cm/s) 31.6 32.3 36.9 37.7

Hw (x/d) 24.0 22.0 32.0 30.0hw (x/d) 20.0 19.0 26.0 25.0ηph (x/d) 21.0 18.0 28.0 23.0

Hs (x/d) 42.2 39.3 54.5 51.0hs (x/d) 35.0 37.5 51.0 49.5ηpl (x/d) 32.0 38.0 50.0 49.0

HL (x/d) 63.8 59.3 82.4 76.9ηex (x/d) 59.0 61.0 88.0 80.0

Note. d = 5 mm.a Proposed in previous work [15].

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butyl acetate filter. In the present study, 2D LIPF and2D Rayleigh imaging devices are applied and trig-gered simultaneously to identify the mixture level offuel and oxidizer in the flame front. A schematic dia-gram of 2D Rayleigh image and 2D LIPF systems isshown in Fig. 1c.

3. Results and discussion

3.1. Manipulation of blowout of a jet flame

Controlling the initial conditions of the jet flameblowout process for repeatability in experiments ispossible with proper acoustic excitation [22]. Con-ventionally, blowout velocity is measured by slowlyincreasing the bulk jet exit velocity until the flameblows out. However, it is very difficult to preciselyrepeat the blowout process due to the inaccuracy ofmanually controlled volumetric flow rates of fuel ormixtures and the inertia associated with the pipelinewhen triggering the blowout process. An effectiveway to overcome this difficulty is to apply acousticexcitation on the jet flame. The effects of acousticexcitation on the jet have been reviewed by Ho andHuerre [27]. They summarized and discussed effectsof major excitation parameters including forcing fre-quency, forcing level, and phase angle on flow struc-tures. Chao and his co-workers have conducted a se-ries of jet flow and flame studies using acoustic ex-citation [16,22,28,29]. They found that the blowoutlimits can be extended and the irregular onset of theblowout process can be controlled and repeated bytuning the excitation frequency. This strategy of us-ing acoustic excitation is to avoid the difficulty ofconvective delays associated with slowly varying theflow rates to reach blowout and to provide alignedinitial conditions for repeated measurements of thedynamic blowout process. The function of turning offthe acoustic excitation is triggered by a slope detector,a counter, and a relay switch. The residual acoustic ef-fects after the acoustic excitation is turned off will notaffect the transition of the blowout process in prac-tice because the sound propagation speed is far higherthan the flow velocity. In other words, it may be as-sumed that the blowout process is triggered as soonas the excitation is turned off.

To obtain the proper excitation frequency for thisstudy, the fundamental frequency of the fuel streamat the blowout velocity must be determined first.A microphone probe is applied to conduct the mea-surements. However, to obtain a well-defined initialcondition, the best excitation frequency and ampli-tude of each flame condition needs to be tested andtuned manually. The chosen acoustic excitation fre-quencies (fe) and the blowout velocities (Ub) for each

flame condition are listed in Table 1. Observation andmeasurement of the flame blowout process are con-ducted with proper acoustic excitation to hold thelifted jet flame at the blowout limits, which have beenmeasured in our previous study [16], and the blowouttransient is triggered when acoustic excitation is elec-tronically turned off at a prescribed phase angle.

3.2. Blowout process of a jet flame

Observation of the blowout of a jet flame can beachieved by tracing the flame base images. The con-tinuous blowout images are recorded by a CCD cam-era with a frame rate of 30 frames per second, and theimages are digitized by the frame grabber for analysis.The flame base is then identified with digital imageprocessing, and the flame-base trace can be expressedas a function of time. According to the dynamic char-acteristics of jet flames, the transient blowout processcan be divided into four regions: pulsating, onsetof receding, receding, and extinction regions [16].In this study, to further understand the characteris-tic blowout processes of different fuel/oxidizer mix-tures, flame-base traces of inert diluted methane andpropane flames are made, and typical traces and flameimages in the four regions of the 30% nitrogen-dilutedmethane jet flame are shown in Fig. 2. Similarly tothe previous study [16], the initial condition of theblowout process is controlled by proper acoustic ex-citation. As soon as the acoustic excitation is turnedoff at a prescribed phase, the process engages andthe flame remains, and then the flame base starts topulsate in the axial direction at roughly 1 Hz [16].The whole flame is blue in color and looks simi-lar to a lifted flame. This is the pulsating region, asseen in photograph α in Fig. 2. The flame tip is obvi-ously affected by the buoyancy-induced vortex. Whenthe flame base reaches the height of x/d = 34, theflame simply moves downstream continuously, beingunable to come back. Red-hot stripes can be foundon the blue flame. This is the onset of the reced-ing region as shown in photograph 2β . When theflame moves further downstream, the flame tip be-comes red, yet the flame base remains blue. This isthe receding region as illustrated in photograph 2γ .Finally, when the height reaches at x/d = 59, theflame becomes shorter and the base is blue, as notedin photograph 2δ. Flame extinction occurs in this re-gion. In the process red stripes and spots are found inthe flames, indicating local low temperature and un-burned regions.

The pulsating frequency of the flame is mea-sured using an ion probe placed at a fixed positionclose to the flame base before blowout. Fig. 3 showsthe pulsating frequency for the 30% nitrogen-dilutedmethane flame. Low pulsating frequencies, around

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Fig. 2. Typical flame-base trace in blowout process of the 30% nitrogen-diluted methane flame with typical flame image in eachregion: pulsating, onset of receding, receding, and extinction.

1–5 Hz, similar to those previously reported [16] areobserved in the flame base. Similar phenomena arealso found in the 30% argon-diluted methane flameand the 50% nitrogen- and argon-diluted propaneflames, despite the apparent differences in Lewisnumber [15]. The measured pulsating frequencyis also similar to the pulsating frequency of non-premixed jet flames near extinction proposed by Füriet al. [17], but less than the characteristic frequency(10–18 Hz) of buoyancy-induced large toroidal vor-tices, which is a type of Kelvin–Helmholtz instabil-ity [30]. For the current turbulent blowout flames, thethermal-diffusive instabilities may not be as dominantin the blowout process [15] as they are in the lami-nar flames [31] and may be suppressed by increasingconvective velocity of flow [17]. The mechanism forinstability as the flame moves downstream of themaximum waist of the stoichiometric contour is stillnot clear and cannot be given based on current results.Other instability modes may exist and play a role inthe blowout process. It is unclear now and needs to beclarified by further experiments.

3.3. Fuel/air mixing and statistics of flame behaviors

It is widely accepted that the universal formulaproposed by Kalghatgi [14] based on the premixedflame model [1] can provide an accurate estimate ofthe blowout velocities for a variety of fuels with dif-ferent degrees of dilution for various inert species [15].In the formula, the position of the flame base closelycorresponds to the locations of the stoichiometric con-tour. Kalghatgi’s model can be extended to predictmajor characteristics of the blowout process. First ofall, to identify the locations of the stoichiometric con-tour, the theoretical model proposed by Birch et al.[32] for the concentration distribution issued froma free jet is adopted. A normalized axial profile, θ̄c/θ̄e,in a free jet can be shown as

(1)θ̄c/θ̄e = k1dε/(z + a1), dε = d(ρe/ρ∞)0.5,

where θ̄ , ρ, z, and dε are the mean fuel mass frac-tion, density, and axial distance from the jet exit andeffective diameter, respectively, and subscripts e and∞ indicate properties at the jet exit and ambient con-

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Fig. 3. Pulsating frequency of the 30% nitrogen-diluted methane flame.

ditions. Reported values of the constants k1 and a1are 4.0 and −5.8d , respectively. Hence, the distancealong the jet axis from the jet exit to the locationwhere the mean fuel concentration falls to the stoi-chiometric level, Hs, can be derived from Eq. (1) as

(2)Hs =[

4θ̄e

θ̄s

(ρe

ρ∞

)1/2+ 5.8

]d,

where subscripts s indicate properties at stoichiomet-ric level. Equation (2) can also be used to calculate thedistance of the lean flammability limit, HL, by replac-ing θ̄s by the lean flammability-limit mass fraction.Distances estimated with Eq. (2) for the stoichiomet-ric level and the lean flammability limits are listed inTable 1. Furthermore, for the radial profiles of the jet,the normalized profiles of mean mass fraction concen-tration are accurately described by the Gaussian-typefunction [32,33], and can be shown as

(3)θ̄/θ̄c = e{−D(r/z)2},where D is found to be 73.6. Substituting Eq. (1) intoEq. (3) and replacing θ̄ by θ̄s, the stoichiometric con-tour can be expressed as a function of r and z and isshown as

(4)r2 + z2

73.6ln

θ̄s(z − 5.8d)

4θ̄0dε= 0.

The stoichiometric contour, Eq. (4), can be used tofind the maximum “waistline” point of the stoichio-metric contour by setting dr/dz to zero, and one can

obtain

(5)2 ln

[θ̄s

4θ̄0dεz − 5.8dθ̄s

4θ̄0dε

]= − z

z − 5.8d.

Equation (5) is solved numerically for z to find themaximum “waistline” distance, Hw, along the axisfrom the jet exit to the location where the radius of theelliptic stoichiometric contour reaches a maximumvalue. These two parameters, Hs and Hw, character-ize the stoichiometric contour and can be shown toplay important roles in the proposed mechanism ofthe blowout process.

The theoretical prediction above can be verifiedby experimental measurements. Two-dimensionalRayleigh scattering imaging is applied to define thestoichiometric contour for the inert-diluted methaneand propane cold jets. Fig. 4a shows the averagedRayleigh imaging of the 30% inert-diluted methanejet. In this figure, each Rayleigh image is an aver-age of 30 single-pulse measurements and the figure iscomposed of 15 images measured at different heights.Due to the limitations of the translation stage, themeasured flame height can only reach up to x/d = 60.Being proportional to the Rayleigh scattering crosssection of mixtures, the color level in stoichiomet-ric conditions, as well as the lean and rich flammablelimits, can be estimated from the binary mixing of theRayleigh scattering signals and is also shown in thefigure. Fuel concentration decreases axially and radi-ally from the jet exit due to entrainment and turbulentmixing with ambient air. Furthermore, the two major

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Fig. 4. (a) Averaged mixing level distribution of the 30% nitrogen-diluted methane flame; (b) probability distribution of transientflame base location during blowout process; (c) flame-base propagation velocity of the 30% nitrogen-diluted methane flamebased on sequential images.

characteristic parameters of the elliptic stoichiometriccontour, hw and hs, in which lower case is used hereto distinguish the measurement data, can be identi-fied via the quantitative Rayleigh measurements. Themeasured hw and hs are listed in Table 1. To fur-ther identify the flame pulsating range, the probabilitydistribution of the transient flame base location dur-ing the blowout process using 10 samples of flamebase tracing is shown in Fig. 4b and is marked as thepulsating region in which the probability is greaterthan 2%. The locations of the upper and lower bound-aries of the flame pulsating range, ηph, and ηpl, arealso tabulated in Table 1 for further comparison withother parameters. Moreover, ηex, defined as the lo-cations where the lifted flame is extinguished com-pletely, are also determined based on statistical resultswith flame base tracing of the blowout process and arelisted in Table 1. To further delineate the flame/flowinteraction during the blowout process, typical flamebase propagation velocities of 30% nitrogen-dilutedmethane flames are plotted in Fig. 4c against the flamebase locations calculated from sequential images. Theresults show that the flame base propagation veloci-ties scatter randomly between 1 and −1 m/s when the

flames are in the pulsating region and x/d is less thanthe onset position of receding flames. The flame baseoscillation may be affected by pulsating instabilitiesand flow interactions so that the propagation veloc-ities jitter irregularly. On the other hand, the flamebase propagation velocity increases almost linearlywith x/d from the onset of receding to extinction.A similar trend was found in the 30% argon-dilutedmethane and the 50% nitrogen- and argon-dilutedpropane flames.

The measured and predicted results of the charac-teristic parameters listed in Table 1 are also comparedto verify the model described above. By careful exam-ination of the data in Table 1, one can find that thereare very good correspondences between the measure-ment maximum “waistline” point hw and the lowerboundary of the pulsating region ηpl, as well as the tipof the stoichiometric contour hs and the upper bound-ary of the pulsating region ηph for all the flame casestested. Even the predicted lean flammability limitdata HL have good agreements with the measurementextinction point data ηex for all the cases. Compar-isons of the measured and predicted results in Ta-ble 1 indicate that the measured maximum “waistline”

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Fig. 5. Simultaneous two-dimensional OH image superposed with Rayleigh scattering image of the 30% nitrogen-dilutedmethane flame in the pulsating region.

point hw and the onset of receding hs takes place atthe axial position slightly upstream of the theoreticalpredictions (Hw and Hs) for all inert-diluted methaneand propane flames. Though the model slightly over-predicted these parameters, fair agreement betweenthe predicted and measurement data is achieved.

The propagation and formation of a lifted non-premixed jet flame was studied by Lyons and Wat-son [34]. It is believed that the propagation of a liftedflame and flame shape is related to the stoichiomet-ric mass fraction contour Ys. For the blowout ofa jet flame, based on the premixed model, Vanquick-enborne and van Tigglen [1] have shown that theblowout process can be triggered when the flame baseis pushed downstream where the stoichiometric massfraction Ys contour reaches its maximum radial width.In view of the good correspondence between hw andηpl, it becomes reasonable to consider the locationwhere the Ys contour reaches its maximum radialwidth as a dividing point separating the stable andunstable regions. Instability in terms of pulsation usu-ally takes place in a flame during the blowout process.The appearance of the pulsating instability may trig-ger the blowout process. In addition, the correspon-dence between hs and ηph may imply that the on-set of receding in the blowout process has a strongconnection with the diminution of the stoichiometricconcentration and the fuel-lean condition plays an im-portant role in the flame recession and extinction ofthe blowout process, as the agreement between HLand ηex may imply.

To verify the level of fuel/air mixing and to iden-tify the reaction zone near the flame base, simulta-neous measurements of LIPF-OH superposed withRayleigh images of the flame base are performed.Single-shot images of Rayleigh and OH for the 30%nitrogen-diluted methane flame in the pulsating re-gion are shown in Fig. 5. With regard to the noise inRayleigh images, the Mie scattering from surround-

ing dust cannot be avoided completely. In a Rayleighimage, the distinct noise dots can be identified withsimple image-processing principles and are removedin the calculation. In other words, the calculation isbased mainly on regions where noise dots do not exist.Despite these limitations of the Rayleigh technique,it is still the best tool to image the distribution offuel/air mixing levels in nonreacting areas. In addi-tion, since the flame thickness in the jet flame baseis thin and the intensity is weak, the LIPF-OH im-age becomes a little bit noisy. Here, we have no in-tention of using the Rayleigh scattering signals fortemperature measurements [35], as the variation ofthe Rayleigh cross section between fuel and air isvery large in the current flames. Simultaneous mea-surements of Rayleigh and OH images provide usefulinformation about fuel/air mixing and its associatedreaction zones. Fig. 5 indicates that fuel and air arewell mixed to stoichiometric levels or even lean con-ditions before approaching the flame base. Similarresults of inert-diluted propane flames are also found,but the images are less noisy because the Rayleighcross-section of methane is much smaller than thatof propane. Large cross-sectional differences betweenfuel and air offer better signal-to-noise ratio and re-duce measurement errors. For simultaneous measure-ments of Rayleigh and OH images, the heat releasefrom the flame base [36,37] may affect the proper-ties of upstream unburned reactants near the flamebase. It is well known that the Rayleigh scattered sig-nal depends on the composition and temperature. Ifthe composition is fixed, the signal is inversely pro-portional to the temperature. Namely, the heat releasemay cause underprediction of equivalence ratio. How-ever, as compared with the major length scale in thejet flame blowout process, which is described in termsof the stoichiometric contour, the length scale of theheated region is much smaller and the induced error

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due to streamline divergence of the heated region canbe neglected for the present purpose.

3.4. Simultaneous fuel/air characteristics nearflame gbase

To further understand the relationship betweenflow velocity and flame base propagation, typical

plots of simultaneous PIV velocity distributions aheadof the flame zone (marked by dashed lines) forthe 30% nitrogen-diluted methane flame in the typ-ical pulsating, onset of receding, and receding re-gions are shown in Figs. 6a–6c, respectively. Simi-larly to the method used by Schefer and Goix [10]and Muñiz and Mungal [7], the decrease in particledensity and scattering cross section in flame can be

Fig. 6. Velocity distribution ahead of flame base of the 30% nitrogen-diluted methane flame: (a) 14 < x/d < 26; (b) 27 <

x/d < 39; (c) 41 < x/d < 53.

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Fig. 6. (continued)

used as a marker for the high-temperature zone. InFig. 6a the velocity of the unburned gases of the cen-terline at this height is about 8 m/s and decreaseswith x/d . The velocity just ahead of the flame basedecreases approximately to the laminar burning ve-locity of a few tens of centimeters per second. InFig. 6b, the flame base is located at the onset posi-tion of recession, and the velocity just ahead of theflame base is also approximately equal to the laminarburning velocity of the 30% nitrogen-diluted methaneflame. However, in Fig. 6c, the velocity ahead of theflame base is about 2–3 m/s higher than the lam-inar burning velocity. Similar results for the 30%argon-diluted methane flame and the 50% nitrogen-and argon-diluted propane flames are also obtained.Based on the PIV measurements, the velocity distri-butions at typical heights of the stabilization point inthe three characteristic blowout regions can be cal-culated. Typical results for the 30% nitrogen-dilutedmethane flames are shown in Fig. 7. Briefly, whenthe flame base is located below the onset position ofrecession, the flame base always stabilizes at a po-sition where local velocities are around the laminarburning velocity (∼0.8–3 Su) and the flame may pul-sate at a frequency of roughly 1 Hz. However, whenthe flame base is pushed downstream beyond the on-set position of recession, the stabilization point ofthe flame moves toward the center of the jet, wherethe flame suffers from much higher flow velocityand becomes thinner and weaker. In addition, in therecession region the local velocity at the flame stabi-lization point increases with the height until the flame

vanishes. These phenomena are also found in otherinert-diluted flames.

The triple flame structure of turbulent lifted flamesis often distorted by vortices [38] so that the threebranches may not always be found. For the purposeof the present study, we do not intend to involve indetailed diagnostics of concentration and velocity oftriple flame (edge flame) structure; instead, resultsfrom others, such as Lyons and Watson [34] for con-centration and Muñiz and Mungal [7] for velocity, areborrowed to compare with the present results. Due tothe high distortion of the flame base, especially in theblowout process, and the difficulty of observation viaplanar laser diagnostics, the major evidence of exis-tence of a triple flame structure in turbulent flame basemay be found from propagation velocity. We havefound that the velocity just ahead of the flame baseis approximately equal to the laminar burning veloc-ity when the flame base is located within the tip of thestoichiometric contour, which agrees with results ofMuñiz and Mungal [7] and the triple flame model ofKioni et al. [9] and Ruetsch et al. [36]

3.5. The blowout mechanism

Similar to the results reported in several previousstudies using PIV [7–10], important evidence that thefluid velocity just ahead of the lifted flame base islow, close to the laminar flame speed, and the factthat the velocity decreases continuously to the laminarflame velocity along the stream up to the triple pointof the triple flame have been described in this study.

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Fig. 7. Velocity distribution just ahead of the 30% nitrogen-diluted methane flame at different heights.

Moreover, in our previous study [15] of an identicalflame configuration, we clearly identified the tripleflame structure in the OH images. Hence, in the pul-sating region, a triple flame structure is identified witha propagation velocity very close to the laminar flamespeed near the stabilization point of the triple flame atthe flame base. The flame base in the pulsating regionis found mostly in the axial locations corresponding tothe range from the maximum “waistline” point to thetip of the stoichiometric “elliptic” contour. However,in the receding region, the flame base is pushed down-stream and follows local flow velocity. Furthermore,the results of simultaneous fuel/air mixing level mea-surements show that the flame base propagates alongthe stoichiometric layer in the pulsating region. Theseinstantaneous and simultaneous 2D mixing level andvelocity results provide specific and sound evidenceto verify the existence of the triple flame structure andthe stabilization point of the triple flame [7] serves toprovide a suitable flame base stabilization mechanismin the pulsating region during the blowout of turbulentjet flames.

The above findings of the flame base behaviorand evolution characteristics in the blowout processcan be used to construct the blowout mechanism ofa turbulent jet diffusion flame. To illustrate the mech-anism of stabilization, propagation, and breakdown of

the triple flame structure in a lifted jet flame in theblowout process, the bold dashed and dotted curvesdenoting contours of the stoichiometric level and thelean flammable limit are compared in Fig. 8, and thecorresponding heights, Hs and HL, as well as the richflammable limit contour are also shown in Fig. 8.

In the proposed mechanism, if the lifted flame isinitially stabilized in the range upstream of the max-imum “waistline” point of the stoichiometric contourwhile the jet exit velocity (Uo) is between the liftoffvelocity (Ul) and the blowout velocity (Ub) (Fig. 8b),the lifted flame is stable and remains lifted (Vanquick-enborne and van Tigglen [1]). In this region, verygood agreement between experimental and theoreti-cal estimates of the liftoff heights has been reportedin previous work [39–42]. In general, the liftoff heightis proportional to jet exit velocity (Uo) and inverselyproportional to the square of the maximum laminarflame speed. As shown in Fig. 8c, if the flame baseof the lifted flame is pushed beyond the maximum“waistline” point while the jet exit velocity is equalto the blowout velocity, the flame will step into thepulsating region of the blowout process and becomeunstable. In either the stable or unstable region, asshown in Figs. 8b and 8c, the jet flame is hollowin structure. In the pulsating region, the triple flamestructure is found in the flame base and the fast sto-

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Fig. 8. Schematic of the proposed blowout process mechanism.

ichiometric branch of the triple flame in the flamebase, which moves along the stoichiometric contour,serves as the stabilization point of the triple flame toprovide the essential element to stabilize the flame.The flame base is bounded by the boundaries of leanand rich limits. The pulsating region corresponds tothe axial range from the maximum “waistline” pointto the tip of the stoichiometric contour. The unsta-ble counterbalance of the local flow velocity and theflame propagation speed at the stabilization point ofthe triple flame result in whole flame pulsation in thepulsating region. As the flame base is pushed down-stream, the flame base moves toward the center alongthe contour and suffers from higher flow velocity. Assoon as the flame is pushed by the flow beyond the tipof the stoichiometric contour (Fig. 8d), the stabiliza-tion point and the stoichiometric branch of the tripleflame structure vanish and the flame becomes lean.In this region, the hollow-cone structure disappearsand the jet flame base becomes disklike. The flamerecedes downstream continuously and finally extin-guishes (Fig. 8e).

Triple flame stabilization plays an important rolein flames in stable liftoff in the pulsating regionsof the blowout process. The stabilization and pul-sation of the stabilization point of the triple flamealong the stoichiometric contour constitute the ma-

jor dynamic behavior in the pulsating region of theblowout process. The stoichiometric contour can bedetermined by the initial gas properties and the initialvelocity at the jet exit. Therefore, for a turbulent flamenot only the blowout limit [15] but also the blowoutprocess and the accompanied dynamic behaviors canbe estimated and characterized based on the initial ve-locity/Reynolds number and gas properties at the jetexit.

4. Conclusion

Through phenomenological observation and de-tailed measurements of the mixing and velocity distri-butions using 2D Rayleigh scattering, 2D LIPF-OH,and PIV techniques, a blowout mechanism is pro-posed to delineate the dynamic flame-base behaviorand evolution characteristics in each characteristic re-gion, i.e., the pulsating, onset of receding, receding,and extinction regions, of the blowout process of a tur-bulent jet diffusion flame. The mechanism is primarybased on the findings that triple flame structures arefound in the flame base in the pulsating and onset ofreceding regions and the correspondence of the flamebase locations in each region with the stoichiometricand lean limit contours of the premixed model. Thestabilization and pulsation of the stabilization point

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of the triple flame along the stoichiometric contourconstitute the major dynamic behavior in the pulsat-ing region and the diminution of the stoichiometricbranch of the triple flame and the fuel-lean conditionlead to the recession and extinction of the flame in theblowout process. The proposed blowout mechanismbased on triple flame and stoichiometric contour alsoprovides an explanation for the fact that the blowoutprocess of a turbulent jet diffusion flame can be es-timated and characterized based on the initial veloc-ity/Reynolds number and gas properties at the jet exitwithout knowing the local flame/flow conditions ofthe liftoff flame near blowout.

Acknowledgment

Financial support by the National Science Coun-cil, Republic of China, through Projects NSC90-2212-E-006-120, NSC91-2212-E-006-039, and NSC-92-2212-E-006-058 is gratefully acknowledged.

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