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An experimental study of the fuel dilution effect on the propagation of methane–air tribrachial flames
Combustion and Flame 153 (2008) 355–366 www.elsevier.com/locate/combustflame
An experimental study of the fuel dilution effect on the propagation of methane–air tribrachial flames Jeong Il Seo a , Nam Il Kim b , Hyun Dong Shin a,∗ a Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong,
Yuseong-gu, Daejeon 305-701, Republic of Korea b School of Mechanical Engineering, Chung-Ang University, 221 Heukseok-dong, Dongjak-gu, Seoul 156-756,
Republic of Korea Received 6 February 2007; received in revised form 28 August 2007; accepted 31 October 2007 Available online 14 March 2008
Abstract The effects of fuel dilution with nitrogen on the propagation of tribrachial flames were studied experimentally using a multislot burner, which can stabilize lifted flames at low concentration gradients. Three fuel dilutions with nitrogen (N2 0%, 25%, and 50% dilution) were employed. The lift-off height and OH-radical content of the flames were measured using an intensified CCD camera and an OH-PLIF scheme. Regardless of the fuel dilution mole fractions, the lift-off height of the tribrachial flames exhibited U-shaped trends with a minimal value during the increase of the concentration gradients. This implies that the propagation velocity is maximized at a specific concentration gradient regardless of the fuel dilution. Overall, the propagation velocity of the tribrachial flame was reduced by the fuel dilution, and the fuel dilution weakly affected the generation of the diffusion flame. The OH radicals in the diffusion branch became prominently active at the critical concentration gradient and these phenomena were more clearly detected at higher fuel dilution mole fractions. The decrease of the three modes of the OH radicals in a streamwise direction is discussed regarding the relation of the diffusion branch to the propagation velocity of the tribrachial flames. It is suggested that the effect of the diffusion branch on the propagation velocity of tribrachial flames needs to be reconsidered, especially when the concentration gradient is small. © 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Tribrachial (triple) flame; Propagation velocity; Enhancement of OH radical; Fuel dilution; Lift-off height
1. Introduction If a flame propagates in a fuel–air mixing layer with a flammable concentration, a tribrachial (or triple) flame is generated. The tribrachial flame consists of a lean premixed branch (or flame), a rich pre* Corresponding author.
E-mail address: [email protected] (H.D. Shin).
mixed branch, and a diffusion branch. The diffusion branch is generated by excess oxygen and unburned fuel passing through the lean premixed branch and the rich premixed branch, respectively. It is known that the structure of a tribrachial flame and its behaviors are important in investigating flame stabilization, flame extinction, reignition of a turbulent flame, etc. [1]. Phillips [2] first observed the tribrachial flame structure in a methane/air stratified mixing layer. He
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showed that the flame propagation velocity decreased with an increase of the concentration gradient. However, the concentration gradient was not clearly quantified in his experiment. The importance of the tribrachial flame structure in combustion was noted by Dold [3], and he concluded that the propagation speed of tribrachial flames primarily depends on the concentration gradient. Numerous studies on the correlation between the concentration gradient and the propagation velocity of tribrachial flames (PVTF) have since been conducted with both numerical and theoretical methods [4–9]. Compared with numerical and theoretical studies, however, experimental studies have rarely been conducted, due to the difficulty of experimental flame stabilization. In most experiments, tribrachial flames were attached to burner surfaces, and thus the experimental results could not eliminate the interaction between the tribrachial flames and the burners. Three representative experimental approaches are introduced as follows. Kioni et al. [10,11] used a multislot burner that could actively control the concentration gradients. They stabilized lifted tribrachial flames within a slightly diverging channel and measured the PVTF. However, their experimental results showed trends that conflicted with those of many other researchers; i.e., the PVTF increased slightly with an increase of the concentration gradient. Extensive studies on the flame propagation of lifted flames have been undertaken by Plessing et al. [12] and Chung’s research group [13–16]. Chung and co-workers [13–16] used capillary nozzles (usually less than 2 mm in diameter) and analyzed the flame stabilization mechanism using the results of the lift-off height. Using coflow jets with highly diluted propane [15], two distinctive lifted flame stabilization modes (stabilization in the developing region and in the developed region of jets) were observed, depending on the initial fuel mole fraction. In Ko and Chung’s study [16], the PVTF of methane was measured using high-speed shadowgraphy and a Schlieren technique. Despite the quality of their research, employing small-diameter jets could not exclude the nonuniform forced velocity field effect around the flame. Furthermore, the experimental conditions were limited to relatively large concentration gradients. Meanwhile, it is notable that a flame under a zero concentration gradient will become a well premixed flame, not a tribrachial flame, and then the propagation velocity of the premixed flame will be comparable to the laminar burning velocity [17,18]. Therefore, there is great interest in the existence of a transition phase between the premixed flame in a zero concentration gradient and the tribrachial flame in a suitably large concentration gradient.
In our previous studies [17,18], the characteristics of tribrachial flames were investigated in a mixing layer with significantly low concentration gradients (less than 0.01 mm) within a uniform velocity flow field. As a result, we found that critical concentration gradients exist at which the PVTF is maximized both in an open jet [17] and in a confined channel [18]. The primary difference between the open jet and the confined channel experiments can be explained by the enhancement of convective diffusion in the confined channel. In these experimental studies, we were interested in the effect of the diffusion branch reaction on the propagation velocity, which had not yet been considered in analytical studies, and found that the reaction rate along the diffusion branch is enhanced at a critical concentration gradient. Even though our experimental results [17,18] cannot be directly compared with those of Kioni et al. [10], both sets of results show a slight increase of the PVTF with the concentration gradient. The main differences between the two experiments are that Kioni’s experiment was conducted using a diluted fuel and that the experimental range of the concentration gradient corresponding to the increase of the PVTF was quite different. These facts imply that the critical concentration gradients are possibly affected by fuel dilution. If this is true, we can focus on investigating the existence of a critical concentration gradient and the characteristics of the tribrachial flame near the critical concentration gradient by only changing the fuel dilution mole fraction. Therefore, we have investigated the effect of fuel dilution on the tribrachial flame characteristics, i.e., the existence and variation of the critical concentration gradient and the variation of the PVTF. Furthermore, this investigation of the fuel dilution effect may also be helpful to understand the flame structure formulated in many practical combustion systems accompanying the fuel dilution effect, e.g., an exhaust gas recirculation system and a multistep reaction system. To examine the dilution characteristics of fire suppression agents (N2 and CO2 ), Briones et al. [19] adopted laminar lifted methane–air nonpremixed flames (NPFs) and partially premixed flames (PPFs) in a coaxial burner and conducted numerical simulations/experiments to investigate the effects of both fuel stream dilution and partial premixing on flame liftoff, stabilization, and blowout. A balance mechanism was reported between the edge-flame speed in the near-field region (or the triple flame speed in the far-field region) and local scalar dissipation rate and local flow velocity according to both diluents. Also, Lock et al. [20] carried out experiments and simulations to contemplate the effect of CO2 dilution on lift-off height and critical CO2 mole fraction required
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for flame blowout for both air stream and fuel stream. Even though several studies have examined the effectiveness of diluents, most previous studies have focused on extinction through flame blowout. In this study, the open jet experimental method introduced in [17] was adopted because it is currently the most effective method for minimizing the flame– burner interaction, and it is also known that the PVTF evaluated in a confined channel agrees well with the flashback conditions in an open jet experiment [18]. Thus, experimental methods quite similar to those in our previous study [17] were adopted to investigate the fuel dilution effects. The following issues are explored in detail in this study: (1) Variation of the flame stabilization conditions for lifted tribrachial flames with fuel dilution. (2) Variation of the PVTF with fuel dilution. (3) Existence and variation of a critical concentration gradient with fuel dilution. (4) Variation of the diffusion branch characteristics with fuel dilution.
2. Experimental apparatus and method Fig. 1 shows a burner that consists of a multislot, a ceramic honeycomb, and a contraction nozzle. The multislot is composed of 14 slots to generate a linear concentration of the mixture and two additional slots for side nitrogen to insulate the flame from ambient air. The honeycomb and contraction nozzle were installed to create a uniform flow. The experimental coordinates were defined at the nozzle exit: x in the flow direction, y in the direction of the concentration gradient, and z in the depth direction. The cross-sectional area of the nozzle exit was 50(y) × 40(z) mm. The experimental parameters are the mean velocity of the mixture (Vm ), the fuel dilution mole fraction (hereafter “dilution ratio”), and the concentration gradient (mass fraction gradient) of the fuel (∇YF = dYF /dy) at the nozzle exit. Here, the mean velocity is defined as the flow rate divided by the cross-sectional area of the nozzle exit. The dilution ratio is defined as the volume fraction of nitrogen in the reactant mixture of methane (99.95% purity) and nitrogen. Three dilution ratios were used: N2 0%, 25%, and 50%. The flames are stabilized in the velocity potential core and this fact was experimentally reconfirmed in the same manner as in the previous study [17]. In the practical experiment, the equivalence ratio of each slot was controlled and the real fuel mass fractions at the nozzle exit were measured with mass spectroscopy (Hiden HPR20, UK) for the three dilution ratios. The symbol φ in the legend of Fig. 2 indicates the difference in equiv-
Fig. 1. Experimental apparatus (PG: particle generator, MC: mixing chamber, HC: honeycomb plate) reprinted from Kim et al. [17]. Dimensions are in units of mm.
alence ratios of neighboring slots. The difference in equivalence of neighboring slots, φ, was directly controlled in the experiment, and the fuel concentration distribution at the nozzle exit was determined experimentally. The results are shown in Figs. 2a and 2b for dilution ratios N2 25% and N2 50%, respectively. Using the fuel concentration distribution in the y-direction, the fuel concentration gradient (∇YF = dYF /dy) was evaluated near the stoichiometric condition given in Fig. 2. A decrease of ∇YF in the x-direction was also tested and was found to be negligibly small (less than 2%) in 100 mm height. Flame images and lift-off heights were obtained using an intensified CCD camera. Also, planar laserinduced fluorescence (PLIF) techniques were used to detect OH radicals with fuel concentration gradients. The laser pulse for LIF excitation was generated by a second harmonic Nd:YAG laser (500 mJ, 532 nm), and a pumped dye laser with a frequency doubler was
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(a)
(b) Fig. 2. Fuel concentration distribution for (a) N2 25% dilution and (b) N2 50% dilution. φ indicates the difference in equivalence ratios of neighboring slots.
tuned nm to excite the Q1 (6) line of the to 283.0 A2 + ← X 2 + transition with a pulse energy of 12 mJ. The laser beam formed a 50-mm laser sheet on the test section using a cylindrical lens (f = 50 mm) and a convex lens (f = 350 mm). An intensified CCD (512 ×512 pixels) camera (100-ns gate) was equipped with a UG-11 filter and a WG-305 filter. More detailed explanations can be found in our previous studies [17,18].
3. Results and discussion The lift-off characteristics of tribrachial flames with their mean velocities and concentration gradi-
ents were examined for the different dilution ratios (N2 0%, 25%, and 50%). Figs. 3a and 3b show the effect of the concentration gradient on both the flame shape and the lift-off height for the given mean velocities and dilution ratios (Vm = 85.0 cm/s for N2 25% and Vm = 62.5 cm/s for N2 50%; shutter speed = 1/60 s and focal length = 1 m). As the concentration gradient increased, the lift-off height initially decreased and then it increased until blow-off occurred. A rich premixed branch and a lean premixed branch were located on the right and left sides of the diffusion branch, respectively. It was shown that the width of the premixed branch and the radius of the flame curvature decreased as the concentration gradient increased. Also, the luminous intensity of the diffusion branch gradually vanished when the concentration gradient was very low (∇YF < 0.00179 for N2 25%, ∇YF < 0.0021 for N2 50%). The luminous intensities of the N2 25% diluted methane flames (Fig. 3a) were stronger than those of the N2 50% diluted methane flames (Fig. 3b), while the trends in Figs. 3a and 3b were similar. The lift-off heights with concentration gradients and mean velocities for the variations in the dilution ratios are shown in Figs. 4a and 4b. Here, the lift-off height is defined as the distance between the nozzle exit and the tribrachial (or triple) point that has the maximum luminous intensity. To measure the lift-off height, 100 images of the lifted flames were taken for each mean velocity and concentration gradient. All trends of the lift-off height for a fixed mean velocity exhibited similar U-shaped trends with an increase of the concentration gradients; i.e., the lift-off height corresponding to a fixed mean velocity were minimized at its corresponding critical concentration gradient. It has been stated that the decrease and increase of the lift-off height imply an increase and decrease of the flame propagation velocity, respectively [17]. Furthermore, the critical concentration gradients increased as the lift-off heights decreased. Similarly to the experimental phenomenon described in our previous study [17], the critical concentration gradients corresponding to the minimum lift-off heights increase as the lift-off heights decrease. The reason for this shift of the critical concentration gradients was explained in our previous study [17,18]. Briefly, when the tribrachial flame is located close to the nozzle, the flame curvature is enlarged by suppression of the flow divergence due to the forced convective boundary condition of the nozzle, similarly to a flame in a confined channel [18]. Thus, the convective diffusion of oxidant and fuel behind the premixed branches is enhanced, thereby shifting the critical concentration gradient to a richer condition. It is well known from research on wrinkled premixed flames [3,4,7] that the PVTF decreases with an
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(a)
(b) Fig. 3. Flame direct photographs with the concentration gradients (brightness was inverted to distinguish the light intensity): (a) N2 25% dilution, Vm = 85.0 cm/s; (b) N2 50% dilution, Vm = 62.5 cm/s.
increasing concentration gradient due primarily to the stretch effect on premixed branches coupled with the increased flame curvature. This general characteristic of tribrachial flames can be applied to explain the increase of the lift-off heights with a concentration gradient greater than the critical value in our experiment. However, the decrease of the lift-off height with a concentration gradient smaller than the critical value implies that the propagation velocity increases with the concentration gradient. Such results have been reported in only a limited number of experimental studies [10,17,18]. This finding is contrary to most theoretical and numerical results of previous research [4–7]. However, this study also shows that critical concentration gradients exist at which the propagation velocity is maximized despite the fuel dilution. This will be discussed in more detail later. The flame stabilization conditions with the mean velocities and concentration gradients for the different dilution ratios are shown in Fig. 5. Tribrachial flames are stabilized at the position where the propagation velocity corresponds closely with the flow velocity. For a given fixed concentration gradient, flashback into the contraction nozzle occurs with a decrease of the mean velocity and blow-off occurs with an increase of the mean velocity. Thus, the flames could be stabilized between these two conditions of flashback
and blow-off. The stable regimes of the lifted flames become narrower with increasing dilution ratio. In this study, the primary mechanism of flame stabilization is the competition between the flow redirection and the forced boundary conditions at the nozzle exit. The volume expansion in both the premixed and diffusion branches causes flow divergence upstream and the local flow velocity decreases near the tribrachial point. This flow redirection varies the velocity profile of the flow field negatively, i.e., a low velocity at the center stream in front of the tribrachial point and a higher velocity at side streams. These flow fields result in a premixed flame shape that is convex upstream. Thus, the total flame surface and total burning rate of the premixed branch increase. A more detailed explanation has been given in a study of premixed flame propagation in a tube [21]. Thus, the PVTF also increases and the flame front moves upstream to the nozzle exit, while the forced boundary condition at the nozzle exit suppresses the flow redirection induced by the flame itself. As a result, the tribrachial flame is stabilized at the location where the propagation velocity is matched with the flow velocity. Normally, the PVTF is defined as the velocity far upstream where the flame does not affect the flow and is occasionally defined in an infinitely uniform flow
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(a)
(b) Fig. 4. Lift-off heights of tribrachial flames with respect to dilution ratio: (a) N2 25% dilution; (b) N2 50% dilution.
to exclude the effects from a nonreactive flow [4,17]. That is, when the lift-off heights are large enough, the flame stabilization mechanism is broken and blowoff occurs. As a result, the mean velocities under the blow-off conditions in this study are close to the definition of the PVTF. Therefore, from the blow-off lines in Fig. 5, we can conclude again that critical concentration gradients exist at which the PVTF values are maximized despite the fuel dilution. As plotted in Fig. 5, the experiment in a confined channel conducted by Kioni et al. [10] shows a slightly larger PVTF compared with this study, de-
spite the fact that Kioni’s dilution ratio of 55% was slightly larger than the value (50%) used in this study. Also, the experimental fuel concentration gradient was much larger in Kioni’s study than in this one. Several possible reasons for these differences can be suggested; however, one notable fact is that the difference of the configuration only is not a major cause. Kioni’s experiment was conducted in a confined channel and it is different from this study, which was conducted in an open jet. According to our previous study [18], conducted in a confined channel, similarly to Kioni’s experiment, the PVTF in a confined channel was close to the flashback conditions in an open jet experiment, as shown in Fig. 5. However, the deviation between Kioni’s results and the flashback conditions in this study at 50% fuel dilution is too large to be explained solely by the configuration difference. Thus, we suspected other reasons, including an experimental limitation that will be explained later. Furthermore, using blow-off limits corresponding to concentration gradients higher than the critical value in Fig. 5, the PVTFs were extrapolated to zero fuel concentration gradients. Three extrapolated values at the zero concentration gradient (Vp,0 ) for three different dilution ratios were then obtained. These values can be compared √ with the asymptotic analytical value Vp ∼ SL0 ρu /ρb proponed by Ruetsch et al. [4], where SL0 is the unstretched flame speed, ρu and ρb are the densities of the unburned and burned gases, respectively, as shown in Fig. 6. For pure methane flames, the experimental asymptotic value (Vp,0 ∼ 110 cm/s) showed very good agreement with the analysis (Vp,0 ∼ 111 cm/s), as shown in Fig. 6. Here, the laminar burning velocities according to the dilution ratios were evaluated from the PREMIX codes using GRI MECH-3.0 [22]. The overall trends with the dilution ratios showed good agreement. The only difference is that the experimental result was less than the analytical value and the deviation between the experimental and theoretical values increased with the increasing fuel dilution. The variation of the flammability limits accounts for this difference, as this factor was not considered in the theoretical prediction. Generally, the PVTF is affected by the total burning rate within the flammability limits, which are determined by the thermal and concentration boundaries. These flammability limits are affected by the dilution ratio, the configuration of the flame, and the experimental methods; thus, deviation between a practical experiment and analysis is unavoidable. Nevertheless, since the previous analytical model did not consider the variation of the flammability limits, modified models of the PVTF are needed to explain the practical flame behavior. Although developing a new analytical model is beyond the scope of this study,
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Fig. 5. Flame stabilization condition of tribrachial flames with respect to dilution ratio (Kioni et al. [10], Kim et al. [18]).
these experimental results for fuel dilution may be used as reference data. Returning to the existence of a maximum PVTF and a critical concentration gradient, two possible reasons were considered in our previous studies [17,18]. One reason was the limitation of the burner, which cannot produce an ideal tribrachial flame; the other was the effect of the diffusion branch on the flow redirection and the propagation velocity. An ideal tribrachial flame is defined as a tribrachial flame formulated in a uniform flow and with a linear concentration profile that covers all flammable concentration ranges [17]. Due to the limited scale of a practical burner, the outer wings of the premixed branches are gradually eliminated as the concentration gradient decreases excessively. It was also shown that the elimination of the premixed wings also affects the PVTF, and the experimental limit of the concentration gradient to guarantee the formation of an ideal tribrachial flame has been derived [17]; ∇YF,ideal ≡
(φrich −φlean )Ystoi min[Wv ,Wc ] , where φrich is the rich flammable limit, φlean is the lean flammable limit, Ystoi is the stoichiometric fuel mass fraction, Wv and Wc are the
widths of the uniform velocity and linear fuel concentration evaluated from the experimental results, respectively. The experimental limits of the concentration gradient for the ideal tribrachial flame (∇YF,ideal ) are 0.00156 for pure methane, 0.00145 for N2 25% diluted methane, and 0.00119 for N2 50% diluted methane. These results are plotted on a logarithmic scale, as shown in Fig. 7. As the dilution ratio increased, the experimental limit of the concentration gradient decreased. This is primarily due to the reduction of the flammable concentration limits. For this, the flammable limits corresponding to the dilution ratios were evaluated using the data for the flammability limits of the representative fuel gases [23]. The flammable limits decreased slightly with the dilution ratio: φrich = 1.58 and φlean = 0.53 for pure methane,
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Fig. 6. Comparison between experimental asymptotic value and theoretical value of propagation velocity of the tribrachial flame at the extremely low concentration gradient.
φrich = 1.53 and φlean = 0.55 for N2 25% diluted methane, and φrich = 1.44 and φlean = 0.59 for N2 50% diluted methane. In contrast, the critical concentration gradients at which the PVTF has maximum values shifted to higher values with increases in the dilution ratios. This result demonstrates that the critical concentration gradients are affected by fuel dilution. Furthermore, the deviation between the experimental limit of the concentration gradient and the critical concentration gradient increased with the fuel dilution increase. This result strongly supports our postulation that the critical concentration gradient in this study is free from the experimental limitation, and thus the existence of the critical concentration gradient is the nature of the PVTF. In our previous studies [17,18], we considered the effect of the diffusion branch on the propagation velocity of tribrachial flames as the primary possible reason for the existence of a critical concentration gradient. Furthermore, the shift of the critical fuel concentration gradient near the nozzle exit was explained by the enhancement of convective diffusion due to the confined structure of the nozzle (or of the channel), which suppresses flow divergence in front of the tribrachial flame [18]. To investigate the variation of the diffusion branch with the fuel concentration gradient, the OH-radical and temperature of the tribrachial flame were investigated. In this study, an extensive investigation on the structure of the diffusion branch was conducted based on the result that the critical concentration gradient is more clearly deviated from the experimental limitations with fuel dilution. For a pure methane flame, the critical concentration gradient (∇YF,cr ) is
between 0.00199 and 0.0052, which corresponds to the blow-off and flashback conditions, respectively. Similarly, for dilution ratios of 25% and 50%, the critical concentration gradients were between 0.00257 and 0.00795 and between 0.00305 and 0.0053, respectively. In most other research, the effect of the diffusion branch on the PVTF has not been considered in detail. This approach is conditionally reasonable when the concentration gradient is sufficiently large, because the flame stretch effect severely affects the PVTF. When the flame curvature is sufficiently small, however, the curvature of the premixed flame is reduced and the PVTF is less affected by the flame stretch. It is possible that the reaction along the diffusion branch may work as an additional volume source. This would provoke a flow redirection and result in an increase of the PVTF if the reaction along the diffusion branch varied sensitively with the concentration gradient, even though it was small. Our studies have therefore been conducted with a relatively smaller fuel concentration gradient than in other researches. One notable experimental indication in our previous study [17,18] was that a dramatic enhancement of the diffusive reaction can be observed near the tribrachial point depending on the concentration gradient. Additional experimental results supporting the contribution of the diffusion branch to the PVTF have been reported. In this study, the reaction rate along the diffusion branch was investigated again with fuel dilution with the postulate that the contribution of the diffusion branch to the PVTF may be enhanced relative to that of a premixed branch with fuel dilution. Such enhancement seems probable because that the fuel concentration gradient dominating the diffusion reaction can be controlled independent of the dilution ratio. The OH-PLIF method was used to qualitatively investigate the variation of flame intensity in the diffusion branch with the concentration gradient. Although it is known that the OH-radical signal is partially retarded behind a premixed flame, its distribution in each flame branch and its variation along the diffusion branch are valuable for assessing whether an addition reaction is involved. The OH-radical distribution with the concentration gradients was measured with the same optical settings, and its intensity is shown in Fig. 8. Even though the overall OH-radical intensity decreased as the dilution ratio increased, the maximum intensity of the OH-signal is near the tribrachial point in all images. Regardless of the dilution ratio, the diffusion branch was not clearly observed behind the premixed flames at very low concentration gradients, but it became clearer with the increasing concentration gradient.
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Fig. 7. Propagation velocity of the tribrachial flame with respect to dilution ratio.
Using the case of N2 25% dilution, the OH-radical distributions at three horizontal (y-direction) lines located 5, 10, and 20 mm downstream from the tribrachial point are shown in Fig. 9. The concentration gradients were 0.00179, 0.00498, and 0.01022 for Figs. 9a, 9b, and 9c, respectively, and the critical concentration gradient in the same conditions was 0.0052. When the concentration gradients were larger than the critical value, the OH-radical obtained its maximum value along the diffusion branch, as shown in Figs. 9b and 9c. However, when the concentration gradient was less than the critical value, the maximum intensity was observed in the lean premixed branch, as shown in Fig. 9a. The OH-radical distribution shown in Fig. 9a is similar to the experimental results obtained by Kioni et al. [11]. This movement of the maximum intensity from a lean premixed branch to a diffusion branch with an increase of the concentration gradient has rarely been reported. Furthermore, it is shown in Fig. 9 that the OH-signal
along the diffusion branch is less affected than that along the lean premixed branch with the concentration gradient. The variation of the OH-signal along the diffusion branch with the streamwise direction from the tribrachial point is depicted in Fig. 10. Several images of the OH-radical for each case are averaged along the diffusion branches with the maximum intensity in the horizontal direction. Three distinctive decrease modes with the concentration gradient were observed. First, for ∇YF = 0.00179, which is smaller than the critical value of ∇YF,cr = 0.0052, the OHsignal of the diffusion branch gradually decreased with the distance from the tribrachial point similar to a typical OH-signal behind a premixed flame. Second, for ∇YF = 0.00498, near the critical value, the OH-signal of the diffusion branch showed an interesting structure. The OH-signal showed another distinctive peak at the downstream. This implies that an additional reaction occurs along the diffusion branch
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(a)
(b)
(c) Fig. 8. Single pulse images of the fluorescence OH-radical with the concentration gradients: (a) N2 0% dilution, Vm = 100.0 cm/s; (b) N2 25% dilution, Vm = 85.0 cm/s; (c) N2 50% dilution, Vm = 60.0 cm/s.
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Fig. 9. OH-radical intensity with the concentration gradients in the y-direction for N2 25% dilution, Vm = 85.0 cm/s.
Fig. 10. OH-radical intensity along the diffusion branch with the concentration gradients in the x-direction for N2 25% dilution, Vm = 85.0 cm/s.
and its location is slightly separated from that of the premixed branch. Even though this phenomenon of a nonmonotonic increase in the OH-radical was not observed in our previous study [17], a similar experimental observation has been reported by Kioni et al. using N2 55% diluted methane [11]. We believe that the experimental observation of the nonmonotonic increase of the OH-radical signal is possible when two conditions are satisfied. The first condition is that the OH-radical of the premixed branch should be small compared with that of the diffusion branch; the second is that the experimental apparatus can formulate a sufficiently small concentration gradient, less than the critical value. To realize both required conditions, fuel dilution is helpful. Finally, for ∇YF = 0.01022, larger than the critical value, the OH signal of the diffusion branch increases
while the location of the second peak is unclear. Thus, the OH signal approximately maintains its specific level continuously at a certain distance from the tribrachial point. Therefore, a typical structure of the tribrachial flame with one peak in the tribrachial point is formulated when the concentration gradient is sufficiently larger than its critical value. From these results, it has been shown that the diffusion flame is drastically enhanced at the conditions near the critical concentration gradient and the separated peak of the OH-radical signal can be observed along the diffusion branch in suitable conditions. Therefore, the critical concentration gradient for the maximum PVTF is strongly coupled with the generation of a diffusion flame. It was shown again that the normal characteristics of a tribrachial flame are not applicable to a flame formulated at a concentration gradient smaller than the critical value, regardless of the dilution ratio. Therefore, our previous suggestion that the critical concentration gradient can be a criterion for the transition from a premixed flame to a typical tribrachial flame remains valid.
4. Conclusions An experimental investigation of the lift-off characteristics of a tribrachial flame with weak concentration gradients was performed. The contribution of the diffusion branch to the propagation velocity of the tribrachial flame (PVTF) was examined in detail by employing three fuel compositions diluted with nitrogen. We have derived the following conclusions from our research: (1) Fuel dilution reduced the experimental conditions for flame stabilization. However, the trends
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of the flame stabilization conditions and lift-off heights were quite similar. (2) Fuel dilution reduced the PVTF for the same concentration gradient. The PVTF was also affected by the velocity profile, particularly near the nozzle exit. (3) The existence of a critical concentration gradient at which the PVTF was maximized was not affected by fuel dilution; however, its value increased with fuel dilution. (4) The enhancement of the OH-radicals near the maximum propagation velocity was more clearly detected in higher fuel dilution mole fractions (dilution ratio). Furthermore, a nonmonotonic increase of the OH signal along the diffusion branch was observed. Therefore, we have suggested that this critical concentration gradient for the maximum PVTF is a criterion of the transition from a premixed flame to a typical tribrachial flame.
Acknowledgment This research was supported by the Combustion Engineering Research Center (CERC) of the Korea Advanced Institute of Science and Technology (KAIST) in Korea.
References [1] P. Domingo, L. Vervisch, Proc. Combust. Inst. 26 (1996) 233–240. [2] H. Phillips, Proc. Combust. Inst. 10 (1965) 1277–1283. [3] J.W. Dold, Combust. Flame 76 (1989) 71–88. [4] G.R. Ruetsch, L. Vervisch, A. Liñán, Phys. Fluids 7 (1995) 1447–1454.
[5] T. Echekki, J.H. Chen, Combust. Flame 114 (1998) 231–245. [6] H.G. Im, J.H. Chen, Combust. Flame 119 (1999) 436– 454. [7] Y. Chen, R.W. Bilger, Combust. Flame 123 (2000) 23– 45. [8] S. Ghosal, L. Vervisch, J. Fluid Mech. 415 (2000) 227– 260. [9] J. Chen, T. Echekki, Combust. Theory Model. 5 (2001) 499–515. [10] P.N. Kioni, B. Rogg, K.N.C. Bray, A. Liñán, Combust. Flame 95 (1993) 276–290. [11] P.N. Kioni, K.N.C. Bray, D.A. Greenhalgh, B. Rogg, Combust. Flame 116 (1999) 192–206. [12] T. Plessing, P. Terhoeven, N. Peters, M.S. Mansour, Combust. Flame 115 (1998) 335–353. [13] S.H. Chung, B.J. Lee, Combust. Flame 86 (1991) 62– 72. [14] B.J. Lee, S.H. Chung, Combust. Flame 109 (1997) 163–172. [15] S.H. Won, S.H. Chung, M.S. Cha, B.J. Lee, Proc. Combust. Inst. 28 (2000) 2093–2099. [16] Y.S. Ko, S.H. Chung, Combust. Flame 118 (1999) 151– 163. [17] N.I. Kim, J.I. Seo, K.C. Oh, H.D. Shin, Proc. Combust. Inst. 30 (2005) 367–374. [18] N.I. Kim, J.I. Seo, Y.T. Guahk, H.D. Shin, Combust. Flame 146 (2006) 168–179. [19] A.M. Briones, S.K. Aggarwal, V.R. Katta, Phys. Fluids 18 (2006) 043603. [20] A.J. Lock, A.M. Briones, S.K. Aggarwal, I.K. Puri, U. Hegde, Combust. Flame 149 (2007) 340–352. [21] N.I. Kim, K. Maruta, Combust. Flame 146 (2006) 283– 301. [22] G.P. Smith, D.M. Golden, M. Frenklach, N.W. Moriarty, B. Eiteneer, M. Goldenberg, C.T. Bowman, R.K. Hanson, S. Song, W.C. Gardiner, V.V. Lissianski, Z. Qin, GRI 3.0 Chemical Kinetic Mechanism, http://www.me.berkeley.edu/gri-mech. [23] B. Lewis, G. von Elbe, Combustion, Flames and Explosions of Gases, Academic Press, USA, 1987, p. 711.
An experimental study of the fuel dilution effect on the propagation of methane–air tribrachial flames Jeong Il Seo a , Nam Il Kim b , Hyun Dong Shin a,∗ a Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, 373-1 Guseong-dong,
Yuseong-gu, Daejeon 305-701, Republic of Korea b School of Mechanical Engineering, Chung-Ang University, 221 Heukseok-dong, Dongjak-gu, Seoul 156-756,
Republic of Korea Received 6 February 2007; received in revised form 28 August 2007; accepted 31 October 2007 Available online 14 March 2008
Abstract The effects of fuel dilution with nitrogen on the propagation of tribrachial flames were studied experimentally using a multislot burner, which can stabilize lifted flames at low concentration gradients. Three fuel dilutions with nitrogen (N2 0%, 25%, and 50% dilution) were employed. The lift-off height and OH-radical content of the flames were measured using an intensified CCD camera and an OH-PLIF scheme. Regardless of the fuel dilution mole fractions, the lift-off height of the tribrachial flames exhibited U-shaped trends with a minimal value during the increase of the concentration gradients. This implies that the propagation velocity is maximized at a specific concentration gradient regardless of the fuel dilution. Overall, the propagation velocity of the tribrachial flame was reduced by the fuel dilution, and the fuel dilution weakly affected the generation of the diffusion flame. The OH radicals in the diffusion branch became prominently active at the critical concentration gradient and these phenomena were more clearly detected at higher fuel dilution mole fractions. The decrease of the three modes of the OH radicals in a streamwise direction is discussed regarding the relation of the diffusion branch to the propagation velocity of the tribrachial flames. It is suggested that the effect of the diffusion branch on the propagation velocity of tribrachial flames needs to be reconsidered, especially when the concentration gradient is small. © 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved. Keywords: Tribrachial (triple) flame; Propagation velocity; Enhancement of OH radical; Fuel dilution; Lift-off height
1. Introduction If a flame propagates in a fuel–air mixing layer with a flammable concentration, a tribrachial (or triple) flame is generated. The tribrachial flame consists of a lean premixed branch (or flame), a rich pre* Corresponding author.
E-mail address: [email protected] (H.D. Shin).
mixed branch, and a diffusion branch. The diffusion branch is generated by excess oxygen and unburned fuel passing through the lean premixed branch and the rich premixed branch, respectively. It is known that the structure of a tribrachial flame and its behaviors are important in investigating flame stabilization, flame extinction, reignition of a turbulent flame, etc. [1]. Phillips [2] first observed the tribrachial flame structure in a methane/air stratified mixing layer. He
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showed that the flame propagation velocity decreased with an increase of the concentration gradient. However, the concentration gradient was not clearly quantified in his experiment. The importance of the tribrachial flame structure in combustion was noted by Dold [3], and he concluded that the propagation speed of tribrachial flames primarily depends on the concentration gradient. Numerous studies on the correlation between the concentration gradient and the propagation velocity of tribrachial flames (PVTF) have since been conducted with both numerical and theoretical methods [4–9]. Compared with numerical and theoretical studies, however, experimental studies have rarely been conducted, due to the difficulty of experimental flame stabilization. In most experiments, tribrachial flames were attached to burner surfaces, and thus the experimental results could not eliminate the interaction between the tribrachial flames and the burners. Three representative experimental approaches are introduced as follows. Kioni et al. [10,11] used a multislot burner that could actively control the concentration gradients. They stabilized lifted tribrachial flames within a slightly diverging channel and measured the PVTF. However, their experimental results showed trends that conflicted with those of many other researchers; i.e., the PVTF increased slightly with an increase of the concentration gradient. Extensive studies on the flame propagation of lifted flames have been undertaken by Plessing et al. [12] and Chung’s research group [13–16]. Chung and co-workers [13–16] used capillary nozzles (usually less than 2 mm in diameter) and analyzed the flame stabilization mechanism using the results of the lift-off height. Using coflow jets with highly diluted propane [15], two distinctive lifted flame stabilization modes (stabilization in the developing region and in the developed region of jets) were observed, depending on the initial fuel mole fraction. In Ko and Chung’s study [16], the PVTF of methane was measured using high-speed shadowgraphy and a Schlieren technique. Despite the quality of their research, employing small-diameter jets could not exclude the nonuniform forced velocity field effect around the flame. Furthermore, the experimental conditions were limited to relatively large concentration gradients. Meanwhile, it is notable that a flame under a zero concentration gradient will become a well premixed flame, not a tribrachial flame, and then the propagation velocity of the premixed flame will be comparable to the laminar burning velocity [17,18]. Therefore, there is great interest in the existence of a transition phase between the premixed flame in a zero concentration gradient and the tribrachial flame in a suitably large concentration gradient.
In our previous studies [17,18], the characteristics of tribrachial flames were investigated in a mixing layer with significantly low concentration gradients (less than 0.01 mm) within a uniform velocity flow field. As a result, we found that critical concentration gradients exist at which the PVTF is maximized both in an open jet [17] and in a confined channel [18]. The primary difference between the open jet and the confined channel experiments can be explained by the enhancement of convective diffusion in the confined channel. In these experimental studies, we were interested in the effect of the diffusion branch reaction on the propagation velocity, which had not yet been considered in analytical studies, and found that the reaction rate along the diffusion branch is enhanced at a critical concentration gradient. Even though our experimental results [17,18] cannot be directly compared with those of Kioni et al. [10], both sets of results show a slight increase of the PVTF with the concentration gradient. The main differences between the two experiments are that Kioni’s experiment was conducted using a diluted fuel and that the experimental range of the concentration gradient corresponding to the increase of the PVTF was quite different. These facts imply that the critical concentration gradients are possibly affected by fuel dilution. If this is true, we can focus on investigating the existence of a critical concentration gradient and the characteristics of the tribrachial flame near the critical concentration gradient by only changing the fuel dilution mole fraction. Therefore, we have investigated the effect of fuel dilution on the tribrachial flame characteristics, i.e., the existence and variation of the critical concentration gradient and the variation of the PVTF. Furthermore, this investigation of the fuel dilution effect may also be helpful to understand the flame structure formulated in many practical combustion systems accompanying the fuel dilution effect, e.g., an exhaust gas recirculation system and a multistep reaction system. To examine the dilution characteristics of fire suppression agents (N2 and CO2 ), Briones et al. [19] adopted laminar lifted methane–air nonpremixed flames (NPFs) and partially premixed flames (PPFs) in a coaxial burner and conducted numerical simulations/experiments to investigate the effects of both fuel stream dilution and partial premixing on flame liftoff, stabilization, and blowout. A balance mechanism was reported between the edge-flame speed in the near-field region (or the triple flame speed in the far-field region) and local scalar dissipation rate and local flow velocity according to both diluents. Also, Lock et al. [20] carried out experiments and simulations to contemplate the effect of CO2 dilution on lift-off height and critical CO2 mole fraction required
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for flame blowout for both air stream and fuel stream. Even though several studies have examined the effectiveness of diluents, most previous studies have focused on extinction through flame blowout. In this study, the open jet experimental method introduced in [17] was adopted because it is currently the most effective method for minimizing the flame– burner interaction, and it is also known that the PVTF evaluated in a confined channel agrees well with the flashback conditions in an open jet experiment [18]. Thus, experimental methods quite similar to those in our previous study [17] were adopted to investigate the fuel dilution effects. The following issues are explored in detail in this study: (1) Variation of the flame stabilization conditions for lifted tribrachial flames with fuel dilution. (2) Variation of the PVTF with fuel dilution. (3) Existence and variation of a critical concentration gradient with fuel dilution. (4) Variation of the diffusion branch characteristics with fuel dilution.
2. Experimental apparatus and method Fig. 1 shows a burner that consists of a multislot, a ceramic honeycomb, and a contraction nozzle. The multislot is composed of 14 slots to generate a linear concentration of the mixture and two additional slots for side nitrogen to insulate the flame from ambient air. The honeycomb and contraction nozzle were installed to create a uniform flow. The experimental coordinates were defined at the nozzle exit: x in the flow direction, y in the direction of the concentration gradient, and z in the depth direction. The cross-sectional area of the nozzle exit was 50(y) × 40(z) mm. The experimental parameters are the mean velocity of the mixture (Vm ), the fuel dilution mole fraction (hereafter “dilution ratio”), and the concentration gradient (mass fraction gradient) of the fuel (∇YF = dYF /dy) at the nozzle exit. Here, the mean velocity is defined as the flow rate divided by the cross-sectional area of the nozzle exit. The dilution ratio is defined as the volume fraction of nitrogen in the reactant mixture of methane (99.95% purity) and nitrogen. Three dilution ratios were used: N2 0%, 25%, and 50%. The flames are stabilized in the velocity potential core and this fact was experimentally reconfirmed in the same manner as in the previous study [17]. In the practical experiment, the equivalence ratio of each slot was controlled and the real fuel mass fractions at the nozzle exit were measured with mass spectroscopy (Hiden HPR20, UK) for the three dilution ratios. The symbol φ in the legend of Fig. 2 indicates the difference in equiv-
Fig. 1. Experimental apparatus (PG: particle generator, MC: mixing chamber, HC: honeycomb plate) reprinted from Kim et al. [17]. Dimensions are in units of mm.
alence ratios of neighboring slots. The difference in equivalence of neighboring slots, φ, was directly controlled in the experiment, and the fuel concentration distribution at the nozzle exit was determined experimentally. The results are shown in Figs. 2a and 2b for dilution ratios N2 25% and N2 50%, respectively. Using the fuel concentration distribution in the y-direction, the fuel concentration gradient (∇YF = dYF /dy) was evaluated near the stoichiometric condition given in Fig. 2. A decrease of ∇YF in the x-direction was also tested and was found to be negligibly small (less than 2%) in 100 mm height. Flame images and lift-off heights were obtained using an intensified CCD camera. Also, planar laserinduced fluorescence (PLIF) techniques were used to detect OH radicals with fuel concentration gradients. The laser pulse for LIF excitation was generated by a second harmonic Nd:YAG laser (500 mJ, 532 nm), and a pumped dye laser with a frequency doubler was
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(a)
(b) Fig. 2. Fuel concentration distribution for (a) N2 25% dilution and (b) N2 50% dilution. φ indicates the difference in equivalence ratios of neighboring slots.
tuned nm to excite the Q1 (6) line of the to 283.0 A2 + ← X 2 + transition with a pulse energy of 12 mJ. The laser beam formed a 50-mm laser sheet on the test section using a cylindrical lens (f = 50 mm) and a convex lens (f = 350 mm). An intensified CCD (512 ×512 pixels) camera (100-ns gate) was equipped with a UG-11 filter and a WG-305 filter. More detailed explanations can be found in our previous studies [17,18].
3. Results and discussion The lift-off characteristics of tribrachial flames with their mean velocities and concentration gradi-
ents were examined for the different dilution ratios (N2 0%, 25%, and 50%). Figs. 3a and 3b show the effect of the concentration gradient on both the flame shape and the lift-off height for the given mean velocities and dilution ratios (Vm = 85.0 cm/s for N2 25% and Vm = 62.5 cm/s for N2 50%; shutter speed = 1/60 s and focal length = 1 m). As the concentration gradient increased, the lift-off height initially decreased and then it increased until blow-off occurred. A rich premixed branch and a lean premixed branch were located on the right and left sides of the diffusion branch, respectively. It was shown that the width of the premixed branch and the radius of the flame curvature decreased as the concentration gradient increased. Also, the luminous intensity of the diffusion branch gradually vanished when the concentration gradient was very low (∇YF < 0.00179 for N2 25%, ∇YF < 0.0021 for N2 50%). The luminous intensities of the N2 25% diluted methane flames (Fig. 3a) were stronger than those of the N2 50% diluted methane flames (Fig. 3b), while the trends in Figs. 3a and 3b were similar. The lift-off heights with concentration gradients and mean velocities for the variations in the dilution ratios are shown in Figs. 4a and 4b. Here, the lift-off height is defined as the distance between the nozzle exit and the tribrachial (or triple) point that has the maximum luminous intensity. To measure the lift-off height, 100 images of the lifted flames were taken for each mean velocity and concentration gradient. All trends of the lift-off height for a fixed mean velocity exhibited similar U-shaped trends with an increase of the concentration gradients; i.e., the lift-off height corresponding to a fixed mean velocity were minimized at its corresponding critical concentration gradient. It has been stated that the decrease and increase of the lift-off height imply an increase and decrease of the flame propagation velocity, respectively [17]. Furthermore, the critical concentration gradients increased as the lift-off heights decreased. Similarly to the experimental phenomenon described in our previous study [17], the critical concentration gradients corresponding to the minimum lift-off heights increase as the lift-off heights decrease. The reason for this shift of the critical concentration gradients was explained in our previous study [17,18]. Briefly, when the tribrachial flame is located close to the nozzle, the flame curvature is enlarged by suppression of the flow divergence due to the forced convective boundary condition of the nozzle, similarly to a flame in a confined channel [18]. Thus, the convective diffusion of oxidant and fuel behind the premixed branches is enhanced, thereby shifting the critical concentration gradient to a richer condition. It is well known from research on wrinkled premixed flames [3,4,7] that the PVTF decreases with an
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(a)
(b) Fig. 3. Flame direct photographs with the concentration gradients (brightness was inverted to distinguish the light intensity): (a) N2 25% dilution, Vm = 85.0 cm/s; (b) N2 50% dilution, Vm = 62.5 cm/s.
increasing concentration gradient due primarily to the stretch effect on premixed branches coupled with the increased flame curvature. This general characteristic of tribrachial flames can be applied to explain the increase of the lift-off heights with a concentration gradient greater than the critical value in our experiment. However, the decrease of the lift-off height with a concentration gradient smaller than the critical value implies that the propagation velocity increases with the concentration gradient. Such results have been reported in only a limited number of experimental studies [10,17,18]. This finding is contrary to most theoretical and numerical results of previous research [4–7]. However, this study also shows that critical concentration gradients exist at which the propagation velocity is maximized despite the fuel dilution. This will be discussed in more detail later. The flame stabilization conditions with the mean velocities and concentration gradients for the different dilution ratios are shown in Fig. 5. Tribrachial flames are stabilized at the position where the propagation velocity corresponds closely with the flow velocity. For a given fixed concentration gradient, flashback into the contraction nozzle occurs with a decrease of the mean velocity and blow-off occurs with an increase of the mean velocity. Thus, the flames could be stabilized between these two conditions of flashback
and blow-off. The stable regimes of the lifted flames become narrower with increasing dilution ratio. In this study, the primary mechanism of flame stabilization is the competition between the flow redirection and the forced boundary conditions at the nozzle exit. The volume expansion in both the premixed and diffusion branches causes flow divergence upstream and the local flow velocity decreases near the tribrachial point. This flow redirection varies the velocity profile of the flow field negatively, i.e., a low velocity at the center stream in front of the tribrachial point and a higher velocity at side streams. These flow fields result in a premixed flame shape that is convex upstream. Thus, the total flame surface and total burning rate of the premixed branch increase. A more detailed explanation has been given in a study of premixed flame propagation in a tube [21]. Thus, the PVTF also increases and the flame front moves upstream to the nozzle exit, while the forced boundary condition at the nozzle exit suppresses the flow redirection induced by the flame itself. As a result, the tribrachial flame is stabilized at the location where the propagation velocity is matched with the flow velocity. Normally, the PVTF is defined as the velocity far upstream where the flame does not affect the flow and is occasionally defined in an infinitely uniform flow
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(a)
(b) Fig. 4. Lift-off heights of tribrachial flames with respect to dilution ratio: (a) N2 25% dilution; (b) N2 50% dilution.
to exclude the effects from a nonreactive flow [4,17]. That is, when the lift-off heights are large enough, the flame stabilization mechanism is broken and blowoff occurs. As a result, the mean velocities under the blow-off conditions in this study are close to the definition of the PVTF. Therefore, from the blow-off lines in Fig. 5, we can conclude again that critical concentration gradients exist at which the PVTF values are maximized despite the fuel dilution. As plotted in Fig. 5, the experiment in a confined channel conducted by Kioni et al. [10] shows a slightly larger PVTF compared with this study, de-
spite the fact that Kioni’s dilution ratio of 55% was slightly larger than the value (50%) used in this study. Also, the experimental fuel concentration gradient was much larger in Kioni’s study than in this one. Several possible reasons for these differences can be suggested; however, one notable fact is that the difference of the configuration only is not a major cause. Kioni’s experiment was conducted in a confined channel and it is different from this study, which was conducted in an open jet. According to our previous study [18], conducted in a confined channel, similarly to Kioni’s experiment, the PVTF in a confined channel was close to the flashback conditions in an open jet experiment, as shown in Fig. 5. However, the deviation between Kioni’s results and the flashback conditions in this study at 50% fuel dilution is too large to be explained solely by the configuration difference. Thus, we suspected other reasons, including an experimental limitation that will be explained later. Furthermore, using blow-off limits corresponding to concentration gradients higher than the critical value in Fig. 5, the PVTFs were extrapolated to zero fuel concentration gradients. Three extrapolated values at the zero concentration gradient (Vp,0 ) for three different dilution ratios were then obtained. These values can be compared √ with the asymptotic analytical value Vp ∼ SL0 ρu /ρb proponed by Ruetsch et al. [4], where SL0 is the unstretched flame speed, ρu and ρb are the densities of the unburned and burned gases, respectively, as shown in Fig. 6. For pure methane flames, the experimental asymptotic value (Vp,0 ∼ 110 cm/s) showed very good agreement with the analysis (Vp,0 ∼ 111 cm/s), as shown in Fig. 6. Here, the laminar burning velocities according to the dilution ratios were evaluated from the PREMIX codes using GRI MECH-3.0 [22]. The overall trends with the dilution ratios showed good agreement. The only difference is that the experimental result was less than the analytical value and the deviation between the experimental and theoretical values increased with the increasing fuel dilution. The variation of the flammability limits accounts for this difference, as this factor was not considered in the theoretical prediction. Generally, the PVTF is affected by the total burning rate within the flammability limits, which are determined by the thermal and concentration boundaries. These flammability limits are affected by the dilution ratio, the configuration of the flame, and the experimental methods; thus, deviation between a practical experiment and analysis is unavoidable. Nevertheless, since the previous analytical model did not consider the variation of the flammability limits, modified models of the PVTF are needed to explain the practical flame behavior. Although developing a new analytical model is beyond the scope of this study,
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Fig. 5. Flame stabilization condition of tribrachial flames with respect to dilution ratio (Kioni et al. [10], Kim et al. [18]).
these experimental results for fuel dilution may be used as reference data. Returning to the existence of a maximum PVTF and a critical concentration gradient, two possible reasons were considered in our previous studies [17,18]. One reason was the limitation of the burner, which cannot produce an ideal tribrachial flame; the other was the effect of the diffusion branch on the flow redirection and the propagation velocity. An ideal tribrachial flame is defined as a tribrachial flame formulated in a uniform flow and with a linear concentration profile that covers all flammable concentration ranges [17]. Due to the limited scale of a practical burner, the outer wings of the premixed branches are gradually eliminated as the concentration gradient decreases excessively. It was also shown that the elimination of the premixed wings also affects the PVTF, and the experimental limit of the concentration gradient to guarantee the formation of an ideal tribrachial flame has been derived [17]; ∇YF,ideal ≡
(φrich −φlean )Ystoi min[Wv ,Wc ] , where φrich is the rich flammable limit, φlean is the lean flammable limit, Ystoi is the stoichiometric fuel mass fraction, Wv and Wc are the
widths of the uniform velocity and linear fuel concentration evaluated from the experimental results, respectively. The experimental limits of the concentration gradient for the ideal tribrachial flame (∇YF,ideal ) are 0.00156 for pure methane, 0.00145 for N2 25% diluted methane, and 0.00119 for N2 50% diluted methane. These results are plotted on a logarithmic scale, as shown in Fig. 7. As the dilution ratio increased, the experimental limit of the concentration gradient decreased. This is primarily due to the reduction of the flammable concentration limits. For this, the flammable limits corresponding to the dilution ratios were evaluated using the data for the flammability limits of the representative fuel gases [23]. The flammable limits decreased slightly with the dilution ratio: φrich = 1.58 and φlean = 0.53 for pure methane,
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Fig. 6. Comparison between experimental asymptotic value and theoretical value of propagation velocity of the tribrachial flame at the extremely low concentration gradient.
φrich = 1.53 and φlean = 0.55 for N2 25% diluted methane, and φrich = 1.44 and φlean = 0.59 for N2 50% diluted methane. In contrast, the critical concentration gradients at which the PVTF has maximum values shifted to higher values with increases in the dilution ratios. This result demonstrates that the critical concentration gradients are affected by fuel dilution. Furthermore, the deviation between the experimental limit of the concentration gradient and the critical concentration gradient increased with the fuel dilution increase. This result strongly supports our postulation that the critical concentration gradient in this study is free from the experimental limitation, and thus the existence of the critical concentration gradient is the nature of the PVTF. In our previous studies [17,18], we considered the effect of the diffusion branch on the propagation velocity of tribrachial flames as the primary possible reason for the existence of a critical concentration gradient. Furthermore, the shift of the critical fuel concentration gradient near the nozzle exit was explained by the enhancement of convective diffusion due to the confined structure of the nozzle (or of the channel), which suppresses flow divergence in front of the tribrachial flame [18]. To investigate the variation of the diffusion branch with the fuel concentration gradient, the OH-radical and temperature of the tribrachial flame were investigated. In this study, an extensive investigation on the structure of the diffusion branch was conducted based on the result that the critical concentration gradient is more clearly deviated from the experimental limitations with fuel dilution. For a pure methane flame, the critical concentration gradient (∇YF,cr ) is
between 0.00199 and 0.0052, which corresponds to the blow-off and flashback conditions, respectively. Similarly, for dilution ratios of 25% and 50%, the critical concentration gradients were between 0.00257 and 0.00795 and between 0.00305 and 0.0053, respectively. In most other research, the effect of the diffusion branch on the PVTF has not been considered in detail. This approach is conditionally reasonable when the concentration gradient is sufficiently large, because the flame stretch effect severely affects the PVTF. When the flame curvature is sufficiently small, however, the curvature of the premixed flame is reduced and the PVTF is less affected by the flame stretch. It is possible that the reaction along the diffusion branch may work as an additional volume source. This would provoke a flow redirection and result in an increase of the PVTF if the reaction along the diffusion branch varied sensitively with the concentration gradient, even though it was small. Our studies have therefore been conducted with a relatively smaller fuel concentration gradient than in other researches. One notable experimental indication in our previous study [17,18] was that a dramatic enhancement of the diffusive reaction can be observed near the tribrachial point depending on the concentration gradient. Additional experimental results supporting the contribution of the diffusion branch to the PVTF have been reported. In this study, the reaction rate along the diffusion branch was investigated again with fuel dilution with the postulate that the contribution of the diffusion branch to the PVTF may be enhanced relative to that of a premixed branch with fuel dilution. Such enhancement seems probable because that the fuel concentration gradient dominating the diffusion reaction can be controlled independent of the dilution ratio. The OH-PLIF method was used to qualitatively investigate the variation of flame intensity in the diffusion branch with the concentration gradient. Although it is known that the OH-radical signal is partially retarded behind a premixed flame, its distribution in each flame branch and its variation along the diffusion branch are valuable for assessing whether an addition reaction is involved. The OH-radical distribution with the concentration gradients was measured with the same optical settings, and its intensity is shown in Fig. 8. Even though the overall OH-radical intensity decreased as the dilution ratio increased, the maximum intensity of the OH-signal is near the tribrachial point in all images. Regardless of the dilution ratio, the diffusion branch was not clearly observed behind the premixed flames at very low concentration gradients, but it became clearer with the increasing concentration gradient.
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Fig. 7. Propagation velocity of the tribrachial flame with respect to dilution ratio.
Using the case of N2 25% dilution, the OH-radical distributions at three horizontal (y-direction) lines located 5, 10, and 20 mm downstream from the tribrachial point are shown in Fig. 9. The concentration gradients were 0.00179, 0.00498, and 0.01022 for Figs. 9a, 9b, and 9c, respectively, and the critical concentration gradient in the same conditions was 0.0052. When the concentration gradients were larger than the critical value, the OH-radical obtained its maximum value along the diffusion branch, as shown in Figs. 9b and 9c. However, when the concentration gradient was less than the critical value, the maximum intensity was observed in the lean premixed branch, as shown in Fig. 9a. The OH-radical distribution shown in Fig. 9a is similar to the experimental results obtained by Kioni et al. [11]. This movement of the maximum intensity from a lean premixed branch to a diffusion branch with an increase of the concentration gradient has rarely been reported. Furthermore, it is shown in Fig. 9 that the OH-signal
along the diffusion branch is less affected than that along the lean premixed branch with the concentration gradient. The variation of the OH-signal along the diffusion branch with the streamwise direction from the tribrachial point is depicted in Fig. 10. Several images of the OH-radical for each case are averaged along the diffusion branches with the maximum intensity in the horizontal direction. Three distinctive decrease modes with the concentration gradient were observed. First, for ∇YF = 0.00179, which is smaller than the critical value of ∇YF,cr = 0.0052, the OHsignal of the diffusion branch gradually decreased with the distance from the tribrachial point similar to a typical OH-signal behind a premixed flame. Second, for ∇YF = 0.00498, near the critical value, the OH-signal of the diffusion branch showed an interesting structure. The OH-signal showed another distinctive peak at the downstream. This implies that an additional reaction occurs along the diffusion branch
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(a)
(b)
(c) Fig. 8. Single pulse images of the fluorescence OH-radical with the concentration gradients: (a) N2 0% dilution, Vm = 100.0 cm/s; (b) N2 25% dilution, Vm = 85.0 cm/s; (c) N2 50% dilution, Vm = 60.0 cm/s.
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Fig. 9. OH-radical intensity with the concentration gradients in the y-direction for N2 25% dilution, Vm = 85.0 cm/s.
Fig. 10. OH-radical intensity along the diffusion branch with the concentration gradients in the x-direction for N2 25% dilution, Vm = 85.0 cm/s.
and its location is slightly separated from that of the premixed branch. Even though this phenomenon of a nonmonotonic increase in the OH-radical was not observed in our previous study [17], a similar experimental observation has been reported by Kioni et al. using N2 55% diluted methane [11]. We believe that the experimental observation of the nonmonotonic increase of the OH-radical signal is possible when two conditions are satisfied. The first condition is that the OH-radical of the premixed branch should be small compared with that of the diffusion branch; the second is that the experimental apparatus can formulate a sufficiently small concentration gradient, less than the critical value. To realize both required conditions, fuel dilution is helpful. Finally, for ∇YF = 0.01022, larger than the critical value, the OH signal of the diffusion branch increases
while the location of the second peak is unclear. Thus, the OH signal approximately maintains its specific level continuously at a certain distance from the tribrachial point. Therefore, a typical structure of the tribrachial flame with one peak in the tribrachial point is formulated when the concentration gradient is sufficiently larger than its critical value. From these results, it has been shown that the diffusion flame is drastically enhanced at the conditions near the critical concentration gradient and the separated peak of the OH-radical signal can be observed along the diffusion branch in suitable conditions. Therefore, the critical concentration gradient for the maximum PVTF is strongly coupled with the generation of a diffusion flame. It was shown again that the normal characteristics of a tribrachial flame are not applicable to a flame formulated at a concentration gradient smaller than the critical value, regardless of the dilution ratio. Therefore, our previous suggestion that the critical concentration gradient can be a criterion for the transition from a premixed flame to a typical tribrachial flame remains valid.
4. Conclusions An experimental investigation of the lift-off characteristics of a tribrachial flame with weak concentration gradients was performed. The contribution of the diffusion branch to the propagation velocity of the tribrachial flame (PVTF) was examined in detail by employing three fuel compositions diluted with nitrogen. We have derived the following conclusions from our research: (1) Fuel dilution reduced the experimental conditions for flame stabilization. However, the trends
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of the flame stabilization conditions and lift-off heights were quite similar. (2) Fuel dilution reduced the PVTF for the same concentration gradient. The PVTF was also affected by the velocity profile, particularly near the nozzle exit. (3) The existence of a critical concentration gradient at which the PVTF was maximized was not affected by fuel dilution; however, its value increased with fuel dilution. (4) The enhancement of the OH-radicals near the maximum propagation velocity was more clearly detected in higher fuel dilution mole fractions (dilution ratio). Furthermore, a nonmonotonic increase of the OH signal along the diffusion branch was observed. Therefore, we have suggested that this critical concentration gradient for the maximum PVTF is a criterion of the transition from a premixed flame to a typical tribrachial flame.
Acknowledgment This research was supported by the Combustion Engineering Research Center (CERC) of the Korea Advanced Institute of Science and Technology (KAIST) in Korea.
References [1] P. Domingo, L. Vervisch, Proc. Combust. Inst. 26 (1996) 233–240. [2] H. Phillips, Proc. Combust. Inst. 10 (1965) 1277–1283. [3] J.W. Dold, Combust. Flame 76 (1989) 71–88. [4] G.R. Ruetsch, L. Vervisch, A. Liñán, Phys. Fluids 7 (1995) 1447–1454.
[5] T. Echekki, J.H. Chen, Combust. Flame 114 (1998) 231–245. [6] H.G. Im, J.H. Chen, Combust. Flame 119 (1999) 436– 454. [7] Y. Chen, R.W. Bilger, Combust. Flame 123 (2000) 23– 45. [8] S. Ghosal, L. Vervisch, J. Fluid Mech. 415 (2000) 227– 260. [9] J. Chen, T. Echekki, Combust. Theory Model. 5 (2001) 499–515. [10] P.N. Kioni, B. Rogg, K.N.C. Bray, A. Liñán, Combust. Flame 95 (1993) 276–290. [11] P.N. Kioni, K.N.C. Bray, D.A. Greenhalgh, B. Rogg, Combust. Flame 116 (1999) 192–206. [12] T. Plessing, P. Terhoeven, N. Peters, M.S. Mansour, Combust. Flame 115 (1998) 335–353. [13] S.H. Chung, B.J. Lee, Combust. Flame 86 (1991) 62– 72. [14] B.J. Lee, S.H. Chung, Combust. Flame 109 (1997) 163–172. [15] S.H. Won, S.H. Chung, M.S. Cha, B.J. Lee, Proc. Combust. Inst. 28 (2000) 2093–2099. [16] Y.S. Ko, S.H. Chung, Combust. Flame 118 (1999) 151– 163. [17] N.I. Kim, J.I. Seo, K.C. Oh, H.D. Shin, Proc. Combust. Inst. 30 (2005) 367–374. [18] N.I. Kim, J.I. Seo, Y.T. Guahk, H.D. Shin, Combust. Flame 146 (2006) 168–179. [19] A.M. Briones, S.K. Aggarwal, V.R. Katta, Phys. Fluids 18 (2006) 043603. [20] A.J. Lock, A.M. Briones, S.K. Aggarwal, I.K. Puri, U. Hegde, Combust. Flame 149 (2007) 340–352. [21] N.I. Kim, K. Maruta, Combust. Flame 146 (2006) 283– 301. [22] G.P. Smith, D.M. Golden, M. Frenklach, N.W. Moriarty, B. Eiteneer, M. Goldenberg, C.T. Bowman, R.K. Hanson, S. Song, W.C. Gardiner, V.V. Lissianski, Z. Qin, GRI 3.0 Chemical Kinetic Mechanism, http://www.me.berkeley.edu/gri-mech. [23] B. Lewis, G. von Elbe, Combustion, Flames and Explosions of Gases, Academic Press, USA, 1987, p. 711.