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Observations on the leading edge in lifted flame stabilization
Brief Communication Observations on the Leading Edge in Lifted Flame Stabilization K. A. WATSON, K. M. LYONS*
Department of Mechanical and Aerospace Engineering, North Carolina State University, Box 7910, Raleigh, NC, 27695-7910 USA
J. M. DONBAR
Air Force Research Laboratory, Wright-Patterson Air Force Base, Dayton, OH, 45433-7103 USA
and C. D. CARTER
Innovative Scientific Solutions, Inc., Dayton, OH, 45440-3696 USA The objective of this paper is to report some of the first experimental evidence for the “leading edge” flame as the stabilization mechanism in lifted jet diffusion flames [1–5]. CH fluorescence has been used to indicate the flame front location (i.e., region of chemical reaction) and thereby characterize features of the stabilization region [5, 6]. The “leading edge” flame phenomenon reported within refers to the outward-extending branch of CH fluorescence at the base of the streamwise CH zones. Whether the “leading edge” flame is a special case of the more general triple flame is a question which remains unanswered. It is evident from previous computational studies [7, 8] that the triple flame, when interacting with a vortex or pair of vortices, can take on characteristics of the “leading edge” flames introduced in the present study. Veynante et al. [8] illustrate the contortion of the premixed branches of the triple flame by the flowfield where the premixed branches are swept into the trailing diffusion flame. These simulated triple flame/vortex interactions are consistent with the results of this study which show a trailing diffusion flame and the leading edge reaction zone structure. © 1999 by The Combustion Institute
The test conditions and measurement locations for this investigation are shown in Fig. 1. The axisymmetric burner consists of a 5-mm inner diameter fuel jet surrounded by a 150-mm i.d. coflow tube. Methane is delivered through the fuel jet, while low-speed air (⬃0.15 m/s) passes through the coflow annulus. The stabilization regions of three lifted flames are investigated by varying the methane and air flow rates and adjusting the burner position accordingly so that the image region includes the leading edge of the reaction zone. The methane exit velocities are 15.8, 21.2, and 27.5 m/s, corresponding to jet Reynolds numbers of 4800, 6400, and 8300, respectively. Both sides of the lifted flame are imaged during the two lower flow rates (Fig. 1a) while the turbulent fluctuations and wider stabilization region resulting from the highest flow * Corresponding author. E-mail: [email protected] COMBUSTION AND FLAME 119:199 –202 (1999) © 1999 by The Combustion Institute Published by Elsevier Science Inc.
rate limit this case to one side of the flame (Fig. 1b). The CH planar laser-induced fluorescence (CH-PLIF) technique has been described elsewhere [5, 6]. The setup includes a Nd:YAGpumped dye laser which excites the Q1(7.5) transition of the B2⌺⫺–X2(0,0) band of CH at ⫽ 390 nm. Fluorescence from the A–X(1,1), (0,0) and B-X(0,1) bands between ⫽ 420 and 440 nm is recorded. This approach has resulted in acceptable CH signal levels and excellent image quality (i.e., spatial resolution and contrast), which the authors find superior to the 431.5 nm laser excitation employed in earlier studies [9, 10]. The authors believe the excellent resolution resulting from the signal levels and laser sheet characteristics in this study are extremely important in uncovering the leading edge premixed branch, which is generally weaker in signal level than the trailing diffusion flame. It is likely that these stabilizing leading 0010-2180/99/$–see front matter PII S0010-2180(99)00056-5
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Fig. 1. Test conditions and measurement locations for (a) the lowest flow rate and (b) the highest flow rate. The intermediate flow rate (not shown) corresponds to a methane velocity of 21.2 m/s while the bottom of the image region is 37.4 mm from the jet exit and includes both sides of the lifted flame.
edge flame observations are not reported in studies with less spatial resolution or are possibly not at all detectable due to limitations in the specific CH excitation/detection scheme. Several diagnostic studies involving lifted flames present the lifted flame structure as a continuous flame surface, similar to a distorted cylindrical object, emanating from a ringshaped structure where the flame is stabilized [4, 5, 11]. Most previous work, however, does not give experimental evidence of the mechanism of lifted flame stabilization. Figure 2 consists of several instantaneous 35.1 mm ⫻ 23.4 mm CH-PLIF images which provide such evidence. The images clearly show a continuous vertical distribution of CH which represents the primary diffusion flame reported in many previous studies. In addition to the vertical trailing diffusion flame, a structure is witnessed near the flame base which curls toward the outside, or fuel-lean, portion of the reaction zone. In comparison to ideal, laminar tribrachial structures, evidence of both rich and lean branches of premixed flame is not present, only the one branch extending outward near the jet edge.
However, the rich branch on the fuel side of the diffusion flame may be overlapped into the diffusion flame by the flowfield as illustrated by Veynante et al. [8]. It is believed that the branch in the CH zone is a leading edge flame, stabilized by opposing the flow in the relatively low-speed region (⬃1.0 m/s) near the outside edge of the jet. This branch of CH is not obviously present in all of the data; it only appears in approximately 30% of the images. The authors reason that the leading edge phenomenon may not be present around all 360 degrees of the stabilizing “ring,” but only in a portion of the flame sufficient to generate enough thermal energy to stabilize the flame globally. The reader must keep in mind that while the laser imaging techniques allow one to investigate the flame in detail locally, global behavior, most notably out-of-sheet activity, may be dominant at any given instant. With the flow inherently three-dimensional and time-dependent [11, 12], sheet imaging techniques often provide data to support a theory, but rarely provide definitive evidence. Since the measurements only investigate the flowfield in
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Fig. 2. CH-PLIF images illustrating the leading edge phenomenon where the CH zone extends outward at the stabilization point. (a)–(c) are from the lowest Re ⫽ 4800 flow condition (Fig. 1a); (d)–(h) are from the intermediate Re ⫽ 6400 flow condition; (i)–(l) are from the highest Re ⫽ 8300 flow condition and only include the right side of the flame (Fig. 1b).
one plane, it is feasible that the leading edge structure could be present outside the measurement slice during the instances when no premixed CH branch is witnessed. In addition, recent cross-sectional images of lifted flames near the stabilization zone clearly render three-
dimensional lobed structures that are consistent with this theory [11, 12]. The extent of mixing and the entrainment of ambient air into the fuel is of central importance to this problem. Based on comparisons with mixture fraction images presented by
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Stårner et al. [13], which illustrate that the portion at the base of a lifted methane flame has a flammable composition, the authors are confident that the leading edge flame lies in a flammable mixture fraction region. Furthermore, fluctuations in the axial location of the leading edge, along with its orientation relative to the trailing diffusion flame, imply axial propagation into the unburned gas region. These observations imply that the physics of flame stabilization is likely a combination of multiple mechanisms based on premixedness, strain rate considerations [6, 14], and flame propagation into nonhomogeneous flowfields with flow separation, scalar gradients, and a range of mixture fractions [13].
2. 3. 4. 5. 6. 7. 8.
9. 10.
11.
This work has been supported by the National Science Foundation (Grant CTS-96-12740) and the Air Force Research Laboratory, Wright-Patterson Air Force Base, OH, under AFOSR Contract No. F33615-97-C-2702, Dr. Julian Tishkoff, Contract Monitor.
12. 13.
14.
Pitts, W. M., Combust. Flame 76:197–212. (1989). Mun ˜iz, L., and Mungal, M. G., Combust. Flame, 111:16 –31 (1997). Schefer, R. W., and Goix, P. J., Combust. Flame 112:559 –574 (1998). Watson, K. A., Lyons, K. M., Donbar, J. M., and Carter, C. D., Combust. Flame 117:257–271 (1991). Carter, C.D., Donbar, J. M., and Driscoll, J. F., Appl. Phys. B. 66:129 –132 (1998). Ruetsch, G. R., Vervisch, L., and Lin ˜´an, A., Phys. Fluids 7:1447–1454 (1995). Veynante, D., Vervisch, L., Poinsot, T., Lin ˜´an, A., and Ruetsch, G. R., Proceedings of the Summer Program, Stanford/Ames Center of Turbulence Research, 1994. Namazian, M., Schmitt, R. L., and Long, M. B., Appl. Optics 27:3597–3600 (1998). Cattolica, R. J., Stepowski, D., Puechberty, D., and Cottereau, M., Quant. Spectrosc. Radiat. Transfer 32: 363–370 (1984). Liepmanr, D., and Gharib, M., J. Fluid Mech. 245:643– 668 (1992). Schefer, R. W., Combust. Sci. Technol. 125:371–394 (1997). Stårner, S. H., Bilger, R. W., Frank, J. H., Marran, D. F., and Long, M. B., Combust. Flame 107:307–313. (1996). Hasselbrink, E. F., and Mungal, M. G., Twenty-Seventh Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, to appear.
REFERENCES 1.
Pitts, W. M., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, pp. 809 – 816.
Received 30 November 1998; revised 26 February 1999; accepted 21 April 1999
Department of Mechanical and Aerospace Engineering, North Carolina State University, Box 7910, Raleigh, NC, 27695-7910 USA
J. M. DONBAR
Air Force Research Laboratory, Wright-Patterson Air Force Base, Dayton, OH, 45433-7103 USA
and C. D. CARTER
Innovative Scientific Solutions, Inc., Dayton, OH, 45440-3696 USA The objective of this paper is to report some of the first experimental evidence for the “leading edge” flame as the stabilization mechanism in lifted jet diffusion flames [1–5]. CH fluorescence has been used to indicate the flame front location (i.e., region of chemical reaction) and thereby characterize features of the stabilization region [5, 6]. The “leading edge” flame phenomenon reported within refers to the outward-extending branch of CH fluorescence at the base of the streamwise CH zones. Whether the “leading edge” flame is a special case of the more general triple flame is a question which remains unanswered. It is evident from previous computational studies [7, 8] that the triple flame, when interacting with a vortex or pair of vortices, can take on characteristics of the “leading edge” flames introduced in the present study. Veynante et al. [8] illustrate the contortion of the premixed branches of the triple flame by the flowfield where the premixed branches are swept into the trailing diffusion flame. These simulated triple flame/vortex interactions are consistent with the results of this study which show a trailing diffusion flame and the leading edge reaction zone structure. © 1999 by The Combustion Institute
The test conditions and measurement locations for this investigation are shown in Fig. 1. The axisymmetric burner consists of a 5-mm inner diameter fuel jet surrounded by a 150-mm i.d. coflow tube. Methane is delivered through the fuel jet, while low-speed air (⬃0.15 m/s) passes through the coflow annulus. The stabilization regions of three lifted flames are investigated by varying the methane and air flow rates and adjusting the burner position accordingly so that the image region includes the leading edge of the reaction zone. The methane exit velocities are 15.8, 21.2, and 27.5 m/s, corresponding to jet Reynolds numbers of 4800, 6400, and 8300, respectively. Both sides of the lifted flame are imaged during the two lower flow rates (Fig. 1a) while the turbulent fluctuations and wider stabilization region resulting from the highest flow * Corresponding author. E-mail: [email protected] COMBUSTION AND FLAME 119:199 –202 (1999) © 1999 by The Combustion Institute Published by Elsevier Science Inc.
rate limit this case to one side of the flame (Fig. 1b). The CH planar laser-induced fluorescence (CH-PLIF) technique has been described elsewhere [5, 6]. The setup includes a Nd:YAGpumped dye laser which excites the Q1(7.5) transition of the B2⌺⫺–X2(0,0) band of CH at ⫽ 390 nm. Fluorescence from the A–X(1,1), (0,0) and B-X(0,1) bands between ⫽ 420 and 440 nm is recorded. This approach has resulted in acceptable CH signal levels and excellent image quality (i.e., spatial resolution and contrast), which the authors find superior to the 431.5 nm laser excitation employed in earlier studies [9, 10]. The authors believe the excellent resolution resulting from the signal levels and laser sheet characteristics in this study are extremely important in uncovering the leading edge premixed branch, which is generally weaker in signal level than the trailing diffusion flame. It is likely that these stabilizing leading 0010-2180/99/$–see front matter PII S0010-2180(99)00056-5
200
K. A. WATSON ET AL.
Fig. 1. Test conditions and measurement locations for (a) the lowest flow rate and (b) the highest flow rate. The intermediate flow rate (not shown) corresponds to a methane velocity of 21.2 m/s while the bottom of the image region is 37.4 mm from the jet exit and includes both sides of the lifted flame.
edge flame observations are not reported in studies with less spatial resolution or are possibly not at all detectable due to limitations in the specific CH excitation/detection scheme. Several diagnostic studies involving lifted flames present the lifted flame structure as a continuous flame surface, similar to a distorted cylindrical object, emanating from a ringshaped structure where the flame is stabilized [4, 5, 11]. Most previous work, however, does not give experimental evidence of the mechanism of lifted flame stabilization. Figure 2 consists of several instantaneous 35.1 mm ⫻ 23.4 mm CH-PLIF images which provide such evidence. The images clearly show a continuous vertical distribution of CH which represents the primary diffusion flame reported in many previous studies. In addition to the vertical trailing diffusion flame, a structure is witnessed near the flame base which curls toward the outside, or fuel-lean, portion of the reaction zone. In comparison to ideal, laminar tribrachial structures, evidence of both rich and lean branches of premixed flame is not present, only the one branch extending outward near the jet edge.
However, the rich branch on the fuel side of the diffusion flame may be overlapped into the diffusion flame by the flowfield as illustrated by Veynante et al. [8]. It is believed that the branch in the CH zone is a leading edge flame, stabilized by opposing the flow in the relatively low-speed region (⬃1.0 m/s) near the outside edge of the jet. This branch of CH is not obviously present in all of the data; it only appears in approximately 30% of the images. The authors reason that the leading edge phenomenon may not be present around all 360 degrees of the stabilizing “ring,” but only in a portion of the flame sufficient to generate enough thermal energy to stabilize the flame globally. The reader must keep in mind that while the laser imaging techniques allow one to investigate the flame in detail locally, global behavior, most notably out-of-sheet activity, may be dominant at any given instant. With the flow inherently three-dimensional and time-dependent [11, 12], sheet imaging techniques often provide data to support a theory, but rarely provide definitive evidence. Since the measurements only investigate the flowfield in
LEADING EDGE IN LIFTED FLAME STABILIZATION
201
Fig. 2. CH-PLIF images illustrating the leading edge phenomenon where the CH zone extends outward at the stabilization point. (a)–(c) are from the lowest Re ⫽ 4800 flow condition (Fig. 1a); (d)–(h) are from the intermediate Re ⫽ 6400 flow condition; (i)–(l) are from the highest Re ⫽ 8300 flow condition and only include the right side of the flame (Fig. 1b).
one plane, it is feasible that the leading edge structure could be present outside the measurement slice during the instances when no premixed CH branch is witnessed. In addition, recent cross-sectional images of lifted flames near the stabilization zone clearly render three-
dimensional lobed structures that are consistent with this theory [11, 12]. The extent of mixing and the entrainment of ambient air into the fuel is of central importance to this problem. Based on comparisons with mixture fraction images presented by
202
K. A. WATSON ET AL.
Stårner et al. [13], which illustrate that the portion at the base of a lifted methane flame has a flammable composition, the authors are confident that the leading edge flame lies in a flammable mixture fraction region. Furthermore, fluctuations in the axial location of the leading edge, along with its orientation relative to the trailing diffusion flame, imply axial propagation into the unburned gas region. These observations imply that the physics of flame stabilization is likely a combination of multiple mechanisms based on premixedness, strain rate considerations [6, 14], and flame propagation into nonhomogeneous flowfields with flow separation, scalar gradients, and a range of mixture fractions [13].
2. 3. 4. 5. 6. 7. 8.
9. 10.
11.
This work has been supported by the National Science Foundation (Grant CTS-96-12740) and the Air Force Research Laboratory, Wright-Patterson Air Force Base, OH, under AFOSR Contract No. F33615-97-C-2702, Dr. Julian Tishkoff, Contract Monitor.
12. 13.
14.
Pitts, W. M., Combust. Flame 76:197–212. (1989). Mun ˜iz, L., and Mungal, M. G., Combust. Flame, 111:16 –31 (1997). Schefer, R. W., and Goix, P. J., Combust. Flame 112:559 –574 (1998). Watson, K. A., Lyons, K. M., Donbar, J. M., and Carter, C. D., Combust. Flame 117:257–271 (1991). Carter, C.D., Donbar, J. M., and Driscoll, J. F., Appl. Phys. B. 66:129 –132 (1998). Ruetsch, G. R., Vervisch, L., and Lin ˜´an, A., Phys. Fluids 7:1447–1454 (1995). Veynante, D., Vervisch, L., Poinsot, T., Lin ˜´an, A., and Ruetsch, G. R., Proceedings of the Summer Program, Stanford/Ames Center of Turbulence Research, 1994. Namazian, M., Schmitt, R. L., and Long, M. B., Appl. Optics 27:3597–3600 (1998). Cattolica, R. J., Stepowski, D., Puechberty, D., and Cottereau, M., Quant. Spectrosc. Radiat. Transfer 32: 363–370 (1984). Liepmanr, D., and Gharib, M., J. Fluid Mech. 245:643– 668 (1992). Schefer, R. W., Combust. Sci. Technol. 125:371–394 (1997). Stårner, S. H., Bilger, R. W., Frank, J. H., Marran, D. F., and Long, M. B., Combust. Flame 107:307–313. (1996). Hasselbrink, E. F., and Mungal, M. G., Twenty-Seventh Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, to appear.
REFERENCES 1.
Pitts, W. M., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, pp. 809 – 816.
Received 30 November 1998; revised 26 February 1999; accepted 21 April 1999