Combustor performance enhancement through direct shear layer excitation

COMBUSTION

AND FLAME

82: 7 5 - 9 2 (1990)

75

Combustor Performance Enhancement Through Direct Shear Layer Excitation K. R. MCMANUS, U. VANDSBURGER, and C. T. BOWMAN Department of Mechanical Engineering, Stanford University, Stanford, CA 94305

Previous studies of turbulent nonreacting shear flows have shown that flow excitation can provide enhanced entrainment and mixing. In the present study, the effects of periodic flow excitation on the performance of a two-dimensional dump combustor were investigated for lean premixed conditions. The flow excitation was in the form of a sinusoidal cross-stream velocity perturbation applied just upstream of the flow separation. The forcing frequencies, chosen such that they corresponded to resonant and off-resonant vortex shedding frequencies indentified in unforced combustion, ranged from 35 to 400 Hz. The effect of forcing on both nonreacting and reacting flowfields was to modulate the formation of vortex structures just downstream of the flow separation. In the nonreacting flowfield, the shear layer spreading rate increased when forcing was applied. In the reacting flow, forcing caused a modulation of the flame structure. Forcing increased the mean CH emission intensity from the flame, which is related to mean volumetric energy release, up to 15%, reduced the rms pressure fluctuation level by up to 30%, and reduced the equivalence ratio at the lean blowoff limit up to 6%. NOx emissions were reduced by up to 20% with forcing. The forcing location and excitation frequency and amplitude are important parameters in gaining effective combustion control. Performance improvements generally increased with increasing excitation amplitude and increasing frequency within the operating constraints of the excitation system. The mean CH emission intensity was found to be proportional to the mean flame surface area, which increased with forcing, suggesting that the observed increase in volumetric energy release was due to an increase in flame area with forcing. The coupling of heat release and the pressure field was investigated using Rayleigh's criterion, and the analysis showed decreased flame driving of the dominant low-frequency modes with forcing applied, resulting in a reduction of the magnitude of rms pressure fluctuations.

INTRODUCTION I n c r e a s i n g d e m a n d s on m o d e r n h i g h - p e r f o r m a n c e propulsion systems with respect t o v o l u m e t r i c heat release and c o m b u s t i o n stability have motivated research c o n c e r n e d with the d y n a m i c control of fluid mechanical processes associated with combustion, Turbulent m i x i n g is a n i m p o r t a n t r a t e - d e t e r m i n i n g process in most practical c o m b u s t i o n systems. The formation o f large-scale coherent structures and their subsequent interactions are k n o w n to play an important role in turbulent transport processes and to have an effect o n m i x i n g rates in turbulent shear flows such as m i x i n g layers, jets, a n d wakes [ 1 , 2 ] . G a i n i n g control over the f o r m a t i o n and evolution o f these structures presents a possible m e a n s to Copyright (~) 1990 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 655 Avenue of the Americas, New York, NY 10010

e n h a n c e the turbulent transport processes affecting m i x i n g and e n t r a i n m e n t rates in these flows and in similar reacting flows. Shear layer control in n o n r e a c t i n g turbulent flows through periodic forcing has b e e n extensively investigated in recent years. In r o u n d jets, periodic s i n g l e - m o d e (axial) excitation has b e e n used to control the frequency o f vortex formation [3-6]. S i n g l e - m o d e forcing can be used to control jet growth by either e n h a n c i n g or suppressing the vortex pairing which occurs in the u n f o r c e d jet. D u a l - m o d e forcing (axial a n d azimuthal), with the p r o p e r c o m b i n a t i o n o f forcing frequencies, amplitudes, and phases, c a n cause the jet to bifurcate ( s e n d i n g vortex rings alternately in two opposite directions) or to b l o o m ( s e n d i n g a shower of rings

0010-2180/90/$3.50

76 in many directions) causing a dramatic increase in jet spreading angle over the unforced case [7, 8]. Periodic forcing of two-dimensional shear layers also has been studied [9-16]. The methods of forcing used in all of these studies were similar in that they produced a flow perturbation which was essentially spatially coherent in the spanwise direction. Control of vortex formation and amalgamation was achieved through low-level monochromatic forcing applied at the upstream end of the developing shear layer. The control of the vortices was most pronounced when the forcing frequency was a subharmonic of the natural shear layer instability frequency. Subharmonic forcing causes vortices which form in the initial shear layer region at a frequency near the natural layer instability frequency to coalesce and form a larger vortex structure with a passage frequency equal to the forcing frequency. These studies have shown that the mean spreading rate of mixing layers can be enhanced through forcing, and in flows with reattachment [9, 16], it was found that the reattachment length could be reduced, The flowfield in many turbulent combustion systems is dominated by large-scale structures as well (see, for example, Refs. [6, 17-23]). Important physical processes which are associated with these structures include mixing and energy release. The unsteady energy release associated with these structures and its influence on combustion stability have been extensively investigated [17-28]. Results from many of these studies suggest a strong coupling between the unsteady energy release rate associated with vortical motion and the acoustic field in the combustor. The resuits from the nonreacting flow studies described above suggest that gaining control over the formation and evolution of these structures in a c o m bustor could improve performance with respect to volumetric energy release and combustion stability. Recent investigations of combustion control through acoustic modulation have been reported [29-32]. These studies were primarily concerned with the suppression of combustion instabilities, and the forcing systems were designed to act directly on the acoustic modes of the facilities. It was found that combustion instabilities could be reduced by actively changing the energy balance

K . R . MCMANUS ET AL. of acoustic waves within the combustor. Schadow et al. [31] reported enhanced mixing when forcing a dump combustor at the bulk mode frequency. In the present study, direct fluid mechanical excitation of the shear layer through periodic forcing is applied to a dump combustor. The objectives of this work are to study the effects of direct shear layer excitation on the overall combustor performance and to understand the mechanism by which the forcing impacts performance. A study of the nonreacting flow using flow visualization and velocity measurements gives an indication of the effects of forcing on the flowfield structure. In the reacting flow tests, the modification of the combustion characteristics through forcing is studied using Schlieren visualization of the flame structure and energy release measurements. Pressure fluctuation measurements are used to determine the stability characteristics of the reacting flow. In addition, pressure measurements in both nonreacting and reacting flows are used to determine the dominant longitudinal acoustic modes of the combustor. EXPERIMENTAL FACILITY The experimental facility used in this study is a subsonic two-dimensional dump combustor (Fig. 1). Ethylene and air are supplied at high pressure and metered with sonic orifices to allow precise control of the flowrates. The ethylene and air streams mix just downstream of the metering oririces and pass through a flow conditioning section fitted with honeycomb and fine mesh screens to straighten the flow and to reduce the scale of the free stream turbulence. The flow passes through a two-dimensional nozzle (2.8:1 contraction ratio) and enters the combustion chamber. The combustor test section is rectangular in cross section (6 cm high and 10 cm wide) and 30 cm in length. The flame is stabilized by means of a flow recirculation zone formed behind a 2 cm high rearward-facing step which blocks one-third of the tunnel cross section. Windows are fitted on the side walls of the combustor test section to allow optical access and the upper and lower walls are water cooled. The products of combustion then pass through an exhaust duct where they are diluted with room air.

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The combustion chamber is operated at a static pressure which was just below atmospheric (0.99 atm) to supply the pressure differential for the air dilution jets. Periodic flow excitation was introduced by means of a loudspeaker-driven cavity. This approach is similar to that used by Fiedler and Mensing [12] for nonreacting flows, and was chosen in order to achieve a wide range of perturbation frequencies and amplitudes. The excitation cavity included a waveguide which directed the perturbation produced by the loudspeaker to a 1.6 mm wide slot, which extended across the entire width of the combustion chamber. For most experiments, the slot was located 3.2 mm upstream of the flow separation; however, some combustion tests were conducted with the slot located 33 mm upstream of the separation to investigate the effect of forcing location on combustor performance. The loudspeaker was driven by an amplified sine wave signal and produced a true sine wave velocity response at the slot for the excitation frequencies used in this study (35-400 Hz). t The perturbation velocity, Up, was uniform across the width of the combustor, providing a spanwise spatially-coherent perturbation to the separating boundary layer. Gain adjustment allowed variation of the perturbation amplitude, t, 'p m,~x, from 0 to 10 m/s. To study the effects of forcing on the structure of the shear layer, flow visualization was per-

formed in the nonreacting flow using a smokewire technique based on light scattering [33]. This technique employed a thin Nichrome wire (0.13 mm diameter) coated with oil droplets which was stretched vertically through the center of the test section near the inlet. Application of an electrical current through the wire heated the oil droplets, causing them to evaporate and create smoke streaks marking the flow streamlines. A strobe light and 35 mm camera, both triggered by time-delay circuits, were used to obtain instantaneous images of the flowfield in the test section. In addition, a single-wire hot-wire anemometer (TSI Model 1050) was employed for mean and fluctuating streamwise velocity measurements in the nonreacting flow and also to characterize the velocity fluctuation at the waveguide exit. To characterize the acoustic field in the cornbustion tunnel under both nonreacting and reacting conditions, fluctuating static pressure measurements were made at six positions along the tunnel (see Fig. 1) to determine the amplitude and relative phase of low-frequency longitudinal acoustic waves. The pressure measurements were made at two locations at a time using the furthest upstream measurement location (tap # 1 ) as the phase reference. In the nonreacting flow, 1/4 in microphones (Bruel & Kjaer Type 4136) were used for these measurements and had a frequency response which was fiat within ± 1 dB from 4 Hz to 2.5 kHz. In the reacting flows, pressure transducers

78 (Kistler Model 211B5) with a flat frequency response from dc to 50 kHz were used. To determine the effects of forcing on flame structure, flow visualization was done using a conventional Schlieren system, with image recording at 4000 frames/s using a video-based digital camera (Spin Physics SP2000). The effects of forcing on volumetric energy release in the combustor were inferred from measurements of radiation from electronically-excited CH (A 2 A ~ X2II) radicals. The proportionality between the emission from CH* and the total energy release in premixed hydrocarbon flames for fixed equivalence ratios has been shown by others [34-36] and was confirmed in experiments on premixed laminar C2I-I4-air flames in our laboratory. The emitted light was focused through neutral density filters and a bandpass filter onto the photocathode of a photomultiplier (Hamamatsu 1P28A). The bandpass filter was centered at 430 nm with a spectral bandwidth of 10 nm (FWHM). The collection optics were arranged such that the field of view could be varied to encompass either most of the combustor test section or a very narrow region to provide increased streamwise spatial resolution, Gas sampling of the exhaust from the combustor test section was performed to determine the effect of forcing on NOx emissions. The samples were extracted at the exit of the test section using an uncooled quartz micro-probe rake which had three probe tips aligned in the vertical direction in the center of the tunnel. This probe configuration allowed a spatially-averaged measurement to be made. The sample pressure was 15 torr, and the samples were analyzed using a calibrated chemiluminescent analyzer (Thermoelectron Model 10 AR). Quenching corrections were made to the NOx analyzer data following the procedure of Matthews et al. [37]. The signals from the velocity, pressure, and emission instrumentation were acquired and processed using a personal computer with a 12 bit resolution A/D converter with a typical sampling frequency of 2 kHz. Various statistical routines were used to compute quantities such as the mean, variance, relative phase, probability density functions, and the power spectral density of the time series data. Low-pass filters were used for acquisi-

K . R . MCMANUS ET AL. tion of data for power spectral density estimations and for the acoustic measurements to reduce aliasing errors. E X P E R I M E N T A L OBSERVATIONS Nonreaeting Flow Nonreacting flow experiments were performed with a nominal inlet air velocity (U0) of 8.5 m/s, which corresponds to a Reynolds number of 11 240 based on the step height, h. The effects of forcing on the velocity field were investigated by measuring profiles of the streamwise component of velocity at seven axial locations in the combustor test section (x/h = 0, 1, 2, 3.5, 5, 7, 9), where x/h = 0 corresponds to the inlet plane). The inlet velocity profile was flat with a thin boundary layer at the separation edge, which in the unforced case fits a Blasius solution. The momentum thickness (0) of the inlet boundary layer for the unforced flow was 0.20 mm, which corresponds to a momentum thickness Reynolds number of 110. The boundary layer turbulence intensity was high (approximately 6%), indicating that the layer was near transition [38]. The inlet boundary layer characteristics and free stream turbulence intensity varied, depending on forcing conditions. Velocity data were taken for two forcing conditions which were identical to two of the reacting flow experiments: f f = 9 8 Hz (v'p max=lO m/s) and f f = 1 6 0 Hz (OCpmax=lOm/s). The boundary layer thickness increased in the forced cases by 33% (98 Hz) and 17% (160 Hz), and the turbulence intensity in the boundary layer with forcing (averaged over many excitation cycles) increased by more than a factor of 10 compared with the unforced case. The inlet free stream turbulence intensity was 0.4% for the unforced flow, 4.5% for ff =98 Hz, and 1.3% f o r f f = 1 6 0 Hz. The increase in free stream turbulence intensity is due primarily to the pulsating mass flow caused by the forcing. The velocity profiles downstream of the separation were used to calculate the shear layer growth rates, and it was found that the layer growth rates with forcing are approximately twice that of the unforced flow. The increase in shear layer growth rate due to forcing has been observed by others [10-13, 15].

COMBUSTOR CONTROL BY SHEAR LAYER EXCITATION To investigate the character of the acoustic field in the facility and to determine the extent of coupiing between the acoustic field and the vortex shedding for unforced nonreacting flows, the dominant longitudinal acoustic modes of the facility were identified using microphone probes. Measurements of the fluctuating static pressure, taken at the six locations shown in Fig. 1, were used to determine the mode structure. An example of the power spectral density for the fluctuating static pressure, obtained using a fast Fourier transform (FFT) algorithm, is shown in Fig. 2. These data, along with the calculated relative phase at specific frequencies, showed a dominant standing quarterwave mode at approximately 64 Hz, with the upstream end of the flow conditioning section acting as an acoustically-closed end. Harmonics of this quarter-wave mode were also present near 192 Hz (first harmonic) and 320 Hz (second harmonte). The one-dimensional turbulence spectra from streamwise velocity measurements made just downstream of the separation (x = 1 mm) in the unforced flow showed slight spectral features near the fundamental acoustic frequency and first harmonic (Fig. 3a); however, flow visualization showed that these features do not correspond to the dominant vortex shedding frequencies. The broad spectral features centered near 550 Hz and 720 Hz indicate that weak velocity fluctuations exist in these frequency bands. The Strouhal numbers (Stro =fOo/Uavg, where Uavg=4.25 m/s in the present study) corresponding to these frequencies are Stro : 0.026 and Stro = 0.034 and are in the lO-' I ~-..i---Fundamental ~to-~ lsIH . . . . O3

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Stro = 0.032, for the natural hydrodynamic instability frequency in plane mixing layers. Figure 4a shows a velocity spectrum taken near the outer edge of the shear layer at a downstream location (x = 40 mm). The spectrum at this location differs from that near the separation edge; in particular, the energy contained in the low-frequency region ( f < ~ 200 Hz) and in the higher frequency bands centered near 550 Hz and 720 Hz is reduced. In addition, the spectrum reveals a dominant broad-band feature average centered near 230 Hz, suggesting that the vortex passage frequency is near 230 Hz. Hence, the passage frequencies of the large-scale structures in the unforced shear layer are different from those in the separating boundary layer, and are thought to be related to a wake instability similar to that found in two-stream mixing layers with thick splitter plates

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[40]. A significant amount of turbulent energy is seen to exist at frequencies lower than the average vortex passage frequency, as reported by others [41, 42]. Eaton and Johnston [42] attributed this broad-band low-frequency unsteadiness to avertical flapping motion of the turbulent shear layer caused by the natural instability of the recirculation zone, a phenomenon also observed in numerical simulations [43, 44]. In the forced flows, the power spectra of the velocity fluctuations in the initial shear layer development region showed a large peak at the forcing frequency and smaller peaks at harmonics of the forcing frequency (Fig. 3b). The spectral features associated with the forcing frequency in the forced flows were more evident than the features described above for the unforced flow. Power spectra taken at downstream locations for the forced flows showed that the spectral feature associated with

the forcing frequency persisted, whereas the harmonies dissipated with increasing downstream distance (Fig. 4b). These results suggest that smallscale turbulent structures are formed at harmonics of the forcing frequency near the separation edge and then dissipate or coalesce with increasing downstream distance. Ghoniem and Ng [15] observed similar behavior in their numerical simulations of a forced confined shear layer, and for high forcing levels, such as those used in the present study, found that a group merging, or collective interaction occurred between the forcing wave and the small vortices formed at its harmonics. Figure 4b also shows a small peak at 50 Hz, which is the first subharmonic of the forcing frequency and may be due to the process of vortex pairing. Smoke streak images of the nonreacting flowthefieldstructure fortheUnforcedand forced conditions are shown in Fig. 5. Large-scale turbulent structures are seen in the shear layer region, and in forced cases, dominant shedding frequency corresponds to the forcing frequency. In the unforced flow, the dominant shedding frequencies are higher than most of the forcing frequencies used in the present experiments. It was observed that the frequency locking of the turbulent structures with forcing was intermittent and that there was some variation in their size and shape. In addition to affecting vortex shedding, flow forcing has altered the structure of the recirculation zone region, suggesting that coupling may exist between the turbulent structures periodically formed in the shear layer and the unsteadiness of the recirculation zone. The flow visualization images also showed that the mean reattachment length was reduced with forcing, as observed by others [9, 16]. Reacting Flow Inlet airflow conditions for most of the reacting flow experiments were identical to those for the nonreacting flow case; U0 was increased to approximately 8.9 m/s due to the addition of fuel. Some experiments were performed with an inlet velocity of 12 m/s, corresponding to a Reynolds number of 15 870. Combustion tests were carried out with lean premixed ethylene/air flames at atmospheric pressure for both unforced and forced

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conditions. Combustion could be stabilized in the combustor test section for a fuel/air equivalence ratio range of approximately 0.48 < ~b < 0.70. Equivalence ratios below ~b ---- 0.48 resulted in blowoff. As the equivalence ratio was increased above ~b = 0.48, pressure oscillations in the combustor grew in amplitude, and increasing the equivalence ratio above $ -- 0.70 caused the onset of a combustion instability which resulted in flame blowoff or flashback. Most of the reacting flow results described below are for an inlet velocity of 8.9 m/s and an equivalence ratio of ~b = 0.63, where in the unforced flow the combustion was nominally stable,

Unforced Combustion A Schlieren image o f the flame structure which is typical of stable lean unforced combustion ($ < 0.70) is shown in Fig. 6a. The flame front exhibits a variety of length scales, which are related to the frequencies at which turbulent structures are shed from the backstep (compare with Fig. 5a). The two-dimensionality of the flame front can be inferred from the degree of contrast and coherence of the structure in the Schlieren images. Analysis of a number of images shows that the flame becomes more three-dimensional as it convects downstream. The shedding frequencies en-

82

K. R. MCMANUS ET AL.

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compassed a broad range (approximately 35-350 Hz); however, certain frequencies were found to dominate the shedding process. These frequencies, identified both in the Schlieren records and in a logarithmic plot of the pressure fluctuation power spectra (Fig. 7), were centered near 50, 100, 250, and 320 Hz and will be referred to as resonant shedding frequencies. The first two frequencies are lower than the shedding frequencies for the unforced nonreacting flow, and it is believed that the dominant 50 Hz shedding frequency is not related to the natural hydrodynamic instabilities, which would occur at higher frequencies than in the nonreacting flow [45]. Pressure

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COMBUSTOR CONTROL BY SHEAR LAYER EXCITATION measurements made along the duct showed that a low-frequency quarter-wave mode was the dominant acoustic mode as was found in nonreacting flows. The frequency of the quarter-wave mode varied depending on operating conditions and was in the range of 45-75 Hz. Higher order lower amplitude acoustic modes were identified at frequencies near 250 and 320 Hz as well. These results suggest that the primary mechanism controlling vortex shedding in the unforced reacting flow is flame-acoustic coupling rather than hydrodynamic instabilities. Due to the complexity of the combustor geometry and the unsteady variation in sound speed throughout the duct, a very complex mode structure was expected. A more comprehensive evaluation of the acoustic field was not pursued because the goal of the acoustic measureme,its was only to determine their influence on the vortex shedding process, Increasing the fuel-air equivalence ratio to ~ = 0.70 gave rise to an instability mode similar to the buzzing instability described by others [18, 19]. The Schlieren flame visualization showed large flapping structures at approximately 45 Hz and corresponded to a strong spike found in the pressure fluctuation power spectra. Rms pressure fluctuations increased to 2.7 mbar (compared to 1.4 mbar at 0 -- 0.55) and a distant buzzing tone was audible.

83

formance of the loudspeaker driver. The upper frequency cutoff values varied from 400 to 230 Hz for V~pmax=-2and l0 m/s, respectively. The specific forcing frequencies were chosen to coincide with both resonant and off-resonant frequencies which were identified in the unforced combustion experiments. This was done to determine if the combustor was more receptive to control when excited at natural resonances. The resonant frequencies are those which were referred to above as resonant shedding frequencies. Off-resonant frequencies are those which were not observed in the pressure fluctuation power spectra of the unforced combustion experiments. The location of the excitation slot relative to the separation point was important in gaining control over the shedding process. Experiments conducted with the slot located 33 mm upstream of the flow separation showed only a marginal control of vortex shedding and combustor performance, demonstrating that the observed performance enhancements were not due to the small pulsating mass flow. In addition, this result suggests that the excitation location should be placed as close as possible to the separation edge. Hence, the slot was positioned 3.2 mm from the separation edge, and all of the combustion experiments discussed below were conducted with this slot location. Flame Structure

Forced Combustion The effects of forcing on the reacting flows were investigated for an equivalence ratio range of approximately 0.48 < ~b < 0.70. The range of forcing frequencies used, 35-400 Hz, was chosen to encompass the range of resonant shedding frequencies which were identified in the unforced combustion experiments. Four forcing amplitudes were used to investigate the effect of forcing level on the combustion characteristics; they were I) Ip max=2, 5, 7 and 10 m/s. These levels are high when compared with excitation levels used in many nonreacting mixing layer studies [39]. However, perturbations of this magnitude were necessary to exert control over the reacting flow at these frequencies. The highest frequency attainable for each forcing level was limited by the per-

When the flow was forced, vortex shedding was triggered by the forcing perturbation and the flame front exhibited a shape similar to a train of vortices convecting downstream (Fig. 6). As in the unforced case, the strong contrast and coherence of the flame front in the Schlieren images implies a strong two-dimensionality of the flame structure, The flame front initially occurs near the outer edge of the vortices as they form near the upstream end of the combustor. As the flame convects downstream, the flame front propagates in an outward direction from the vortices, and in some cases was seen to impinge on the upper wall of the test section. Heat release causes an overall acceleration of the gas in the downstream direction which can account for the apparent streamwise stretching of the flame front with increas-

84

K . R . MCMANUS ET AL.

ing downstream distance. The control over vortex shedding, and hence, flame structure, increased with increased forcing amplitude. With the lowest

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became the dominant shedding frequency, At some forcing frequencies, smaller flame structures corresponding to the first harmonic of the forcing frequency were found interlaced with the structures formed at the fundamental. C H Emission/Energy Release To determine the impact of forcing on combustion intensity, the time-averaged CH emission intensity was used to infer the mean volumetric energy release in the observed region of the combustor. Measurements were obtained to determine the elfect of forcing on the CH emission intensity from a large region of the combustor volume which encompassed most of the flame (the field of view extended from the extreme upstream end of the viewing window to a position 15 cm downstream of the step and was the full height of the test section). Figure 8 shows the effect of forcing on the time-averaged CH emission intensity from this region for a fixed equivalence ratio. As discussed earlier, these data reflect the change in the mean volumetric energy release. The time-averaged CH emission level increased by as much as 15% over the unforced case for the forcing frequencies investigated, which implies a 15% increase in energy release in the observed combustor volume. The effect of forcing on the time-averaged CH emission intensity is more pronounced as the foreing amplitude is increased. This behavior is consistent with the effect of forcing amplitude on the ability to control the flame structure which was de-

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averagedCH emission intensity (0 = 0.63). scribed above. The trends also show an increased CH emission intensity with increasing frequency for frequencies greater than 50 Hz. To determine the axial variation of the volumetric energy release in the combustor test section, CH emission measurements were taken with an aperture that allowed emission collection from 2 cm wide slices (extending the full height of the test section) of the combustor and could be traversed along the combustor test section. Figure 9 shows the magnitude of the time-averaged CH emission intensity versus the nondimensional downstream distance. For the unforced flow, the emission intensity reaches a maximum at approximately x/h = 6, and at x/h = 12 decreases to values on the order of those found at the first 0.20

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Hz band is dominant, with a strong component at 50 Hz, and the 60-120 Hz band almost vanishes. The magnitude of the peak in the 20-60 Hz band reaches a maximum at x/h = 9.0 in the unforced flow (Fig. 1 la), and the magnitude of this peak is significantly reduced with forcing (Fig. lib). With forcing, a discrete peak at the forcing frequency is observed at all measurement locations and corresponds to the "forced" unsteadiness of the energy release.

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Downstream Distance, xJh

Fig. 10. Effect of forcing on the fluctuation intensity of CH emission (~ : 0.63). • Unforced; • ff=280 Hz, v Ip m~x=7

m/s. measurement station just downstream of the step. The intensity distribution with forcing ( f f = 2 8 0 Hz, o Ip max =7 m/s) shows the maximum value occurring at x/h = 4.5. These data show that the net effect of forcing is to increase the volumetric energy release in the upstream region of the test section, The effect of forcing on the fluctuating heat release is shown in Figs. 10 and 11. Forcing reduced the fluctuation intensity (llrrns/Iavg) associated with the CH emission (Fig. 10) and implies a more steady energy release rate in the combustor volume, which can be attributed to the increased regularity of the flame structure with forcing. Figure 11 shows the power spectra of CH emission at six locations along the combustor test section (x/h -- 1.5, 2.75, 4.25, 6.5, 9, and 11.25). The emission measurements for these spectra were made using a 1 mm wide slit which was oriented vertically and extended the entire height of the test section. The power spectra for the unforced flow, Fig. 1 la, and the forced flow, Fig. 1 lb (ff =280 Hz, v ~pmax:7 m/s), exhibit similar trends in the low-frequency region between approximately 20 and 120 Hz. In the upstream region of the combustor (x/h < 2.75), spectral features appear in the frequency bands of 20-60 Hz and 60-120 Hz (these frequencies also are present in the pressure fluctuation power spectra, Fig. 7). In this region, the 60-120 Hz band contains more energy. This is in contrast to the region further downstream in the combustor (x/h >_6.5) where the 20-60

Pressure Fluctuations R m s pressure fluctuation amplitudes in t h e c o m bustor test section were used as a measure of combustion instability. The amplitude of pressure fluetuations decreased when forcing was applied at all frequencies higher than 50 Hz (Fig. 12). Forcing at 50 Hz, which is in the frequency range of the dominant longitudinal acoustic mode, caused a significant increase in the amplitude of pressure fluctuations. Figure 12 shows that the reduction in pressure fluctuations increases with increased forcing frequency, for frequencies higher than 50 Hz, and with increased excitation amplitude. The power spectra of the pressure fluctuations with forcing, Fig. 13b, show that the features of the unforced spectrum, Fig. 13a, are preserved with the addition of a sharp peak at the forcing frequency and with a reduction of the peak associated with the quarter-wave acoustic frequency near 50 Hz. These spectra are plotted with a linear scale to accent the large peaks in the low-frequency region; however, the higher frequency lower amplitude spectral features do not appear. The peak near 50 Hz is the dominant feature in the low-frequency region as was observed in the CH emission spectra. Both the pressure spectra and the CH emission spectra (Fig. 11) show a reduced amplitude in the low-frequency components at x/h = 4.25, which corresponds approximately to the location of the mean flow reattachment. Reduction in the amplitude of the buzzing instability could be achieved through the application of forcing while operating the combustor in the unstable mode. When operating the combustor in the unforced mode at an equivalence ratio of 4> = 0.70, application of 160 Hz forcing pro-

86

K . R . M C M A N U S ET A L . 1.0

t-

O -~

0.8

0.6

15 ~.

0.4

I~.

-

Wh=1.5 _

_. . . .

v'%o ~ ~ ~ ,

~~::i

. . . . .

~

J~k

'

.

Ja.

11.2S"~',J~" "Y'~r'l ~ 0 100

.... 200

I 300

400

Frequency (Hz) (a) 1.0 "N c a ~

0.8

0.6

15 ~.

0.4

0.2

t ,

_

r'/ooi~r~.O ~0 _

: 0

100

200

?,: 300

400

Frequency (Hz) (b) Fig. 11. CH emission power spectra at six axial locations along the combustor test section (4~ = 0.63). (a) Unforced; (b) f/=280 ttz, o Iu m,x =7 m/s.

duced a decrease in the rms pressure fluctuation level by 30% due primarily to a decrease in the amplitude o f the low-frequency pressure fluctuations. Schlieren visualization showed that forcing reduced the flapping motion o f the flame associated with the instability; in addition, small vortices

modulated at the forcing frequency were superimposed on larger structures. The equivalence ratio at lean blowoff is another measure o f combustion stability and was found to decrease by up to 6% with forcing (from 4) = 0.48 for the unforced flow to 4) = 0.45 for f f = 2 0 0

COMBUSTOR CONTROL BY SHEAR LAYER EXCITATION 10 Excitation

Amr~,,,oo=a m/s

,v -10 o~_=

~

-2o

5 -30

0

10m/s I 100

7,vs I

200

I

ao0

I

400

so0

Excitation Frequency (Hz)

Fig. 12. Effect of amplitude and frequency of forcing on the rms pressure fluctuation level in the combustor ($ = 0.63); pressure tap # 3 .

HZ, l)tp m a x z 10 m/s). This result is thought to be related to the decreased pressure fluctuation level with forcing which could allow improved stability, The importance of the periodic nature of the forcing perturbation was demonstrated by an experiment in which constant air injection was applied at the excitation slot. The results from this experiment showed that the constant injection tended to drive the combustion unstable, in contrast to the results reported by Choudhury [46]. NOx Emissions The exhaust gas sampling to determine the effect of forcing on NOx concentrations revealed a small decrease in the spatially-averaged NO at the exit of the test section due to forcing. The concentration of NO was low in all cases and was reduced from 13 ppmv in the unforced flow to 10.5 ppmv in the case of forcing at f f =280 Hz, u tp m a x =7 m/s. The amount of NO2 present was negligible for all cases ( < 1 ppmv), DISCUSSION The results of the nonreacting flow experiments show that forcing in the frequency range of 50 < f f < 320 Hz leads to the formation of turbulent structures at the forcing frequency. The calculations of Ghoniem and Ng for high forcing levels [15], such as those used in the present exper-

87

iments, suggest that a group merging of vortices occurs early in the shear layer development. In this group merging, or collective interaction, a group of smaller vortices, formed at a harmonic of the forcing frequency, coalesces to form a larger vortex with a passage frequency equal to the forcing frequency. In the calculations, this coalescence was found to occur within a distance on the order of one convective wavelength of the forcing perturbation. In the present work, the turbulence spectra and the smoke streak flow visualization support the observations of Ghoniem and Ng that a collective interaction occurs during the initial formation of large v o r t e x s t r u c t u r e s with forcing; however, the resolution in the smoke streak images is not adequate to resolve the presence of the vortices formed at harmonics. These vortex structures dominate the flowfield and interact strongly with the flow in the recirculation zone. This process is accompanied by a dramatic increase in the shear layer spreading rate. The flame propagation in the combustor is influenced by the presence of large-scale structures. In the present study, Schlieren images show that near the upstream end of the combustor, the flame initially occurs near the outer edge of the vortices shed from the backstep, where the hot recirculated combustion products interface with the unburned mixture. The flame front then propagates in a direction which is approximately normal to the edge of the vortices (at the burned/unburned interface) as they convect downstream. The net effect of controlling the vortex formation frequency is to control the scale of each vortex structure and, hence, the flame geometry. A computer-aided analysis of the Schlieren images was used to measure the approximate instantaneous flame surface area assuming a continuous two-dimensional flame front within the measurement region. The measurement field was identical to that used in the large volume CH emission intensity measurements. Measurements from 50 instantaneous Schlieren images were used to compute ensemble-averaged values of the flame surface area for each experimental condition analyzed. The results from this analysis showed that the average flame surface area per unit volume increased with forcing. The increased flame surface area with forcing is a result

88

K . R . MCMANUS ET AL. 1.0_ ¢n t-

0.6_

£3 .~

0.6_

~=1.5

_

't'/",s ~eO' 9"~

~ 9.0 ~ "",.,,..,~..~.~ , . i . I

0

. 100

_

"'". _ "'=.

200

300

400

300

4110

Frequency (Hz) (a) 1.0_ "~

0.8_

~

o.6_

~

0.4_

~ o.2_ x/h-1.5 ~

.

I

.

, .

! 1 "''""''. "..

°..

i,A= 0

1O0

200

Frequency (Hz) (b)

Fig. 13. Pressurefluctuation power spectra at six axial locations along the combustortest section ($ = 0.63). (a) Unforced; (b) f/:280 Hz, v tp raox:7 mJs. of the ability to control the scale of turbulent structures which affect the flame structure. Hurle et al. [36] have speculated that in turbulent flames, observed increases in CH emission intensity are the consequence of increased flame surface area. A comparison of the flame area measurements with

the mean CH emission intensity measurements in the present work shows a proportionality between the increased flame surface area and the increased CH emission intensity due to forcing (Fig. 14), and this implies a correlation between flame surface area and volumetric energy release. There

COMBUSTOR CONTROL BY SHEAR LAYER EXCITATION 25 _~ 'c= G) --

20 -r t.) ~o® o~ 15 -

/

/ S0ope= 1 ~ /// \ /

F-

/f

c° • ~

•

/ f j

•

1/11

E 10 .'5

/

,

/

.//~ /

0 f

~

*

~ I

5

I

10

I

15

I

20

25

% Increase in Mean Flame Area Fig. 14. Correlationbetween time-averaged CH emission intensity and mean flame surface area. • jr/=160 Hz, o/p max =7

fy=160 Hz, Vtp,.ox=lO m/s; • . / / = 2 3 0 Hz, O/pmax=lO m/s; • . / / = 2 8 0 Hz, I)/pmax=5 m/s; • f f = 2 8 0 Hz, u rp max =7 m/s. The solid line is a linear least squares fit

m/s; @

of the data constrained to pass through the origin (unforced

condition),

89

coupling can give rise to resonant vortex shedding frequencies at acoustic modes of the combustion facility. Flow forcing provides a means by which the vortex formation can be decoupled from, or more strongly coupled to, the acoustic modes since the vortex shedding frequency can be locked to the forcing frequency. Since forcing can decouple the combustion process from the acoustic modes of the combustor, it can attenuate the acoustically-driven pressure fluctuations in the combustor. The reduction in pressure fluctuations with out-of-phase forcing has been noted by others [30, 32]. Rayleigh's criterion has often been used to quantify the magnitude of the coupiing between unsteady heat release and acoustic pressure fields particularly in studies pertaining to combustion instabilities (see, for example, Refs. [21, 22, 32, 49]). This criterion states that if the local unsteady heat release, q'(x, t), is in phase with the local pressure fluctuation, pr(x, t), the pressure wave associated with the fluctuation will

be locally amplified. Using the notation from Ref. [21], a Rayleigh index, G(x), c a n b e d e f i n e d a s the mean value of the product of these two quantities:

appears to be a greater increase in mean flame surface area than in the mean CH emission in-

G(x) -

tensity for a given forcing condition, suggesting that forcing may be affecting the flame propagation process (e.g., quenching due to flame stretch [47] or modulation in flame speed due to vortex shedding [48]). John and Summerfield [34] and

Rayleigh's criterion states that if G(x) > 0, local amplification of flame-acoustic interaction ex-

Hurle et el. [36] reported a deviation (decrease) from the linear dependence of CH emission intensity on volume flow rate of combustible mixture for Reynolds numbers greater than 13000, suggesting that turbulence may alter the basic flame propagation process, In contrast to the unforced nonreacting flow, vortex formation in the unforced reacting flow is strongly influenced by the coupling of the unsteady combustion process with the pressure field in the combustor. The unsteady combustion process is associated with the formation of large vortex structures which form at the backstep as a result of velocity fluctuations which are induced by the oscillating pressure field [19, 21]. This

q (x, t)p~(x, t)dt.

(l)

ists, and if G(x) < 0, damping exists. Furthermore, the magnitude of G(x) will give an indication as to the extent of coupling between the unsteady energy release and the pressure fluctuations. The expression for the Rayleigh index, Eqn. 1, may be expressed in the frequency domain as follows:

G(x)-= 2

fo x

ISpq(x, f ) l cos C,pq(X, f ) d f

(2)

where Spqfj') are the Fourier coefficients of the cross-spectrum of p~(x, t) and q'(x, t) and thpq(X,f ) are the relative phase angles. Evaluation of the integrand in Eqn. 2 allows the Rayleigh index to be calculated at a specific frequency, and thereby determines the flame-pressure field coupling at that frequency. In the present study,

90

K . R . MCMANUS ET AL.

the Rayleigh index was evaluated at six locations along the combustor test section to encompass most of the flame zone (x/h = 1.5, 2.75, 4.25, 6.5, 9, 11.25). The CH emission measurement for this evaluation was made using the 1 mm slit. The fluctuating pressure measurement was made by placing the pressure transducer on the upper wall at the same streamwise location as the CH emission. For the purpose of analysis, the integrand in Eqn. 2 was approximated by I = ISpl(x, f)l cos ~px(X, f)

df

= 2

ISpi(x, f)lcos~bpi(x,

f)df.

(4)

~

Forc

~ 0 ~" -0.1

Freq. Band= 60 < f < 120 Hz

I

I

L

I

I

2

4

6

8

10

(3)

where the subscript, I, denotes the fluctuating voltage signal corresponding to the CH emission intensity. Substituting Eqn. 3 into Eqn. 2, and setting the integration limits to encompass the frequency band of fl _< f < f 2 , the following equation is obtained:

G'(x)l

01 ~

~ unfor~eq .( ~ e q

12

DownstreamDistance,x/h (a)

0.1

Unforced

~

* 0

.-

I

-0.1

Equation 4 was used to calculate

Gl(x)

Freq. Band= 20 < f < 60 Hz

in the

frequency bands where the primary heat release and pressure fluctuations were observed: 20-60 Hz and 60-120 Hz (see Figs. 11 and 13). A plot of G'(x) for the frequency band 60-120 Hz is given in Fig. 15a. Large positive values of G~(x) are found in the upstream region of the combustor (1.5 < x/h < 3.5), corresponding to the region where the largest energy release fluctuations were observed for this frequency band (Fig. 11). The addition of forcing causes a reduction in the amplitude of G I(x) in this region, as well as in the downstream region of the combustor (x/h >_6.5). A corresponding plot of G'(x) for the frequency band of 20-60 Hz is shown in Fig. 15b. This frequency band contains the largest pressure fluctuation levels in the unforced case (Fig. 13a) and is associated with the low-frequency combustion instability. Large positive values of G'(x) occur in the unforced flow near x/h = 6.5, the region of maximum heat release. With forcing, the amplitude of G'(x) is reduced near x/h = 6.5; however, a positive value appears near x/h = 4.25, which is associated with the observed upstream shift of the unsteady energy release in this frequency band (Fig. 1 lb). These results suggest

aI

4I

6I

8I

I 10

la

Downstream Distance, x/h

(b) Fig. 15. Effectofforcing on Rayleighindex, Gl(x)fromEqn. 4, vs. downstreamdistance (~b.=0.63).

that amplification of pressure fluctuations due to unsteady heat release occurs in two different regions of the combustor and that different frequency bands are associated with each of these regions. One zone is confined to the upstream end of the combustor (x/h < 4.25) and is associated with strong energy release fluctuations in the 60-120 Hz frequency band. A second zone is located downstream (x/h > 4.25) and is associated with strong energy release fluctuations in the 20-60 Hz band. These two zones are separated by a region (x/h = 4.25) where minima are observed at all frequencies in both the pressure fluctuation spectra (Fig. 13) and the CH emission spectra (Fig. 11). This region of low fluctuation activity is near the point of mean flow reattachment. The shift in the primary heat release and pressure fluctuation frequencies at this location suggests that different

COMBUSTOR CONTROL BY SHEAR LAYER EXCITATION

mechanisms may be involved in the coupling between the heat release and the pressure field in the two zones. The results also suggest that the reduced pressure fluctuations with forcing can be attributed to reduced flame driving by a partial decoupling of the unsteady heat release from the pressure field.

der for his help in conducting the CH emission calibration experiments. REFERENCES 1. Townsend,A. A., The Structure o f Turbulent Shear 2. 3.

SUMMARY Experiments performed in a two-dimensional dump combustor with lean premixed flames have shown that direct shear layer excitation in the form

4.

of a periodic perturbation to the boundary layer at separation can provide enhancement of combustor performance including:

6.

5.

7.

i) ii) iii) iv)

Increased volumetric energy release. Decreased rms pressure fluctuations. Improved lean blowoff stability. Reduced NOx emissions.

The magnitude of the effect of forcing on combustor performance depends on both frequency and amplitude of the excitation, and the present results show increased effects with increasing frequency and amplitude. The performance improvements are a result of the control of vortex shedding which forcing provides and the subsequent modification of the flame structure. The flame area in the upstream region of the combustor test section was increased for all forced cases studied and accounted for the observed increase in volumetric energy release. Coupling of the heat release with the pressure field was reduced with forcing, which in turn reduced the rms pressure fluctuations in the combustor. The results of this study suggest the possibility of using direct shear layer excitation in active control systems to improve the performance of practical combustion systems.

This work was supported by the Office o f

91

8.

Flow. Cambridge University Press, 1956. Roshko, A., A I A A J. 14:1349 (1976). Parikh, P. G. and Moffat, R. J., in Fluid Mechanics o f Combustion Systems (T. Morel, R. P. Lohman, and J. M. Rackley, Eds.), ASME, New York, 1981. Zaman, K. B. M. Q. and Hussain, A. K. M. F., J. Fluid Mech. 101:449 (1980). Bouchard,E. E. and Reynolds, W. C., in Unsteady Turbulent Shear Flows, IU-TAM Symp., 1981, p. 370. Schadow, K. C., Gutmark, E., Parr, T. P., Parr, D. M., and Wilson, K. J., in AIAA 25th Aerospace Sciences Meeting, AIAA Paper 87-0376, January 1987. Lee, M. and Reynolds, W. C., in Fifth Symposium on Turbulent Shear Flows, 1985, p. 1.7. Parekh, D. E., Reynolds, W. C., and Mungal, M. G., in AIAA 25th Aerospace Sciences Meeting, AIAA Paper

9.

87-0164, January 1987. Viets, H. and Piatt, M., Progress in Astronautics and

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Aeronautics 76:611-624, 1981. Ho, C-M. and Huang, L-S., J. Fluid Mech. 119:443 (1982). Oster, D. and Wygnanski, 1., J. Fluid Mech. 123:91

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(1982). 12.

Fiedler, H. E. and Mensing, P., J. FluidMech. 150:281

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(1985). Roberts, F. A. and Roshko, A., in A I A A Shear Flow Control Conference, AIAA Paper 85-0570, March

1985.

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Choudhury, P. R., Gerstein, M., and Mojaradi, R., in 22nd J A N N A F Combustion Meeting, Pasadena, CA,

October 1985. 15.

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16.

Roos, F. W. and Kegelman, J. T., in A I A A 24th Aerospace Sciences Meeting, AIAA Paper 86-0112,

January 1986. 17. 18.

19.

N a v a l R e s e a r c h u n d e r C o n t r a c t s NOOO14-84-K-

0383 and NOOO14-89-J-1664. The authors wish to acknowledge Dr.

20.

Thierry Poinsot and Prof. Sebastien Candel for many helpful discussions during the preparation o f this manuscript and Mr. Gregory Kry-

21.

Parker, L. J., Sawyer, R. F., and Ganji, A. R., Combust. Sci. and Tech. 20:235 (1979). Keller, J. O., Vaneveld, L., Korschelt, D., Hubbard, G. L., Ghoniem, A. F., Daily, J. W., and Oppenheim, A. K., A I A A J. 20:254 (1982). Smith, D. A., Ph.D. Thesis, California Institute of Technology, Pasadena, 1985. Schadow, K. C., presented at ONR/NAVAIR Program Review, Johns Hopkins University, Baltimore, MD, October 1985. Poinsot, T. J., Trouve, A. C., Veynante, D. P., Candel, s . M . , and Esposito, E. J., J. Fluid Mech. 177:265 (1987).

92 22.

23.

24. 25. 26. 27.

28. 29. 30. 31.

32. 33.

34. 35. 36.

K . R . M C M A N U S ET AL. Hegde, U. G., Reuter, D., Zinn, B. T., and Daniel, B. R., in A I A A 25th Aerospace Sciences Meeting, AIAA Paper 87-0216, January 1987. Lovett, J. A., Turns, S. R., and Merkle, C. L., presented at the Spring Technical Meeting, Central States Section, The Combustion Institute, Argonne, IL, May 1987. Jarosinski, J. and Wojcicki, S., Acta Astronautica 3:567 (1976). Crump, J. E., Schadow, K. C., Yang, V., and Culick, F. E. C., J. Propulsion 2:105 (1986). Sivasegaram, S. and Whitelaw, J. H., Cornbust. Flame 68:121 (1987). Sterling, J. D. and Zukoski, E. E., in A I A A 25th Aerospace Sciences Meeting, AIAA Paper 87-0220, January 1987. Flandro, G. A., J. Propulsion 2:206 (1986). Pitz, R. W., Ph.D. Thesis, University of California, Berkeley, 1981. Lang, W., Poinsot, T. and Candel, S., Combust. Flame 70:281 (1987). Schadow, K. C., Gutmark, E., Wilson, K. J., Parr, D. M., and Mahan, V. A., Combust. Sci. and Tech. 54:103 (1987). Bloxsidge, G. J., Dowling, A. P., Hooper, N., and Langhorne, P. J., A I A A J. 26:783 (1988). Corke, T. C., Koga, D. J., Drubka, R. E., and Nagib, H. M., in ICIASF 1977 Record, Shrivenham, England, 1977, IEEE Pub. 77, p. 74. John, R. R. and Summerfi~ld, M., Jet Propulsion 27:169 (1957). Drederichsen, J. and Gould, R. D., Cornbust. Flame 2:25 (1965). Hurle, I. R., Price, R. B., Sugden, T. M., and Thomas, A., Proc. R. Soc. Lond. A 303:409 (1968).

37. 38. 39. 40. 41.

42.

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Matthews, R. D., Sawyer, R. F., and Schefer, R. W., Environ. Sci. & Tech. 11:1092 (1977). Batt, R. G., A I A A J. 13:245 (1975). Ho, C-M. and Huerre, P., Ann. Rev. Fluid Mech. 16:365 (1984). Dziomba, B. and Fiedler, H. E., J. Fluid Mech. 152:419 (1985). Tani, I., Iuchi, M., and Komoda, H., Aeronautical Research Institute, University of Tokyo, Japan, Report No. 364, 1961. Eaton, J. K. and Johnston, J. P., in Third Int. Syrnp. on Turb. Shear Flows--Selected Papers (L. J. Bradbury, F. Durst, B. E. Launder, F. W. Schmidt, and J. H. Whitelaw, Eds.), Springer-Verlag, 1981. Sethian, J. and Ghoniem, A. F., J. Comput. Phys., 74:283 (1988). Kailasanath, K., Gardner, J., Boris, J., and Oran, E., in A I A A 25th Aerospace Sciences Meeting, AIAA Paper 86-1609, January 1987. Trouve, A., Candel, S. M., and Daily, J. W., in A I A A 26th Aerospace Sciences Meeting, Paper 88-0149, January 1988. Choudhury, P. R., AFOSR Report No. AFOSR-TR-771234, August 1977. Law, C. K., Twenty-Second Symposium (Int.) Cornbust., The Combustion Institute, Pittsburgh, PA, 1989, pp. 1381-1402. Reuter, D. M., Hegde, U. G., and Zinn, B. T., presented at the 25th JANNAF Combustion Meeting, October 1988. Culick, F. E. C., Combust. Sci. and Tech. 7:165 (1973).

Received 4 January 1989; revised 25 August 1989