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An experimental study of probe distortions to the structure of one-dimensional flames
C O M B U S T I O N A N D F L A M E 6 3 : 1 9 - 2 9 (1986)
19
An Experimental Study of Probe Distortions to the Structure of One-Dimensional Flames O W E N I. S M I T H Department of Chemical Engineering, UCLA, Los Angeles, California
and D A V I D W. C H A N D L E R Sandia National Laboratories, Livermore, California
In this study we have examined the effect of a molecular-beam mass-spectrometer sampling probe on the structure of a fiat H2-O2-Ar flame doped with a small amount of HCN. Relative CN concentration and CN rotational temperature fields were obtained by laser-induced fluorescence using the R(0) through R(24) lines of the B-X transition around 385 nm. Measurements were made for several probe positions, both upstream and downstream of the location of peak unperturbed concentration. Reduced CN concentrations were observed within five orifice hydraulic diameters of the probe tip; however, a corresponding perturbation of rotational temperature was not. Suction at the probe orifice did not measurably effect either result, indicating that residence time perturbations do not play a major role. Perturbations of the CN concentration field are discussed in terms of catalyzed recombination of the major flame radicals on the probe surface, coupled with diffusive transport and gas phase chemistry applicable to the CN radical.
INTRODUCTION The influence o f sampling probes on flame structure has long been o f c o n c e r n to those interested in flame c h e m i s t r y . Most p r e v i o u s work in this area has i n v o l v e d e x p e r i m e n t a l studies o f the p r o b e ' s effect on either the O H radical or t e m p e r a t u r e profiles, or potential flow calculations designed to study its influence on the flame flowfield. Most studies o f radical profile distortions have been based on c o m p a r i s o n o f O H c o n c e n trations measured by v ar io u s s p e c tr o s c o p i c techniques and m o l e c u l a r b e a m - m a s s s p e c t r o m e t r y ( M B M S ) in similar flames. Cattolica et al. [1] e x a m i n e d the O H profile a s t o i c h i o m e t r i c methCopyright © 1986 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017
a n e - a i r flame at at m o sp h er i c pressure by laser absorption and M B M S . They found that except very close to the burner, the M B M S profile was shifted five orifice diameters d o w n s t r e a m in c o m p a r i s o n with the p r e s u m a b l y undistorted absorption m e a s u r e m e n t s . In a m o r e recent study, Stepowski et al. [2] c o m p a r e d laser induced f l u o r e s c e n c e (LIF), laser absorption, and M B M S m e a s u r e m e n t s o f O H in a low pressure p r o p a n e - o x y g e n flame. In the region o f the flame w h e r e O H diffuses toward the burner surface, the probe was found to l o w e r the O H co n cen t r at i o n just upstream of its tip. Spatially r e s o l v e d L I F m e a s u r e m e n t s made as a function o f radial position just upstream o f the nozzle tip support this observa-
0010-2180/86/$03.50
20
OWEN I. SMITH and DAVID W. CHANDLER
tion. The degree of perturbation to the gas at this point was found to decrease with increasing burner-probe separation. Although the MBMS OH profile was displaced downstream in this region, the displacement was itself a function of burner-probe separation. As a result, no simple correlation was found between the optical and MBMS measurements. Downstream of the reaction zone, where the OH concentration gradient is small, no significant perturbations were observed. This suggests that the effects observed in the preheat and reaction zones might be due to the lowered diffusive flux of OH caused by recombination on the downstream probe surface. In this case, no perturbation would be expected in the region where the diffusive flux is either small or in the direction of the flow. There have been several investigations of the effect of probes on the temperature field in flames. These include direct measurements of the temperature profile in the presence and absence of the probe by means of thermocouples [1, 3, 4] and time of flight (TOF) techniques [1]. Thermocouple measurements of the temperature field near the probe have produced mixed results. One study shows temperature decreases of 100-200K immediately upstream of the cone
[3]. No thermal perturbations were observed in a more recent study [2]. The temperature just upstream of the probe tip may also be determined by TOF in a molecular beam system, that is, by interpretation of the moments of the beam's velocity distribution downstream of the point of translational freezing. Comparison of TOF temperature profiles with those taken by a thermocouple in the absence of the probe shows much the same behavior observed previously for the OH profile [1]; e.g., the TOF profile is shifted downstream about five orifice diameters. As before, this correlation fails to hold near the burner surface. The result is typically as depicted in Fig. 1. Aside from the processes of radical diffusion and heat conduction to the probe walls, distortions in radical and temperature profiles could also result from coupling between the flame chemistry and hydrodynamic distortions induced by the probe. Potential flow calculations based on an infinite cone with a point sink embedded in the tip indicate that the mean residence time of the sampled gas can be significantly different from that corresponding to the same position in an unperturbed flame [5]. One finds that the residence time for a fluid
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Fig. 1. Time-of-flight and thermocouple temperature profiles for a H202/Ar = 0 . 1 7 0 / 0 . 0 6 6 / 0 . 7 6 4 flame at 88 Torr. Thermocouple measurements were taken without the probe present. Probe orifice diameter is 0.04 cm.
FLAME DISTORTION BY A PROBE
21
element moving along the probe centerline is much lower than that for the unperturbed flame. However, because of axial symmetry, most of the sampled gas travels along streamlines near the stagnation streamline, where the local residence time is much larger than that of the unperturbed flame. MBMS systems usually extract a small portion of the sample jet originating at the probe orifice by means of a skimmer. This is then passed to the mass spectrometer for 20
i
analysis. In a typical configuration, this would consist of the central 2.5% (by volume) of the jet. For this reason, one would expect the residence time of the portion of the sample available for analysis to be more characteristic of that along the centerline than that of the sample as a whole. The mean residence time for any portion of the sampled gas may be obtained by integrating over the appropriate streamlines. Figure 2a
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(b) Fig. 2. (a) Plots of mean residence time as a function of burner-probe separation for several core fractions of the sampled gas (solid curves), ranging from 0.25 (by volume) to 1. Unperturbed residence time profile (broken curve). All data refer to the system described in Fig. 1. (b) Time-of-flight temperature profile of Fig. 1 corrected for residence time distortions as outlined in the text.
22
OWEN I. SMITH and DAVID W. CHANDLER
depicts mean residence time as a function of probe-burner separation for the system of Fig. 1. The solid curves represent the mean residence time profiles corresponding to various fractions of the sampled gas. The residence time profile for the unperturbed flame is given for comparison. The calculations indicate that very large residence time distortions are to be expected for the central core of the sample. Where nonequilibrated chemical reactions occur on this time scale, radial concentration gradients will be induced and the resulting radial diffusion will work to counteract the effect of the broad residence time distribution. Essentially, the flame is no longer one-dimensional and considerable diffusive mixing will take place upstream of the orifice. Thus, the mean residence time characterizing the gas which is analyzed should be larger than that predicted for the core 2.5 %. To a first approximation, one could correct for composition or temperature distortions arising from residence time perturbations by mapping the perturbed residence time profile into the unperturbed profile. In Fig. 2b we have applied this correction to the TOF temperature measurements of Fig. 1, using the residence time profile for the core 75% of the sample. Except very near the burner surface, agreement with the thermocouple profile is much improved. It seems that the concept of the sample being effectively drawn from some distance upstream of the orifice, or the "actual mean sampling distance of the sampling p r o b e " [2], might well be a manifestation of hydrodynamic distortions. To summarize, previous work in this area suggests at least two possible mechanisms for the probe induced distortion of one-dimensional, premixed flames.
sample transmitted to the analytical instrument. Other data suggest that the body of the cone dominates, and that this disturbance propagates upstream of the probe by reduced diffusive fluxes of chain carriers. In order to distinguish between these mechanisms, measurements of probe induced perturbations to a sharply peaked radical profile would be extremely valuable. It would also be helpful to evaluate the role of probe suction (sink strength) in the perturbations.
I. The probe acts as a radical and/or heat sink. 2. Hydrodynamic distortions couple with flame chemistry to produce the observed distortions. Within the former category, some data seem to indicate that the probe tip dominates, e.g., that the species and/or thermal boundary layers originating at the stagnation point penetrate far enough toward the orifice centerline to effect the
EXPERIMENTAL In this study we have examined the effect of a probe on the CN concentration field in a nearly stoichiometric hydrogen-oxygen-argon flame doped with HCN. The flame, of initial composition O2/H2/Ar/HCN = 0 . 1 4 3 / 0 . 2 5 9 / 0 . 5 8 5 / 0.013, was stabilized on a cooled porous plug burner at 25 Torr. The burner and gas handling system have been described previously [6]. A number of factors motivate our choice of the CN radical to characterize probe distortions to the flame's structure. First, the CN profile in this flame is relatively narrow in comparison with those of the major flame radicals. It exhibits both strong positive and negative gradients, and peaks somewhat nearer the burner surface. It is also highly reactive and strongly coupled to the major flame radicals through fast chemistry. Second, spatially resolved relative CN concentrations can be measured by laser induced fluorescence with relative ease. CN rotational temperatures can also be obtained from the spectra. The hydrogen-oxygen system was chosen as the matrix for this study because the applicable chemistry is fairly well understood [6]. Atomic hydrogen is present in concentrations of a few percent in this flame, and dominates most of the H atom abstraction steps. For this reason, diffusive transport plays an important role in determining the flame structure. These factors, taken together, should make the CN concentration field very sensitive to probe disturbances (probably unusually so). A conical quartz probe of 60* included angle was used in this study. The orifice diameter was
FLAME DISTORTION BY A PROBE 0.5 mm. This design is representative of those used in MBMS systems with fairly large pumping capacity in the first stage [1]. The probe could be positioned along the burner axis independently of the burner by means of a dial gauge. The fixture used to move the probe also served to evacuate the region downstream of the orifice. The pressure drop across the orifice was at least a factor of 3. This is normally sufficient to ensure choked flow. For a microprobe with much smaller orifice diameter and longer entry length, a pressure ratio of 5 is thought to be necessary at high temperatures [7]. However, the frictional effects responsible for this behavior should be much less important in our case. At worst, they will be no more severe than in the microprobe. Colket et al. [7] indicate that for the microprobe the mass flowrate at a pressure ratio of 3 is 95 % of the choked value, so that our sink strength should be representative of those achieved in MBMS systems. The laser detection system used in this study was a unique tunable continuous wave ultraviolet laser developed at Sandia for flame studies [6]. A schematic of the apparatus is shown in Fig. 3. The source is capable of scanning from about 360 to 400 nm with 10 mW power and 0.006 nm resolution without resorting to frequency doubling. This is accomplished by directly exciting Polyphenyl 1 laser dye with all ultraviolet lines from an argon ion laser. At
23 these power levels it is easy to show that saturation is not important. Further, the spectral resolution of 0.006 nm allowed individual rotaOonal lines in the 0-0, 1-1, 2-2 bands of the CN B2y. +-X2Y~ + transition to be selectively excited. Fluorescence was detected at right angles to the laser propagation through a 500 #m pinhole and a broad band filter that removed the visible light. Spatial resolution of the optical system was checked experimentally by looking at scattered light from a thin wire. It was determined that the spatial resolution of the combined excitation and detection system was about 0.7 mm radially, by about 0.2 mm axially. Absorption studies on this flame indicated that radiative trapping of the fluorescence was negligible. Less than 3% of the light is absorbed in 12 passes through the flame. Laser induced fluorescence spectra of the R branch of the B - X transition of CN were used to obtain rotational temperatures. The Doppler width of an individual rotational line at temperatures between 1100 and 1400K is approximately 0.15 c m - l , a factor of 3 smaller than the laser resolution. Since the spectrum is normalized by the laser power, the height of an individual LIF line is proportional to the population in the corresponding ground state. The rotational temperature is obtained from a Boltzmann plot of the logarithm of the intensity (normalized by the rotational line strength and the degeneracy)
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24 against the energy of the corresponding l o w e r vibrational-rotational energy level. Not all of the lines between R(0) and R(24) were used in the Boltzmann plot. The R(3), R(6), R(10), and R(14) lines are spectroscopically perturbed by the A state [8], and therefore emit with characteristics of the A state. The detection optics discriminate against this light; therefore, those lines appear smaller than they should. Also, lines which overlap lines of the 1-1 or 2-2 bands were not used. Most of the relative CN concentration data were obtained by twice scanning the R(4) through R(12) transitions at discrete locations within the flame. The line intensities for each measurement were corrected for rotational state distribution by the temperature profile as determined above. To aid in interpolation between these points, a number of CN profiles were taken by sitting on a rotational line and traversing the burner axially. This procedure allowed us to locate the point of maximum concentration quite accurately, and to determine the general shape of the profile. While these traverses could be accomplished quite rapidly, the dye laser often drifted off the line center during the process. This frequency instability was associated with vibration of the optics induced by the stepping motor driving the burner. In general, we found that the discrete data were much more reproducible. Collisional quenching is not considered in analysis of the data, as the flame is dilute and number density varies by only 30% over the region of interest. RESULTS Figure 4 displays relative CN concentration profiles at several radial positions for a constant probe-burner distance of 1.08 cm. The CN profile obtained in the absence of the probe is also given. The coefficient of variation among measurements taken at the same position was usually around 0.07. Where measurements were made at points of low CN concentration, values as high as 0.15 were occasionally observed. Based on this, we estimate the uncertainty in the concentration data to be no more than + 15%.
OWEN I. SMITH and DAVID W. CHANDLER
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Fig. 4. Relative CN concentration profiles by LIF in an HCN doped H2-O2-Ar flame at 25 Torr. Solid circles denote
the profile in the absence of the probe. Perturbed CN profiles at various radial positions were obtained for a probe-burner separation of 1.08 cm. Open circles and closed squares represent two independent measurements of the CN rotational temperature profile. The broken curve denotes the calculated flame temperature, obtained as outlined in the text. The data presented in Fig. 4 were obtained with the probe positioned just downstream of the point of maximum CN concentration in the undistorted flame. All are normalized by the maximum undistorted concentration. Along the axis of the probe (r = 0), considerable attenuation is observable within five orifice diameters (0.25 cm) of the probe tip. Just upstream of the orifice the CN concentration is about 40% lower than that at the same position in the undistorted flame. Profiles taken off the axis (r > 0) approach the undistorted profile slowly with increasing radial distance. Two independent measurements of the CN rotational temperature profile (taken with the probe removed) are displayed in Fig. 4. In most cases, these measurements differ by less than 70K. At the downstream edge of the CN profile, where the concentration is small and the high temperature broadens the rotational state distri-
FLAME DISTORTION BY A PROBE
25 accurate since additional data (described in the previous section) were used for this purpose. No additional data were used in the radial interpolations, so that where large radial gradients exist the contour positions are less certain. With the probe positioned just upstream of the undistorted profile peak (Fig. 5), some degree of perturbation is observed on the centerline over the entire profile. Strong radial gradients are present further downstream near the probe wall. With the probe just downstream of the undistorted peak (Fig. 6), probe distortions no longer reach the upstream edge of the profile. Radial gradients downstream of the orifice are not as strong. As the probe is moved well downstream of the undistorted peak (Fig. 7), the upstream portion of the CN profile becomes practically free of distortion. Radial gradients further downstream are much smaller. Figures 5-7 indicate that while probe distortions are reduced in magnitude as the probe is moved downstream, significant perturbations
bution, larger discrepancies are observed. The temperature profile obtained from a numerical simulation of this flame shows reasonable agreement with the measurements. Details of the simulation are given later. Measurements were also made with the probe present; however, any thermal perturbation present was too small to detect. To the best of our knowledge, these are the first optical temperature measurements obtained near the probe surface. One can obtain a two-dimensional contour map of the CN concentration field in the vicinity of the probe by radial interpolation of the data given in Fig. 4. Figures 5-7 show such results at probe-burner separations of 0.93, 1.08, and 1.42 cm respectively. The unperturbed concentration, which would apply at large distances from the centerline, is shown as a function of axial distance along the edge of each figure. The filled circles denote the points at which discrete measurements were taken. The axial interpolations depicted by the contours should be quite QUARTZ S A M P L I N G
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Fig. 5. Contour map of the relative CN concentration field near the probe for a probeburner separation of 0.93 cm. The unperturbed profile is shown along the left-hand edge.
26
OWEN I. SMITH and DAVID W. CHANDLER
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Fig. 6. Contour map of the relative CN concentration field near the probe for a probeburner separation of 1.08 cm.
can be present even well downstream of the undistorted profile peak. One can put this on a quantitative basis by mild extrapolations of data such as that presented in Fig. 4. The result, expressed in terms of percentage reduction in CN concentration at the orifice entrance, is given in Table I. Several CN concentration measurements were made along the centerline upstream of the probe in the absence of suction for probe-burner separations of 0.93 and 1.43 cm. In each case the results were not significantly different from those obtained with suction applied. DISCUSSION In view of the large residence time distortions predicted on the basis of the simple theory
presented previously, we were greatly surprised at the total lack of sensitivity shown by our results to suction on the probe. Even within two orifice diameters upstream of the probe tip and in a region of large CN concentration gradient, our results indicate that very little of the distortion noted in the previous section can be attributed to suction. Since the CN chemistry is fairly rapid here, yet is far from equilibrated, it is clear that the distortions observed in this study cannot be explained solely on the basis of hydrodynamic perturbations. Our results show that the probe has no strong influence on the temperature field within the flame. This indicates that the flame structure as a whole is not grossly distorted, as would be expected where flame attachment is important, and is in good agreement with the results of
F L A M E D I S T O R T I O N BY A PROBE
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Probe-Burner Separation (cm)
CN Concentration Distortion (%)
0.93 1.08 1.43
>48 42 33
Stepowski et al. [2]. Near the probe surface our LIF signal is typically small (due to the low CN concentrations observed here) and is subject to interference by scattered light. As a result, the uncertainty in our temperature measurements (which is already fairly large) grows rapidly.
Nevertheless, it is certain that temperature reductions at the orifice entrance o f the magnitude depicted in Fig. 1 ( > 400K at some points) would be noticeable. Our results indicate that the temperature just upstream of the orifice is largely unperturbed while the T O F measurements (Fig. 1) typically show large reductions in the sampled gas temperature. Comparison of the incompressible flowfield in the presence of a point sink and the size o f our optical volume indicate that our measurements should be representative o f the sample passed through the skimmer for T O F analysis; however, subsequent calculations based on a more realistic model very near the orifice (compressible flow and a disk sink [9]) cast some doubt on this
28 conclusion. This may explain the discrepancy between our observations and those made by TOF methods. Our most interesting data are those involving perturbations to the CN concentration field. At first glance, these results seem to disagree with those of Stepowski et al. [2]. Probe perturbations definitely extend downstream of the location of peak CN concentration, indicating that lowered upstream diffusive flux of CN cannot be wholly responsible for our observations. However, as noted previously, CN is closely coupled with the major flame radicals (particularly H and O) by fast chemical reactions. Thus, reduced upstream fluxes of the major radicals would be expected to influence the CN profile. To investigate the role of gas phase chemistry in our system, we have performed a numerical calculation of the unperturbed flame structure using a package of codes developed at Sandia [9, 10]. The mechanism proposed by Miller et al. [6], which does an excellent job of predicting reactant and product profiles in very similar flames, was used without modification. Calculated profiles for the major flame radicals and CN are presented in Fig. 8. Our experimental CN profile (normalized to the peak calculated CN mole fraction) is also presented. Its location shows fairly good agreement with the calculations. The peak concentration of CN occurs slightly upstream of those of H and O, and well upstream of that of OH. As mentioned previously, the calculated and measured temperature profiles also show good agreement (Fig. 4). At 0.7 cm, well upstream of the profile peak, the calculations show that chemistry has little influence on the CN concentration. The temperature is relatively low, and diffusive and convective fluxes dominate. We attribute the perturbations observed here to reduced diffusive flux from downstream, caused by lowered CN concentrations near the probe. Figures 5-7 indicate that the degree of probe induced distortion at this point correlates well with the decrease in concentration gradient. In the region of peak concentration (1.05 cm), chemical terms dominate the species equation for CN. The CN concentration is heavily influ-
OWEN I. SMITH and DAVID W. CHANDLER
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Fig. 8. Calculated profiles for H, O, OH, and CN (solid curves), obtained as outlined in the text. Measured CN profile (broken curve) obtained without the probe present and normalized to the peak calculated mole fraction for CN.
enced by the reaction between HCN and atomic hydrogen, H + HCN = H2 + CN,
(1)
which proceeds rapidly in both directions. Reaction (2), O + HCN = OH + CN,
(2)
also contributes significantly to CN formation, and reactions of CN with 02 and OH contribute to its destruction: CN + 02 = NCO + O,
(3)
CN + OH = NCO + H.
(4)
At this point in the flame, CN formation is dominated by reaction (1) (which is nearly balanced). As previous studies have shown that stable species are influenced little by the probe, the lower CN concentrations observed here in the presence of the probe suggest that the H atom profile is probably being perturbed. If this is the case, then the H atom profile is distorted
FLAME DISTORTION BY A PROBE
29
by a probe positioned downstream of its peak (Fig. 7). This supposition is not necessarily inconsistent with the results of Stepowski and coworkers, since the H and OH concentrations are coupled by the rapid reaction OH + He = n 2 0 + H.
(5)
In this study the probe is always positioned upstream of the peak OH concentration. Still further downstream, at 1.40 cm, both downstream transport and chemical reaction of CN are important. Reaction (1) still dominates both the formation and destruction rates, but is actually further from being balanced due to greatly increased consumption via reactions (4) and (6): CN + O = CO + N.
(6)
Both lowered flux of CN from upstream and decreased concentrations of O and H seem likely to play a role in probe distortions here. Decreased O and H concentrations could result indirectly from reduced OH concentration, since the hydrogen-oxygen chemistry is close to being partially equilibrated here. The data presented in Figs. 5-7 would seem to favor wall catalyzed radical recombination as the ultimate cause of CN concentration distortions. Nevertheless, the data are not conclusive. Reaction (1) is fairly endothermic (AH ° = 25 kcal/mole). Where it is rapid compared with other processes effecting CN, a temperature decrease of 100K could reduce the CN concentration by about a factor of 2. Although we observed no thermal perturbations, a temperature change of this magnitude cannot be ruled out close to the probe surface.
CONCLUSIONS The conclusions reached in the course of this study can be summarized as follows. 1. Any large perturbations ( > 100K) to the temperature field of the flame must be localized near the central portion of the orifice entrance.
2. Suction does not play a critical role in probe induced distortions of CN concentration observed in this study. 3. Composition distortions decrease with increasing distance from the burner but can persist downstream of the point of peak concentration for some radicals, particularly those appearing early in the reaction zone. 4. Chemistry and diffusion both play important roles in perturbation of the CN profile. Thermal perturbations, although not detected in this study, may also contribute. When sampling close to the burner, where distortions are the largest, upstream diffusion blockage of the radical itself seems to dominate.
This study was supported by the United States Department o f Energy under Contract Number DEACO476DPO0789, and by the National Science Foundation under Grant Number CPE82-0983 7. REFERENCES 1. 2.
3. 4. 5. 6.
7.
8. 9. 10. 11.
Cattolica, R. J., Yoon, S., and Knuth, E. L., Comb. Sci. and Tech. 28:225 (1982). Stepowski, D., Puechberty, D., and Cottereau, M. J., Eighteenth Syrup. (Int.) on Combustion, 1981, p. 1567. Biordi, J. C., Lazzara, C. D., and Papp, J. F., Combust. Flame 23:73 (1974). Bowman, C. T., in Progress in Astronautics and Aeronautics, Vol. 53 (B. Zinn, Ed.), 1977, p. 3. Smith, O. I., Combust. Flame 40:187 (1981). Miller, J. A., Branch, M. C., McLean, W. J., Chandler, D. W., Smooke M. D., and Kee, R. J., Proceedings o f the Twentieth Syrup. (Int.) on Combustion (in press). Colket, M. B., Chippetta, L., Guile, R. N., Zabielski, M. F., and Serry, D. J., Combust. Flame 44:3 (1982). Wagner, A. T., Phys. Rev. 64:18 (1943). Yi, C. H., and Knuth, E. L., submitted to Combust. Flame May (1985). Smooke, M. D., Report SANDSI-8040, Sandia National Laboratories, Livermore, CA, 1982. Kee, R. J., Miller, J. A., and Jefferson, T. J., Report SAND80-8003, Sandia National Laboratories, Livermore, CA, 1980.
Received 31 January 1985; revised 16 May 1985
19
An Experimental Study of Probe Distortions to the Structure of One-Dimensional Flames O W E N I. S M I T H Department of Chemical Engineering, UCLA, Los Angeles, California
and D A V I D W. C H A N D L E R Sandia National Laboratories, Livermore, California
In this study we have examined the effect of a molecular-beam mass-spectrometer sampling probe on the structure of a fiat H2-O2-Ar flame doped with a small amount of HCN. Relative CN concentration and CN rotational temperature fields were obtained by laser-induced fluorescence using the R(0) through R(24) lines of the B-X transition around 385 nm. Measurements were made for several probe positions, both upstream and downstream of the location of peak unperturbed concentration. Reduced CN concentrations were observed within five orifice hydraulic diameters of the probe tip; however, a corresponding perturbation of rotational temperature was not. Suction at the probe orifice did not measurably effect either result, indicating that residence time perturbations do not play a major role. Perturbations of the CN concentration field are discussed in terms of catalyzed recombination of the major flame radicals on the probe surface, coupled with diffusive transport and gas phase chemistry applicable to the CN radical.
INTRODUCTION The influence o f sampling probes on flame structure has long been o f c o n c e r n to those interested in flame c h e m i s t r y . Most p r e v i o u s work in this area has i n v o l v e d e x p e r i m e n t a l studies o f the p r o b e ' s effect on either the O H radical or t e m p e r a t u r e profiles, or potential flow calculations designed to study its influence on the flame flowfield. Most studies o f radical profile distortions have been based on c o m p a r i s o n o f O H c o n c e n trations measured by v ar io u s s p e c tr o s c o p i c techniques and m o l e c u l a r b e a m - m a s s s p e c t r o m e t r y ( M B M S ) in similar flames. Cattolica et al. [1] e x a m i n e d the O H profile a s t o i c h i o m e t r i c methCopyright © 1986 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017
a n e - a i r flame at at m o sp h er i c pressure by laser absorption and M B M S . They found that except very close to the burner, the M B M S profile was shifted five orifice diameters d o w n s t r e a m in c o m p a r i s o n with the p r e s u m a b l y undistorted absorption m e a s u r e m e n t s . In a m o r e recent study, Stepowski et al. [2] c o m p a r e d laser induced f l u o r e s c e n c e (LIF), laser absorption, and M B M S m e a s u r e m e n t s o f O H in a low pressure p r o p a n e - o x y g e n flame. In the region o f the flame w h e r e O H diffuses toward the burner surface, the probe was found to l o w e r the O H co n cen t r at i o n just upstream of its tip. Spatially r e s o l v e d L I F m e a s u r e m e n t s made as a function o f radial position just upstream o f the nozzle tip support this observa-
0010-2180/86/$03.50
20
OWEN I. SMITH and DAVID W. CHANDLER
tion. The degree of perturbation to the gas at this point was found to decrease with increasing burner-probe separation. Although the MBMS OH profile was displaced downstream in this region, the displacement was itself a function of burner-probe separation. As a result, no simple correlation was found between the optical and MBMS measurements. Downstream of the reaction zone, where the OH concentration gradient is small, no significant perturbations were observed. This suggests that the effects observed in the preheat and reaction zones might be due to the lowered diffusive flux of OH caused by recombination on the downstream probe surface. In this case, no perturbation would be expected in the region where the diffusive flux is either small or in the direction of the flow. There have been several investigations of the effect of probes on the temperature field in flames. These include direct measurements of the temperature profile in the presence and absence of the probe by means of thermocouples [1, 3, 4] and time of flight (TOF) techniques [1]. Thermocouple measurements of the temperature field near the probe have produced mixed results. One study shows temperature decreases of 100-200K immediately upstream of the cone
[3]. No thermal perturbations were observed in a more recent study [2]. The temperature just upstream of the probe tip may also be determined by TOF in a molecular beam system, that is, by interpretation of the moments of the beam's velocity distribution downstream of the point of translational freezing. Comparison of TOF temperature profiles with those taken by a thermocouple in the absence of the probe shows much the same behavior observed previously for the OH profile [1]; e.g., the TOF profile is shifted downstream about five orifice diameters. As before, this correlation fails to hold near the burner surface. The result is typically as depicted in Fig. 1. Aside from the processes of radical diffusion and heat conduction to the probe walls, distortions in radical and temperature profiles could also result from coupling between the flame chemistry and hydrodynamic distortions induced by the probe. Potential flow calculations based on an infinite cone with a point sink embedded in the tip indicate that the mean residence time of the sampled gas can be significantly different from that corresponding to the same position in an unperturbed flame [5]. One finds that the residence time for a fluid
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Fig. 1. Time-of-flight and thermocouple temperature profiles for a H202/Ar = 0 . 1 7 0 / 0 . 0 6 6 / 0 . 7 6 4 flame at 88 Torr. Thermocouple measurements were taken without the probe present. Probe orifice diameter is 0.04 cm.
FLAME DISTORTION BY A PROBE
21
element moving along the probe centerline is much lower than that for the unperturbed flame. However, because of axial symmetry, most of the sampled gas travels along streamlines near the stagnation streamline, where the local residence time is much larger than that of the unperturbed flame. MBMS systems usually extract a small portion of the sample jet originating at the probe orifice by means of a skimmer. This is then passed to the mass spectrometer for 20
i
analysis. In a typical configuration, this would consist of the central 2.5% (by volume) of the jet. For this reason, one would expect the residence time of the portion of the sample available for analysis to be more characteristic of that along the centerline than that of the sample as a whole. The mean residence time for any portion of the sampled gas may be obtained by integrating over the appropriate streamlines. Figure 2a
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(b) Fig. 2. (a) Plots of mean residence time as a function of burner-probe separation for several core fractions of the sampled gas (solid curves), ranging from 0.25 (by volume) to 1. Unperturbed residence time profile (broken curve). All data refer to the system described in Fig. 1. (b) Time-of-flight temperature profile of Fig. 1 corrected for residence time distortions as outlined in the text.
22
OWEN I. SMITH and DAVID W. CHANDLER
depicts mean residence time as a function of probe-burner separation for the system of Fig. 1. The solid curves represent the mean residence time profiles corresponding to various fractions of the sampled gas. The residence time profile for the unperturbed flame is given for comparison. The calculations indicate that very large residence time distortions are to be expected for the central core of the sample. Where nonequilibrated chemical reactions occur on this time scale, radial concentration gradients will be induced and the resulting radial diffusion will work to counteract the effect of the broad residence time distribution. Essentially, the flame is no longer one-dimensional and considerable diffusive mixing will take place upstream of the orifice. Thus, the mean residence time characterizing the gas which is analyzed should be larger than that predicted for the core 2.5 %. To a first approximation, one could correct for composition or temperature distortions arising from residence time perturbations by mapping the perturbed residence time profile into the unperturbed profile. In Fig. 2b we have applied this correction to the TOF temperature measurements of Fig. 1, using the residence time profile for the core 75% of the sample. Except very near the burner surface, agreement with the thermocouple profile is much improved. It seems that the concept of the sample being effectively drawn from some distance upstream of the orifice, or the "actual mean sampling distance of the sampling p r o b e " [2], might well be a manifestation of hydrodynamic distortions. To summarize, previous work in this area suggests at least two possible mechanisms for the probe induced distortion of one-dimensional, premixed flames.
sample transmitted to the analytical instrument. Other data suggest that the body of the cone dominates, and that this disturbance propagates upstream of the probe by reduced diffusive fluxes of chain carriers. In order to distinguish between these mechanisms, measurements of probe induced perturbations to a sharply peaked radical profile would be extremely valuable. It would also be helpful to evaluate the role of probe suction (sink strength) in the perturbations.
I. The probe acts as a radical and/or heat sink. 2. Hydrodynamic distortions couple with flame chemistry to produce the observed distortions. Within the former category, some data seem to indicate that the probe tip dominates, e.g., that the species and/or thermal boundary layers originating at the stagnation point penetrate far enough toward the orifice centerline to effect the
EXPERIMENTAL In this study we have examined the effect of a probe on the CN concentration field in a nearly stoichiometric hydrogen-oxygen-argon flame doped with HCN. The flame, of initial composition O2/H2/Ar/HCN = 0 . 1 4 3 / 0 . 2 5 9 / 0 . 5 8 5 / 0.013, was stabilized on a cooled porous plug burner at 25 Torr. The burner and gas handling system have been described previously [6]. A number of factors motivate our choice of the CN radical to characterize probe distortions to the flame's structure. First, the CN profile in this flame is relatively narrow in comparison with those of the major flame radicals. It exhibits both strong positive and negative gradients, and peaks somewhat nearer the burner surface. It is also highly reactive and strongly coupled to the major flame radicals through fast chemistry. Second, spatially resolved relative CN concentrations can be measured by laser induced fluorescence with relative ease. CN rotational temperatures can also be obtained from the spectra. The hydrogen-oxygen system was chosen as the matrix for this study because the applicable chemistry is fairly well understood [6]. Atomic hydrogen is present in concentrations of a few percent in this flame, and dominates most of the H atom abstraction steps. For this reason, diffusive transport plays an important role in determining the flame structure. These factors, taken together, should make the CN concentration field very sensitive to probe disturbances (probably unusually so). A conical quartz probe of 60* included angle was used in this study. The orifice diameter was
FLAME DISTORTION BY A PROBE 0.5 mm. This design is representative of those used in MBMS systems with fairly large pumping capacity in the first stage [1]. The probe could be positioned along the burner axis independently of the burner by means of a dial gauge. The fixture used to move the probe also served to evacuate the region downstream of the orifice. The pressure drop across the orifice was at least a factor of 3. This is normally sufficient to ensure choked flow. For a microprobe with much smaller orifice diameter and longer entry length, a pressure ratio of 5 is thought to be necessary at high temperatures [7]. However, the frictional effects responsible for this behavior should be much less important in our case. At worst, they will be no more severe than in the microprobe. Colket et al. [7] indicate that for the microprobe the mass flowrate at a pressure ratio of 3 is 95 % of the choked value, so that our sink strength should be representative of those achieved in MBMS systems. The laser detection system used in this study was a unique tunable continuous wave ultraviolet laser developed at Sandia for flame studies [6]. A schematic of the apparatus is shown in Fig. 3. The source is capable of scanning from about 360 to 400 nm with 10 mW power and 0.006 nm resolution without resorting to frequency doubling. This is accomplished by directly exciting Polyphenyl 1 laser dye with all ultraviolet lines from an argon ion laser. At
23 these power levels it is easy to show that saturation is not important. Further, the spectral resolution of 0.006 nm allowed individual rotaOonal lines in the 0-0, 1-1, 2-2 bands of the CN B2y. +-X2Y~ + transition to be selectively excited. Fluorescence was detected at right angles to the laser propagation through a 500 #m pinhole and a broad band filter that removed the visible light. Spatial resolution of the optical system was checked experimentally by looking at scattered light from a thin wire. It was determined that the spatial resolution of the combined excitation and detection system was about 0.7 mm radially, by about 0.2 mm axially. Absorption studies on this flame indicated that radiative trapping of the fluorescence was negligible. Less than 3% of the light is absorbed in 12 passes through the flame. Laser induced fluorescence spectra of the R branch of the B - X transition of CN were used to obtain rotational temperatures. The Doppler width of an individual rotational line at temperatures between 1100 and 1400K is approximately 0.15 c m - l , a factor of 3 smaller than the laser resolution. Since the spectrum is normalized by the laser power, the height of an individual LIF line is proportional to the population in the corresponding ground state. The rotational temperature is obtained from a Boltzmann plot of the logarithm of the intensity (normalized by the rotational line strength and the degeneracy)
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24 against the energy of the corresponding l o w e r vibrational-rotational energy level. Not all of the lines between R(0) and R(24) were used in the Boltzmann plot. The R(3), R(6), R(10), and R(14) lines are spectroscopically perturbed by the A state [8], and therefore emit with characteristics of the A state. The detection optics discriminate against this light; therefore, those lines appear smaller than they should. Also, lines which overlap lines of the 1-1 or 2-2 bands were not used. Most of the relative CN concentration data were obtained by twice scanning the R(4) through R(12) transitions at discrete locations within the flame. The line intensities for each measurement were corrected for rotational state distribution by the temperature profile as determined above. To aid in interpolation between these points, a number of CN profiles were taken by sitting on a rotational line and traversing the burner axially. This procedure allowed us to locate the point of maximum concentration quite accurately, and to determine the general shape of the profile. While these traverses could be accomplished quite rapidly, the dye laser often drifted off the line center during the process. This frequency instability was associated with vibration of the optics induced by the stepping motor driving the burner. In general, we found that the discrete data were much more reproducible. Collisional quenching is not considered in analysis of the data, as the flame is dilute and number density varies by only 30% over the region of interest. RESULTS Figure 4 displays relative CN concentration profiles at several radial positions for a constant probe-burner distance of 1.08 cm. The CN profile obtained in the absence of the probe is also given. The coefficient of variation among measurements taken at the same position was usually around 0.07. Where measurements were made at points of low CN concentration, values as high as 0.15 were occasionally observed. Based on this, we estimate the uncertainty in the concentration data to be no more than + 15%.
OWEN I. SMITH and DAVID W. CHANDLER
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Fig. 4. Relative CN concentration profiles by LIF in an HCN doped H2-O2-Ar flame at 25 Torr. Solid circles denote
the profile in the absence of the probe. Perturbed CN profiles at various radial positions were obtained for a probe-burner separation of 1.08 cm. Open circles and closed squares represent two independent measurements of the CN rotational temperature profile. The broken curve denotes the calculated flame temperature, obtained as outlined in the text. The data presented in Fig. 4 were obtained with the probe positioned just downstream of the point of maximum CN concentration in the undistorted flame. All are normalized by the maximum undistorted concentration. Along the axis of the probe (r = 0), considerable attenuation is observable within five orifice diameters (0.25 cm) of the probe tip. Just upstream of the orifice the CN concentration is about 40% lower than that at the same position in the undistorted flame. Profiles taken off the axis (r > 0) approach the undistorted profile slowly with increasing radial distance. Two independent measurements of the CN rotational temperature profile (taken with the probe removed) are displayed in Fig. 4. In most cases, these measurements differ by less than 70K. At the downstream edge of the CN profile, where the concentration is small and the high temperature broadens the rotational state distri-
FLAME DISTORTION BY A PROBE
25 accurate since additional data (described in the previous section) were used for this purpose. No additional data were used in the radial interpolations, so that where large radial gradients exist the contour positions are less certain. With the probe positioned just upstream of the undistorted profile peak (Fig. 5), some degree of perturbation is observed on the centerline over the entire profile. Strong radial gradients are present further downstream near the probe wall. With the probe just downstream of the undistorted peak (Fig. 6), probe distortions no longer reach the upstream edge of the profile. Radial gradients downstream of the orifice are not as strong. As the probe is moved well downstream of the undistorted peak (Fig. 7), the upstream portion of the CN profile becomes practically free of distortion. Radial gradients further downstream are much smaller. Figures 5-7 indicate that while probe distortions are reduced in magnitude as the probe is moved downstream, significant perturbations
bution, larger discrepancies are observed. The temperature profile obtained from a numerical simulation of this flame shows reasonable agreement with the measurements. Details of the simulation are given later. Measurements were also made with the probe present; however, any thermal perturbation present was too small to detect. To the best of our knowledge, these are the first optical temperature measurements obtained near the probe surface. One can obtain a two-dimensional contour map of the CN concentration field in the vicinity of the probe by radial interpolation of the data given in Fig. 4. Figures 5-7 show such results at probe-burner separations of 0.93, 1.08, and 1.42 cm respectively. The unperturbed concentration, which would apply at large distances from the centerline, is shown as a function of axial distance along the edge of each figure. The filled circles denote the points at which discrete measurements were taken. The axial interpolations depicted by the contours should be quite QUARTZ S A M P L I N G
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Fig. 5. Contour map of the relative CN concentration field near the probe for a probeburner separation of 0.93 cm. The unperturbed profile is shown along the left-hand edge.
26
OWEN I. SMITH and DAVID W. CHANDLER
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can be present even well downstream of the undistorted profile peak. One can put this on a quantitative basis by mild extrapolations of data such as that presented in Fig. 4. The result, expressed in terms of percentage reduction in CN concentration at the orifice entrance, is given in Table I. Several CN concentration measurements were made along the centerline upstream of the probe in the absence of suction for probe-burner separations of 0.93 and 1.43 cm. In each case the results were not significantly different from those obtained with suction applied. DISCUSSION In view of the large residence time distortions predicted on the basis of the simple theory
presented previously, we were greatly surprised at the total lack of sensitivity shown by our results to suction on the probe. Even within two orifice diameters upstream of the probe tip and in a region of large CN concentration gradient, our results indicate that very little of the distortion noted in the previous section can be attributed to suction. Since the CN chemistry is fairly rapid here, yet is far from equilibrated, it is clear that the distortions observed in this study cannot be explained solely on the basis of hydrodynamic perturbations. Our results show that the probe has no strong influence on the temperature field within the flame. This indicates that the flame structure as a whole is not grossly distorted, as would be expected where flame attachment is important, and is in good agreement with the results of
F L A M E D I S T O R T I O N BY A PROBE
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Probe-Burner Separation (cm)
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>48 42 33
Stepowski et al. [2]. Near the probe surface our LIF signal is typically small (due to the low CN concentrations observed here) and is subject to interference by scattered light. As a result, the uncertainty in our temperature measurements (which is already fairly large) grows rapidly.
Nevertheless, it is certain that temperature reductions at the orifice entrance o f the magnitude depicted in Fig. 1 ( > 400K at some points) would be noticeable. Our results indicate that the temperature just upstream of the orifice is largely unperturbed while the T O F measurements (Fig. 1) typically show large reductions in the sampled gas temperature. Comparison of the incompressible flowfield in the presence of a point sink and the size o f our optical volume indicate that our measurements should be representative o f the sample passed through the skimmer for T O F analysis; however, subsequent calculations based on a more realistic model very near the orifice (compressible flow and a disk sink [9]) cast some doubt on this
28 conclusion. This may explain the discrepancy between our observations and those made by TOF methods. Our most interesting data are those involving perturbations to the CN concentration field. At first glance, these results seem to disagree with those of Stepowski et al. [2]. Probe perturbations definitely extend downstream of the location of peak CN concentration, indicating that lowered upstream diffusive flux of CN cannot be wholly responsible for our observations. However, as noted previously, CN is closely coupled with the major flame radicals (particularly H and O) by fast chemical reactions. Thus, reduced upstream fluxes of the major radicals would be expected to influence the CN profile. To investigate the role of gas phase chemistry in our system, we have performed a numerical calculation of the unperturbed flame structure using a package of codes developed at Sandia [9, 10]. The mechanism proposed by Miller et al. [6], which does an excellent job of predicting reactant and product profiles in very similar flames, was used without modification. Calculated profiles for the major flame radicals and CN are presented in Fig. 8. Our experimental CN profile (normalized to the peak calculated CN mole fraction) is also presented. Its location shows fairly good agreement with the calculations. The peak concentration of CN occurs slightly upstream of those of H and O, and well upstream of that of OH. As mentioned previously, the calculated and measured temperature profiles also show good agreement (Fig. 4). At 0.7 cm, well upstream of the profile peak, the calculations show that chemistry has little influence on the CN concentration. The temperature is relatively low, and diffusive and convective fluxes dominate. We attribute the perturbations observed here to reduced diffusive flux from downstream, caused by lowered CN concentrations near the probe. Figures 5-7 indicate that the degree of probe induced distortion at this point correlates well with the decrease in concentration gradient. In the region of peak concentration (1.05 cm), chemical terms dominate the species equation for CN. The CN concentration is heavily influ-
OWEN I. SMITH and DAVID W. CHANDLER
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Fig. 8. Calculated profiles for H, O, OH, and CN (solid curves), obtained as outlined in the text. Measured CN profile (broken curve) obtained without the probe present and normalized to the peak calculated mole fraction for CN.
enced by the reaction between HCN and atomic hydrogen, H + HCN = H2 + CN,
(1)
which proceeds rapidly in both directions. Reaction (2), O + HCN = OH + CN,
(2)
also contributes significantly to CN formation, and reactions of CN with 02 and OH contribute to its destruction: CN + 02 = NCO + O,
(3)
CN + OH = NCO + H.
(4)
At this point in the flame, CN formation is dominated by reaction (1) (which is nearly balanced). As previous studies have shown that stable species are influenced little by the probe, the lower CN concentrations observed here in the presence of the probe suggest that the H atom profile is probably being perturbed. If this is the case, then the H atom profile is distorted
FLAME DISTORTION BY A PROBE
29
by a probe positioned downstream of its peak (Fig. 7). This supposition is not necessarily inconsistent with the results of Stepowski and coworkers, since the H and OH concentrations are coupled by the rapid reaction OH + He = n 2 0 + H.
(5)
In this study the probe is always positioned upstream of the peak OH concentration. Still further downstream, at 1.40 cm, both downstream transport and chemical reaction of CN are important. Reaction (1) still dominates both the formation and destruction rates, but is actually further from being balanced due to greatly increased consumption via reactions (4) and (6): CN + O = CO + N.
(6)
Both lowered flux of CN from upstream and decreased concentrations of O and H seem likely to play a role in probe distortions here. Decreased O and H concentrations could result indirectly from reduced OH concentration, since the hydrogen-oxygen chemistry is close to being partially equilibrated here. The data presented in Figs. 5-7 would seem to favor wall catalyzed radical recombination as the ultimate cause of CN concentration distortions. Nevertheless, the data are not conclusive. Reaction (1) is fairly endothermic (AH ° = 25 kcal/mole). Where it is rapid compared with other processes effecting CN, a temperature decrease of 100K could reduce the CN concentration by about a factor of 2. Although we observed no thermal perturbations, a temperature change of this magnitude cannot be ruled out close to the probe surface.
CONCLUSIONS The conclusions reached in the course of this study can be summarized as follows. 1. Any large perturbations ( > 100K) to the temperature field of the flame must be localized near the central portion of the orifice entrance.
2. Suction does not play a critical role in probe induced distortions of CN concentration observed in this study. 3. Composition distortions decrease with increasing distance from the burner but can persist downstream of the point of peak concentration for some radicals, particularly those appearing early in the reaction zone. 4. Chemistry and diffusion both play important roles in perturbation of the CN profile. Thermal perturbations, although not detected in this study, may also contribute. When sampling close to the burner, where distortions are the largest, upstream diffusion blockage of the radical itself seems to dominate.
This study was supported by the United States Department o f Energy under Contract Number DEACO476DPO0789, and by the National Science Foundation under Grant Number CPE82-0983 7. REFERENCES 1. 2.
3. 4. 5. 6.
7.
8. 9. 10. 11.
Cattolica, R. J., Yoon, S., and Knuth, E. L., Comb. Sci. and Tech. 28:225 (1982). Stepowski, D., Puechberty, D., and Cottereau, M. J., Eighteenth Syrup. (Int.) on Combustion, 1981, p. 1567. Biordi, J. C., Lazzara, C. D., and Papp, J. F., Combust. Flame 23:73 (1974). Bowman, C. T., in Progress in Astronautics and Aeronautics, Vol. 53 (B. Zinn, Ed.), 1977, p. 3. Smith, O. I., Combust. Flame 40:187 (1981). Miller, J. A., Branch, M. C., McLean, W. J., Chandler, D. W., Smooke M. D., and Kee, R. J., Proceedings o f the Twentieth Syrup. (Int.) on Combustion (in press). Colket, M. B., Chippetta, L., Guile, R. N., Zabielski, M. F., and Serry, D. J., Combust. Flame 44:3 (1982). Wagner, A. T., Phys. Rev. 64:18 (1943). Yi, C. H., and Knuth, E. L., submitted to Combust. Flame May (1985). Smooke, M. D., Report SANDSI-8040, Sandia National Laboratories, Livermore, CA, 1982. Kee, R. J., Miller, J. A., and Jefferson, T. J., Report SAND80-8003, Sandia National Laboratories, Livermore, CA, 1980.
Received 31 January 1985; revised 16 May 1985