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Investigation of sooting in microgravity droplet combustion
Twenty-Sixth Symposium (International) on Combustion/The Combustion Institute, 1996/pp. 1243–1249
INVESTIGATION OF SOOTING IN MICROGRAVITY DROPLET COMBUSTION MUN YOUNG CHOI and KYEONG-OOK LEE Department of Mechanical Engineering University of Illinois at Chicago Chicago, IL 60607, USA
This investigation describes experiments performed at the 2.2-s microgravity facility at NASA-Lewis Research Center using n-heptane droplets burning in atmospheric pressure air. The transient soot distributions within the region bounded by the droplet surface and the flame were measured using a full-field light-extinction technique and subsequent tomographic inversion using Abel transforms. It has been speculated that under microgravity conditions, the absence of buoyancy and the effects of thermophoresis create a situation in which high concentrations of soot accumulate into a soot cloud. This study presents the first quantitative measurements of the degree of sooting for microgravity droplet combustion. Results indicate that the soot concentrations for microgravity heptane droplet flames (with maximum soot volume fractions ; 60 ppm) are significantly higher than corresponding values that are reported for normal-gravity flames (which are typically ; 1 ppm). Since the accumulated soot represents incomplete combustion and can also modify the heat-transfer mechanism by altering the local temperature distributions within the fuel-rich region, sooting effects can significantly influence the burning behavior under microgravity conditions. Experiments were also performed to assess the droplet-size–dependent effects on the sooting behavior. Initial experiments using 1.0 and 1.75 mm initial diameter droplets indicate that while the distribution of soot volume fractions are comparable for the two cases (for the observation times that were available), the ratio of the instantaneous mass of soot contained within the fuel-rich region for the 1.75-mm droplet compared to the 1.0-mm droplet was more than a factor of 3. This ratio is also expected to increase for longer observations.
Introduction The investigation of the burning of a single droplet in microgravity is an ideal problem from which to gain fundamental understanding of diffusion flame characteristics [1,2]. The pioneering microgravity studies provided important insights regarding steady-state burning behavior that were used to calibrate theoretical predictions. However, the effects of sooting and thermal radiation on the overall burning characteristics were not considered [3,4]. Recent microgravity experiments and numerical calculations indicate that soot/soot-shell formation and thermal radiation may affect all phases of steady-state and transient burning characteristics. For example, Choi et al. [5] reported that the burning rate of n-heptane droplets were as much as 30–40% lower than the classically accepted results [4]. The main conclusion drawn from that investigation was that the formation and accumulation of the soot shell and its axisymmetric configuration caused by small degrees of relative droplet/gas convection may affect the heat transfer from the flame to the droplet. Recent numerical studies have investigated the effects of radiative heat transfer on droplet combustion. Saitoh
et al. [6] found that thermal radiation from n-heptane droplet flames can reduce the maximum flame temperature by 25%. Chao et al. [7] determined that thermal dissipative losses from gas-phase products can promote the onset of extinction of droplet flames. For sooting droplet flames, it is likely that soot emission will dominate thermal radiation from the flame and therefore may enhance these effects [8]. If radiation represents a viable heat-transfer mechanism to the droplet, the magnitude will be determined not only by the soot concentration near the flame but throughout the entire fuel-rich region, which can attenuate the radiation. Recent work by Chang and Shieh [8] suggests that the droplet burning rate is sensitive to the soot concentration within the flame and the opacity of the liquid droplet. Therefore, in order to accurately assess the importance of sooting on droplet combustion, it is necessary to obtain transient soot concentration distributions under microgravity conditions. Under normal-gravity conditions, several techniques have been used to determine the degree of sooting in droplet flames. Randolph and Law [9] studied the sooting characteristics of small, freely falling droplets, using both visual and sampling
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Fig. 1. Schematic of experimental apparatus: B1-B4: detection camera, lens, and filters; B5: pinhole; B6, O1: 39-diameter mirrors; C1: ignitors; C2: deployment needles; C3: silica carbide fiber posts; C4: 200-mm plano-convex lens; O2: incandescent lamp; O3: high-speed camera; T1: diode laser; T2: collimating lens.
techniques. They found excellent correlation of soot index with the luminous region of the flame. The luminosity was then successfully used as a nonintrusive measure of the degree of sooting to study the effects of droplet spacing, fuel blending, and convection. However, since the luminosity is expected to be dominated by soot concentrations just within the vicinity of the flame front, it does not provide detailed information regarding the entire structure of the soot-containing region. Kadota and Hiroyasu [10] first measured soot concentration in droplet flames, using optical techniques under normal-gravity conditions. In their study, light-extinction measurements were performed at a fixed location below the droplet, and the radial variation was deduced by assuming that the regressing flame maintained constant spherical structure. In the region below the droplet, this procedure may provide accurate information since the flame geometry is nearly spherical. However, the same approach cannot be performed for locations above the droplet (this is the more important region that produce the bulk of the particulates), where it displays both spatial and temporal variations. The transient nature of droplet flames requires diagnostic techniques that can instantaneously examine the entire soot field. Recently, several groups have used laser-induced incandescense (LII) to measure the soot concentrations for droplet flames in normal gravity. The incandescense results from the rapid heating of the soot particles by a shortduration, high-powered laser. Vander Wal and coworkers [11,12] performed two-dimensional LII measurements to obtain temporally and spatially resolved soot volume fraction, fv, for suspended droplets of various alkane and chlorinated alkane fuels. Gupta and Santoro [13] performed similar LII experiments using freely falling droplets of benzene/ methanol mixtures. The advantage of using this tech-
nique is that soot concentration is proportional to the intensity of the incandescence (provided that the intensities have been calibrated using an independent technique [14]). However, this technique has not been applied for droplet combustion studies under microgravity conditions. An alternative diagnostic approach employing fullfield light extinction and tomographic inversion was used by Lee et al. [15] to study the burning characteristics of suspended toluene droplets under normal-gravity conditions. Greenberg and co-workers first used this technique to measure soot concentrations in laminar gas-jet diffusion flames in microgravity [16–18]. The motivation for the present study is to use this technique to measure the transient soot concentrations for droplets burning under microgravity conditions. In previous microgravity experiments, the degree of sooting was estimated by observing the “darkness” of the sooting region [5,19,20]. This study presents the first quantitative measurements of the soot volume fractions for microgravity droplet flames. The ultimate goal of this study is to control the degree of sooting using pressure, inert variation, oxygen concentration, fuel type, and droplet dimensions and determine their effects on the steady-state and transient burning characteristics. In the following sections, experiments performed using n-heptane droplets of different initial diameters are presented.
Experimental Description Experiments were performed at the 2.2-s microgravity drop tower at NASA-LeRC in Cleveland, OH. Figure 1 displays the schematic of the experimental apparatus. (The combustion chamber and apparatus design was based on the experimental rig used by the NASA Droplet Combustion
INVESTIGATION OF SOOTING IN MICROGRAVITY DROPLET COMBUSTION
Fig. 2. Photograph of the deployment syringes, suspending fiber, hot-wire ignitors, and droplet.
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sitioned at 458. The beam was transmitted through the chamber and was then focused using a 200-mm plano-convex lens and redirected using a second 75mm mirror (also positioned at 458) and imaged through a spatial filter to a high-resolution CCD camera located on the bottom optical plate. An image quality interference filter and an absorption neutral density (o.d., 3.0) filter were placed directly in front of the camera to discriminate against flame emission or stray light. The video output from the camera was transmitted through a fiber optic cable and recorded onto a BETACAM recorder at 30 frames/s. A second CCD camera placed on the middle optical plate was used to capture the flame image to denote the time of ignition. Figure 2 displays a laser-backlit photograph of the droplet and the deployment and ignition apparatus. The droplet was formed by using two opposing hypodermic needles of 0.2 mm in diameter (shown as vertical lines) that were attached to two separate rotary motors. Prior to release into free fall, the droplet was deposited on a 10-lm silica-carbide–suspending filament. The filament was needed to prevent droplet motion during the experiment. (Due to the degree of magnification used in the present experiment, movement of several droplet diameters in either directions can clip the sooting regions out of the field of view. Due to its small size, the presence of the filament did not affect the sooting behavior near the region of analysis, which is depicted by the black dotted line in Fig. 3a.) Deployment of the droplet was actuated by the rapid retraction of the needles, which causes some oscillations in the droplet. However, it has been shown in a previous study that it decays within 0.2–0.3 s for droplet dimensions that are typical of microgravity experiments [5]. Approximately 0.3 s after deployment, the droplet was ignited using two kanthol hot wires (shown as two circular lines) that were attached to translational motors. The igniters were then retracted away from the field of view to allow for undisturbed burning.
Soot Volume Fraction Measurements in Microgravity Fig. 3. Time sequence of 1.75-mm n-heptane droplet burning in atmospheric pressure air. (a) 0.2 s and (b) 0.5 s.
Experiment.) On the top optical plate, light from a 635-nm diode laser was attenuated by a variable neutral density filter and expanded using a Newport LCV expander/collimator to produce a beam of approximately 50 mm in diameter. The expanded beam was directed through the top optical port of the 12-l stainless steel combustion chamber (which was fitted with a 50-mm-diameter quartz window with antireflection coating) using a 75-mm-diameter mirror po-
Figures 3a and 3b display photographs of a 1.75mm initial diameter heptane droplet in atmospheric pressure air at various times during the burning process. Shortly after ignition, soot particles are transported from regions just inside the flame toward the droplet (as shown by the very diffuse and broad soot containing region surrounding Fig. 3a compared with Fig. 3b). The transported soot particles eventually form a soot cloud that is concentric with the droplet (see Fig. 3b). The term “soot cloud” is used here instead of “soot shell” since the region is comprised of a porous dispersion of soot rather than a structurally rigid shell. For all burning times that
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Fig. 4. Soot volume fraction distributions plotted as a function of nondimensional radius for 0.2 to 0.8 s in 0.1-s increments for 1.75-mm droplet.
were analyzed, the images indicate the presence of soot particles at radial positions beyond the darkest regions of the soot cloud, indicating a continuous production and transport of soot. The intensity distributions were measured by digitizing the Betacam images with a high-resolution frame acquisition and processing board and a custom image-processing algorithm [15]. The images were filtered using a linear, 3- by 3-pixel mean filter. The intensity ratio distributions were calculated by dividing the gray-level values for the sooting image along the line of analysis (black dotted line in Fig. 3a) by corresponding intensities measured for the background image (which was captured prior to droplet ignition and therefore unattenuated by soot). The intensity ratios were then averaged using a moving 5-pt operator. The minimum intensity ratios were measured closer to the droplet surface than the flame front. Since the majority of soot is expected to be formed near the flame [2], it is evident that the transport toward the droplet surface is facilitated by thermophoretic forces [18,21–24]. Due to vibrations of the various optical components caused by the release into free fall, the background image was not obtained until approximately 0.8 s after free fall. Alternatively, the use of background images captured prior to release into free fall would have increased the overall observation time to beyond 1.0 s. However, the associated spatial fluctuations in the intensity ratios would have been an order of magnitude larger. The projected light-extinction ratio distributions obtained as a function of time were used to determine the soot volume fraction, fv(r), using a 3-pt Abel deconvolution technique [25] with soot optical property determined using the light-extinction/gravimetric-calibration technique [26]. Figure 4 displays the deconvolved soot volume fraction distributions calculated as a function of time for the 1.75-mm hep-
tane droplet. It is speculated that the combination of increased residence time due to lack of buoyancy [2], which enhances soot formation, and thermophoretic forces, which transports the soot toward the droplet surface (preventing oxidation of the soot particles), creates conditions in which a significant amount of soot is accumulated into a soot cloud within the fuel-rich region [5,19]. Since the initiation of soot-cloud formation is observed within 0.15 s after ignition, it is expected that the average fuel residence time within the fuel-rich region will be comparable to this value, which suggests that the residence time in microgravity is only 3–4 times longer than that observed under normal-gravity conditions [2]. However, the total residence time for soot formation and growth must take into account the unique behavior observed for droplet combustion in which the soot within the soot cloud persists throughout the burning lifetime (which can last up to 5 s for large droplets ; 2 mm). Due to this accumulation effect, the maximum fv is shown to increase significantly as a function of time from a value of ;20 ppm at 0.2 s to ;60 ppm at 0.8 s. The concentrations of soot measured for the microgravity heptane flames are significantly higher than the nominal values of 1 ppm measured under normalgravity conditions by Vander Wal and co-workers using LII [11,12]. Similar observations of enhanced sooting in microgravity was reported in an earlier study by Bahadori et al. [27] using laminar gas-jet flames. Figure 4 also displays the transient character of the soot standoff ratios (SSR; radial position of the maximum soot volume fraction divided by instantaneous droplet radius). It is also interesting to note that due to significant accumulation as a result of thermophoresis, the region containing the bulk of the soot concentrations is rather narrow compared to the flame dimension (the flame standoff ratio is on the order of five to six). The measured SSR for the initial phases of burning are significantly lower (;1.5) than those measured in previous studies [28,29]. The measurements approach values measured by Jackson and Avedisian [29] only during the latter stages of burning. This may be due to the fact that for droplets of this size, heat-up periods can account for up to 20% of the total lifetime [30] (which corresponds to approximately 1.1 s). Therefore, it is unlikely that the droplet has reached quasisteady-state conditions during the relatively short observation times that were available (;1 s). In many of the early droplet combustion studies, the effects of sooting were disregarded due to a combination of the complexity involved in incorporating the soot formation mechanisms and the belief that sooting for fuels such as heptane was not an important factor (based on degree of sooting observed under normal-gravity conditions). However, the importance of sooting in microgravity droplet
INVESTIGATION OF SOOTING IN MICROGRAVITY DROPLET COMBUSTION
combustion has been recognized in more recent investigations [5–8,19–24]. One mechanism for the variations in the burning behavior as a function of sooting is the possible modification in the heat transfer from the flame to the droplet surface [5,24,31]. Chang and Shieh [8] reported that the burning rate is enhanced when thermal radiation from soot was incorporated into their n-heptane droplet combustion model. The degree of enhancement was sensitive to the chosen soot concentration (the distribution was assumed to be constant and uniform) and the opacity of the liquid droplet. However, a more accurate assessment of thermal radiation can be determined by using the experimental measurements of the transient soot distributions and the spectral absorption characteristics of the liquid fuel. In addition to modifying the flame temperature through enhanced radiative dissipation, soot concentrations within the fuel-rich region may attenuate the radiative and conductive heat transfer to the droplet. Spectral attenuations of the thermal radiation (from the flame front to the droplet surface) as high as 30% were calculated in the range of 1–2 lm using RADCAL (a narrowband radiation model) [32] and typical flame temperatures between 1800 and 2100 K. The presence of soot can also modify the thermophysical properties of the soot-laden gas. For example, the heat capacity of the soot/gas mixture will be higher than that of the surrounding gas [29]. Although the thermal conductivity is higher for soot compared to the surrounding gas, the overall conductivity of the mixture is not expected to be much different from the gas since contact among soot aggregates is not likely. The resulting changes in the gas-phase temperature gradient at the droplet surface (due to the presence of soot) can directly affect the burning rate [33]. However, detailed temperature measurements are needed to ascertain the degree of modification of heat transfer to the droplet as a result of the above-mentioned effects. The accumulation of soot particles within the fuelrich region can affect the burning behavior in other ways. The formation of soot through pyrolysis reactions (which are endothermic) serves as a heat sink, and accumulated particles within the soot-cloud region represent incomplete burning since they are not oxidized [5,24]. Both of these factors will reduce the effective heat of combustion and the transfer number [33]. Although the transfer number appears in the natural log term, reductions in the burning rate are expected for conditions producing significant soot concentrations [31]. Jackson and Avedisian [29] have also included the growth of soot particles (within the soot cloud) through surface reactions with the vaporized fuel as a possible mechanism for enhancing the degree of sooting, which further reduces the efficiency of the combustion process.
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Droplet Size Effects Classical theories of droplet combustion characteristics indicate negligible size-dependent effects on the overall burning parameters [34]. For example, the predictions of the vaporization rate and flame standoff ratio are insensitive to the initial droplet diameter. There are dimensional effects related to natural and forced convections that can be used in conjunction with the “d2-law” analysis [35]. These effects are not expected to be important for microgravity experiments with small degrees of negligible forced convection. Recently, Jackson and co-workers [24,29] speculated that the burning behavior is affected by the initial droplet size through variations in the degree of sooting. Although the effects of sooting on droplet burning behavior (including the steady-state burning rate) were established in earlier studies [5,36], Jackson and co-workers were the first to use the initial diameter as a parameter for controlling the degree of sooting under microgravity conditions. The effects of initial droplet diameter on the sooting behavior were investigated under normal gravity by Kitano et al. [37] by measuring soot emitted from the opentipped flame. By varying the initial diameter, they were able to determine the sooting limit, which was the minimum droplet size that produces particulate emission. Under microgravity conditions, the envelope flame is proportional to the droplet size, and therefore, it is expected that longer residence time (which is proportional to the square of the diameter, d2) will be available for larger droplets, during which the fuel molecules undergo pyrolysis reactions leading to soot formation [29]. However, experimental verification of the size-dependent variations in the sooting characteristics for microgravity droplet combustion was not available. For these reasons, experiments were performed by varying the initial diameter of the n-heptane droplet. Experiments were performed using a 1.0- and a 1.75-mm initial diameter droplet burning in atmospheric pressure air for comparisons. (It is to be noted that convection can also affect the soot formation/ accumulation behavior [5]. However, only the experiments with very low degrees of convection were analyzed for this study.) Figure 5 displays the timevarying maximum soot volume fraction, fv,max, and the SSR for the 1.0- and the 1.75-mm droplet plotted as a function of time divided by the square of the initial diameter (which is proportional to the fractional burning time). The measured soot standoff ratios for the 1.0-mm droplet are in good agreement with those reported by Jackson and Avedisian [29]. For the smaller droplet, the available microgravity times are sufficient to observe that the soot volume fractions have achieved a maximum value and begin to decrease as a function of time. As the droplet size decreases due to burning, it is likely that
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Conclusions
Fig. 5. Comparisons of maximum soot volume fraction, fv,max, and soot standoff ratio, SSR, as a function of reduced time, t/d2o for 1.75- and 1.0-mm droplets.
the rate of soot formation at the flame front is also decreasing through combinations of reduced rate of mass vaporization and reductions in maximum temperature due to increased reactant leakage through the flame. Simultaneously, the Stefan flux near the droplet surface increases (assuming that the burning rate is constant), while the thermophoretic force is expected to decrease due to reductions in the temperature gradient, causing soot to be transported toward the flame [23,28]. These combined effects can reduce the amount of soot accumulated within the soot cloud as a function of time. The maximum soot volume fractions measured for the two cases are similar to within the experimental uncertainty. However, quasi-steady conditions were not attained for the larger droplet, and therefore, comparable values of maximum soot volume fractions for the two cases do not indicate that sooting behaviors are similar. A more relevant parameter to compare is the instantaneous soot mass within the region bounded by the droplet and the flame, which is calculated by integrating the product of the soot density and fv distributions with respect to volume. (Soot density of 1.8 g/cc was used.) Since the volume of the flame will scale as d3, soot mass within the envelope flame is likely to be significantly larger for the 1.75 mm droplet. Comparison of the maximum instantaneous soot mass for the 1.75-mm droplet is approximately three times larger than the corresponding value for the smaller droplet. Considering that the instantaneous mass ratios between the two droplets are likely to increase with longer observation times available to analyze the larger droplet, it is evident that there is a strong size-dependent effect on the degree of sooting. However, additional experiments performed for longer observation times are needed to determine the quantitative correlations between sooting and burning characteristics in microgravity droplet combustion.
The transient soot distributions within the region bounded by the droplet surface and the flame were measured using a full-field light-extinction technique and subsequent tomographic inversion using Abel transforms. The soot volume fraction results for n-heptane droplets represent the first quantitative assessment of the degree of sooting for isolated droplets burning under microgravity condition. The absence of buoyancy (which produces longer residence times) and the effects of thermophoresis produce a situation in which a significant concentration of soot is produced and accumulated into a soot cloud. Results indicate that indeed the soot concentrations within the microgravity droplet flames (with maximum soot volume fractions as high as ;60 ppm) are significantly higher than corresponding values that are reported for normal-gravity flames, which are typically on the order of 1 ppm. Experiments were also performed to determine the size-dependent effects on the sooting behavior. Initial experiments using 1.0- and 1.75-mm initial diameter droplets indicate that while the distribution of soot volume fractions are comparable for the two cases (for the observation times that were available), the total mass of soot contained within the fuel-rich region is significantly larger for the 1.75mm droplet. The maximum instantaneous soot mass for the 1.75-mm droplet was nearly three times larger than the maximum mass measured for the 1.0mm droplet. Results analyzed for longer observation times under microgravity conditions will likely produce larger mass ratios and increase the already significant size-dependent effects. Acknowledgments The authors gratefully acknowledge helpful advice regarding the full-field light-extinction technique provided by P. Greenberg, D. Griffin, B. Whiteside, and D. Urban and the experimental apparatus design team led by F. Gati and A. Birchenough of NASA-LeRC. We would also like to thank P. Ferkul, D. Schultz, and R. L. Vander Wal for valuable insights regarding this work. This work was supported by NASA-LeRC through Grant #NAG3-1631 with P. Ferkul and D. Schultz serving as project scientist and project monitor, respectively. REFERENCES 1. Williams, F. A. and Dryer, F. L., Science Requirements Document for Droplet Combustion Experiment, NASA-LeRC, 1994. 2. Law, C. K. and Faeth, G. M., Prog. Energy Combust. Sci. 20:65–113 (1994). 3. Kumagai, S. and Isoda, H., Sixth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1957, pp. 726–731.
INVESTIGATION OF SOOTING IN MICROGRAVITY DROPLET COMBUSTION 4. Kumagai, S., Sakai, T., and Okajima, S., Thirteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1971, pp. 779–785. 5. Choi, M. Y., Dryer, F. L., and Haggard, J. B., TwentyThird Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, pp. 1597– 1605. 6. Saitoh, T., Yamazaki, K., and Viskanta, R., J. Thermophys. Heat Transfer 7:94–100 (1993). 7. Chao, B. H., Law, C. K., and Tien, J. S., Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, pp. 523–531. 8. Chang, K.-C. and Shieh, J.-S., Int. J. Heat Mass Transfer 38:2611–2621 (1995). 9. Randolph, A. L. and Law, C. K., Combust. Flame 64:267–284 (1986). 10. Kadota, T. and Hiroyasu, H., Combust. Flame 55:195– 201 (1984). 11. Vander Wal, R. L., Dietrich, D. L., and Choi, M. Y., “Relative Soot Volume Fractions in Droplet Combustion via Laser-Induced Incandescense,” Presented at the Eastern States Section Meeting of the Combustion Institute, Clearwater Beach, FL, December 1994. 12. Vander Wal, R. L. and Dietrich, D. L., Appl. Opt. 34(6):1103 (1995). 13. Gupta, S. B., Ni, T., and Santoro, R. J., “Spatial and Time Resolved Soot Volume Fraction Measurements in Methanol/Benzene Droplet Flames,” Presented at the Eastern States Section Meeting of the Combustion Institute, Clearwater Beach, FL, December 1994. 14. Vander Wal, R. L., Zhou, Z., and Choi, M. Y., Combust. Flame 105:462 (1996). 15. Lee, K. O., Jensen, K., and Choi, M. Y., Twenty-Sixth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1996, pp. 2397–2404. 16. Griffin, D. W. and Greenberg, P. S., “Selected Microgravity Combustion Diagnostics Techniques,” 2nd Int’l Microgravity Combustion Workshop Proceedings, September 1992. 17. Greenberg, P. S., “Laser Doppler Velocimetry and Full-Field Soot Volume Fraction Measurements in Microgravity,” presented at the Third Int’l Microgravity Combustion Workshop, Cleveland, OH, 1995. 18. Greenberg, P. S. and Ku, J. C., Appl. Opt., 1995, submitted. 19. Shaw, B. D., Dryer, F. L., Williams, F. A., and Haggard, Jr., J. B., Acta Astronaut. 17:1195–1120 (1988).
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20. Jackson, G. S., Avedisian, C. T., and Yang, J. C., Proc. R. Soc. London A 435:359–369 (1991). 21. Knight, B. and Williams, F. A., Combust. Flame 38:111–119 (1980). 22. Green, G. J., Dryer, F. L., and Sangiovanni, J. J., Fall Eastern States Meeting of the Combustion Institute, Gaithersburg, MD, 1984. 23. Choi, M. Y., Ph.D. Thesis, Droplet Combustion Characteristics under Microgravity and Normal-Gravity Conditions, Dept. of Mechanical and Aerospace Engineering, Princeton University, MAE-T1937, 1992. 24. Jackson, G. S., Avedisian, C. T., and Yang, J. C., Int. J. Heat Mass Transfer 35:2017–2033 (1992). 25. Dasch, C. J., Appl. Opt. 31:146 (1992). 26. Choi, M. Y., Mulholland, G. W., Hamins, A., and Kashiwagi, T., Combust. Flame 102:161–169 (1995). 27. Bahadori, M. Y., Stocker, D. P., and Edelman, R. B., AIAA Paper No. 90-0651 presented in Reno, NV, January 1990. 28. Choi, M. Y., Dryer, F. L., Green, G. J., and Sangiovanni, J. J., AIAA Paper 93-0823, 1993. 29. Jackson, G. S. and Avedisian, C. T., Proc. R. Soc. London A 446:255–276 (1994). 30. Hubbard, G. L., Denny, V. E., and Mills, A. F., Int. J. Heat Mass Transfer 18:1003 (1975). 31. Choi, M. Y., Cho. S. Y., Dryer, F. L., and Haggard, Jr., J. B., Some Further Observations On Droplet Combustion Characteristics: NASA-LeRC-Princeton Results, AIAA/IKI Microgravity Science Symposium, Moscow, USSR, 1991. 32. Grosshandler, W. L., RADCAL: A Narrow Band Model for Radiation Calculation in a Combustion Environment, NIST Technical Note 1402, 1992. 33. Glassman, I., Combustion, Academic Press, Orlando, 1987. 34. Spalding, D. B., Some Fundamentals of Combustion, Butterworths, London, 1955. 35. Law, C. K. and Williams, F. A., Combust. Flame 19:393 (1972). 36. Choi, M. Y., Dryer, F. L., Haggard, Jr., J. B., and Borowski, B. A., “Burning Behavior Of Hydrocarbon Droplets In Reduced Pressure Environments,” Eastern States Section Of The Combustion Institute, Orlando, FL, 1990. 37. Kitano, M., Kobayashi, H., and Sugimoto, T., Combust. Sci. Technol. 78:19–31 (1991).
INVESTIGATION OF SOOTING IN MICROGRAVITY DROPLET COMBUSTION MUN YOUNG CHOI and KYEONG-OOK LEE Department of Mechanical Engineering University of Illinois at Chicago Chicago, IL 60607, USA
This investigation describes experiments performed at the 2.2-s microgravity facility at NASA-Lewis Research Center using n-heptane droplets burning in atmospheric pressure air. The transient soot distributions within the region bounded by the droplet surface and the flame were measured using a full-field light-extinction technique and subsequent tomographic inversion using Abel transforms. It has been speculated that under microgravity conditions, the absence of buoyancy and the effects of thermophoresis create a situation in which high concentrations of soot accumulate into a soot cloud. This study presents the first quantitative measurements of the degree of sooting for microgravity droplet combustion. Results indicate that the soot concentrations for microgravity heptane droplet flames (with maximum soot volume fractions ; 60 ppm) are significantly higher than corresponding values that are reported for normal-gravity flames (which are typically ; 1 ppm). Since the accumulated soot represents incomplete combustion and can also modify the heat-transfer mechanism by altering the local temperature distributions within the fuel-rich region, sooting effects can significantly influence the burning behavior under microgravity conditions. Experiments were also performed to assess the droplet-size–dependent effects on the sooting behavior. Initial experiments using 1.0 and 1.75 mm initial diameter droplets indicate that while the distribution of soot volume fractions are comparable for the two cases (for the observation times that were available), the ratio of the instantaneous mass of soot contained within the fuel-rich region for the 1.75-mm droplet compared to the 1.0-mm droplet was more than a factor of 3. This ratio is also expected to increase for longer observations.
Introduction The investigation of the burning of a single droplet in microgravity is an ideal problem from which to gain fundamental understanding of diffusion flame characteristics [1,2]. The pioneering microgravity studies provided important insights regarding steady-state burning behavior that were used to calibrate theoretical predictions. However, the effects of sooting and thermal radiation on the overall burning characteristics were not considered [3,4]. Recent microgravity experiments and numerical calculations indicate that soot/soot-shell formation and thermal radiation may affect all phases of steady-state and transient burning characteristics. For example, Choi et al. [5] reported that the burning rate of n-heptane droplets were as much as 30–40% lower than the classically accepted results [4]. The main conclusion drawn from that investigation was that the formation and accumulation of the soot shell and its axisymmetric configuration caused by small degrees of relative droplet/gas convection may affect the heat transfer from the flame to the droplet. Recent numerical studies have investigated the effects of radiative heat transfer on droplet combustion. Saitoh
et al. [6] found that thermal radiation from n-heptane droplet flames can reduce the maximum flame temperature by 25%. Chao et al. [7] determined that thermal dissipative losses from gas-phase products can promote the onset of extinction of droplet flames. For sooting droplet flames, it is likely that soot emission will dominate thermal radiation from the flame and therefore may enhance these effects [8]. If radiation represents a viable heat-transfer mechanism to the droplet, the magnitude will be determined not only by the soot concentration near the flame but throughout the entire fuel-rich region, which can attenuate the radiation. Recent work by Chang and Shieh [8] suggests that the droplet burning rate is sensitive to the soot concentration within the flame and the opacity of the liquid droplet. Therefore, in order to accurately assess the importance of sooting on droplet combustion, it is necessary to obtain transient soot concentration distributions under microgravity conditions. Under normal-gravity conditions, several techniques have been used to determine the degree of sooting in droplet flames. Randolph and Law [9] studied the sooting characteristics of small, freely falling droplets, using both visual and sampling
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Fig. 1. Schematic of experimental apparatus: B1-B4: detection camera, lens, and filters; B5: pinhole; B6, O1: 39-diameter mirrors; C1: ignitors; C2: deployment needles; C3: silica carbide fiber posts; C4: 200-mm plano-convex lens; O2: incandescent lamp; O3: high-speed camera; T1: diode laser; T2: collimating lens.
techniques. They found excellent correlation of soot index with the luminous region of the flame. The luminosity was then successfully used as a nonintrusive measure of the degree of sooting to study the effects of droplet spacing, fuel blending, and convection. However, since the luminosity is expected to be dominated by soot concentrations just within the vicinity of the flame front, it does not provide detailed information regarding the entire structure of the soot-containing region. Kadota and Hiroyasu [10] first measured soot concentration in droplet flames, using optical techniques under normal-gravity conditions. In their study, light-extinction measurements were performed at a fixed location below the droplet, and the radial variation was deduced by assuming that the regressing flame maintained constant spherical structure. In the region below the droplet, this procedure may provide accurate information since the flame geometry is nearly spherical. However, the same approach cannot be performed for locations above the droplet (this is the more important region that produce the bulk of the particulates), where it displays both spatial and temporal variations. The transient nature of droplet flames requires diagnostic techniques that can instantaneously examine the entire soot field. Recently, several groups have used laser-induced incandescense (LII) to measure the soot concentrations for droplet flames in normal gravity. The incandescense results from the rapid heating of the soot particles by a shortduration, high-powered laser. Vander Wal and coworkers [11,12] performed two-dimensional LII measurements to obtain temporally and spatially resolved soot volume fraction, fv, for suspended droplets of various alkane and chlorinated alkane fuels. Gupta and Santoro [13] performed similar LII experiments using freely falling droplets of benzene/ methanol mixtures. The advantage of using this tech-
nique is that soot concentration is proportional to the intensity of the incandescence (provided that the intensities have been calibrated using an independent technique [14]). However, this technique has not been applied for droplet combustion studies under microgravity conditions. An alternative diagnostic approach employing fullfield light extinction and tomographic inversion was used by Lee et al. [15] to study the burning characteristics of suspended toluene droplets under normal-gravity conditions. Greenberg and co-workers first used this technique to measure soot concentrations in laminar gas-jet diffusion flames in microgravity [16–18]. The motivation for the present study is to use this technique to measure the transient soot concentrations for droplets burning under microgravity conditions. In previous microgravity experiments, the degree of sooting was estimated by observing the “darkness” of the sooting region [5,19,20]. This study presents the first quantitative measurements of the soot volume fractions for microgravity droplet flames. The ultimate goal of this study is to control the degree of sooting using pressure, inert variation, oxygen concentration, fuel type, and droplet dimensions and determine their effects on the steady-state and transient burning characteristics. In the following sections, experiments performed using n-heptane droplets of different initial diameters are presented.
Experimental Description Experiments were performed at the 2.2-s microgravity drop tower at NASA-LeRC in Cleveland, OH. Figure 1 displays the schematic of the experimental apparatus. (The combustion chamber and apparatus design was based on the experimental rig used by the NASA Droplet Combustion
INVESTIGATION OF SOOTING IN MICROGRAVITY DROPLET COMBUSTION
Fig. 2. Photograph of the deployment syringes, suspending fiber, hot-wire ignitors, and droplet.
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sitioned at 458. The beam was transmitted through the chamber and was then focused using a 200-mm plano-convex lens and redirected using a second 75mm mirror (also positioned at 458) and imaged through a spatial filter to a high-resolution CCD camera located on the bottom optical plate. An image quality interference filter and an absorption neutral density (o.d., 3.0) filter were placed directly in front of the camera to discriminate against flame emission or stray light. The video output from the camera was transmitted through a fiber optic cable and recorded onto a BETACAM recorder at 30 frames/s. A second CCD camera placed on the middle optical plate was used to capture the flame image to denote the time of ignition. Figure 2 displays a laser-backlit photograph of the droplet and the deployment and ignition apparatus. The droplet was formed by using two opposing hypodermic needles of 0.2 mm in diameter (shown as vertical lines) that were attached to two separate rotary motors. Prior to release into free fall, the droplet was deposited on a 10-lm silica-carbide–suspending filament. The filament was needed to prevent droplet motion during the experiment. (Due to the degree of magnification used in the present experiment, movement of several droplet diameters in either directions can clip the sooting regions out of the field of view. Due to its small size, the presence of the filament did not affect the sooting behavior near the region of analysis, which is depicted by the black dotted line in Fig. 3a.) Deployment of the droplet was actuated by the rapid retraction of the needles, which causes some oscillations in the droplet. However, it has been shown in a previous study that it decays within 0.2–0.3 s for droplet dimensions that are typical of microgravity experiments [5]. Approximately 0.3 s after deployment, the droplet was ignited using two kanthol hot wires (shown as two circular lines) that were attached to translational motors. The igniters were then retracted away from the field of view to allow for undisturbed burning.
Soot Volume Fraction Measurements in Microgravity Fig. 3. Time sequence of 1.75-mm n-heptane droplet burning in atmospheric pressure air. (a) 0.2 s and (b) 0.5 s.
Experiment.) On the top optical plate, light from a 635-nm diode laser was attenuated by a variable neutral density filter and expanded using a Newport LCV expander/collimator to produce a beam of approximately 50 mm in diameter. The expanded beam was directed through the top optical port of the 12-l stainless steel combustion chamber (which was fitted with a 50-mm-diameter quartz window with antireflection coating) using a 75-mm-diameter mirror po-
Figures 3a and 3b display photographs of a 1.75mm initial diameter heptane droplet in atmospheric pressure air at various times during the burning process. Shortly after ignition, soot particles are transported from regions just inside the flame toward the droplet (as shown by the very diffuse and broad soot containing region surrounding Fig. 3a compared with Fig. 3b). The transported soot particles eventually form a soot cloud that is concentric with the droplet (see Fig. 3b). The term “soot cloud” is used here instead of “soot shell” since the region is comprised of a porous dispersion of soot rather than a structurally rigid shell. For all burning times that
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Fig. 4. Soot volume fraction distributions plotted as a function of nondimensional radius for 0.2 to 0.8 s in 0.1-s increments for 1.75-mm droplet.
were analyzed, the images indicate the presence of soot particles at radial positions beyond the darkest regions of the soot cloud, indicating a continuous production and transport of soot. The intensity distributions were measured by digitizing the Betacam images with a high-resolution frame acquisition and processing board and a custom image-processing algorithm [15]. The images were filtered using a linear, 3- by 3-pixel mean filter. The intensity ratio distributions were calculated by dividing the gray-level values for the sooting image along the line of analysis (black dotted line in Fig. 3a) by corresponding intensities measured for the background image (which was captured prior to droplet ignition and therefore unattenuated by soot). The intensity ratios were then averaged using a moving 5-pt operator. The minimum intensity ratios were measured closer to the droplet surface than the flame front. Since the majority of soot is expected to be formed near the flame [2], it is evident that the transport toward the droplet surface is facilitated by thermophoretic forces [18,21–24]. Due to vibrations of the various optical components caused by the release into free fall, the background image was not obtained until approximately 0.8 s after free fall. Alternatively, the use of background images captured prior to release into free fall would have increased the overall observation time to beyond 1.0 s. However, the associated spatial fluctuations in the intensity ratios would have been an order of magnitude larger. The projected light-extinction ratio distributions obtained as a function of time were used to determine the soot volume fraction, fv(r), using a 3-pt Abel deconvolution technique [25] with soot optical property determined using the light-extinction/gravimetric-calibration technique [26]. Figure 4 displays the deconvolved soot volume fraction distributions calculated as a function of time for the 1.75-mm hep-
tane droplet. It is speculated that the combination of increased residence time due to lack of buoyancy [2], which enhances soot formation, and thermophoretic forces, which transports the soot toward the droplet surface (preventing oxidation of the soot particles), creates conditions in which a significant amount of soot is accumulated into a soot cloud within the fuel-rich region [5,19]. Since the initiation of soot-cloud formation is observed within 0.15 s after ignition, it is expected that the average fuel residence time within the fuel-rich region will be comparable to this value, which suggests that the residence time in microgravity is only 3–4 times longer than that observed under normal-gravity conditions [2]. However, the total residence time for soot formation and growth must take into account the unique behavior observed for droplet combustion in which the soot within the soot cloud persists throughout the burning lifetime (which can last up to 5 s for large droplets ; 2 mm). Due to this accumulation effect, the maximum fv is shown to increase significantly as a function of time from a value of ;20 ppm at 0.2 s to ;60 ppm at 0.8 s. The concentrations of soot measured for the microgravity heptane flames are significantly higher than the nominal values of 1 ppm measured under normalgravity conditions by Vander Wal and co-workers using LII [11,12]. Similar observations of enhanced sooting in microgravity was reported in an earlier study by Bahadori et al. [27] using laminar gas-jet flames. Figure 4 also displays the transient character of the soot standoff ratios (SSR; radial position of the maximum soot volume fraction divided by instantaneous droplet radius). It is also interesting to note that due to significant accumulation as a result of thermophoresis, the region containing the bulk of the soot concentrations is rather narrow compared to the flame dimension (the flame standoff ratio is on the order of five to six). The measured SSR for the initial phases of burning are significantly lower (;1.5) than those measured in previous studies [28,29]. The measurements approach values measured by Jackson and Avedisian [29] only during the latter stages of burning. This may be due to the fact that for droplets of this size, heat-up periods can account for up to 20% of the total lifetime [30] (which corresponds to approximately 1.1 s). Therefore, it is unlikely that the droplet has reached quasisteady-state conditions during the relatively short observation times that were available (;1 s). In many of the early droplet combustion studies, the effects of sooting were disregarded due to a combination of the complexity involved in incorporating the soot formation mechanisms and the belief that sooting for fuels such as heptane was not an important factor (based on degree of sooting observed under normal-gravity conditions). However, the importance of sooting in microgravity droplet
INVESTIGATION OF SOOTING IN MICROGRAVITY DROPLET COMBUSTION
combustion has been recognized in more recent investigations [5–8,19–24]. One mechanism for the variations in the burning behavior as a function of sooting is the possible modification in the heat transfer from the flame to the droplet surface [5,24,31]. Chang and Shieh [8] reported that the burning rate is enhanced when thermal radiation from soot was incorporated into their n-heptane droplet combustion model. The degree of enhancement was sensitive to the chosen soot concentration (the distribution was assumed to be constant and uniform) and the opacity of the liquid droplet. However, a more accurate assessment of thermal radiation can be determined by using the experimental measurements of the transient soot distributions and the spectral absorption characteristics of the liquid fuel. In addition to modifying the flame temperature through enhanced radiative dissipation, soot concentrations within the fuel-rich region may attenuate the radiative and conductive heat transfer to the droplet. Spectral attenuations of the thermal radiation (from the flame front to the droplet surface) as high as 30% were calculated in the range of 1–2 lm using RADCAL (a narrowband radiation model) [32] and typical flame temperatures between 1800 and 2100 K. The presence of soot can also modify the thermophysical properties of the soot-laden gas. For example, the heat capacity of the soot/gas mixture will be higher than that of the surrounding gas [29]. Although the thermal conductivity is higher for soot compared to the surrounding gas, the overall conductivity of the mixture is not expected to be much different from the gas since contact among soot aggregates is not likely. The resulting changes in the gas-phase temperature gradient at the droplet surface (due to the presence of soot) can directly affect the burning rate [33]. However, detailed temperature measurements are needed to ascertain the degree of modification of heat transfer to the droplet as a result of the above-mentioned effects. The accumulation of soot particles within the fuelrich region can affect the burning behavior in other ways. The formation of soot through pyrolysis reactions (which are endothermic) serves as a heat sink, and accumulated particles within the soot-cloud region represent incomplete burning since they are not oxidized [5,24]. Both of these factors will reduce the effective heat of combustion and the transfer number [33]. Although the transfer number appears in the natural log term, reductions in the burning rate are expected for conditions producing significant soot concentrations [31]. Jackson and Avedisian [29] have also included the growth of soot particles (within the soot cloud) through surface reactions with the vaporized fuel as a possible mechanism for enhancing the degree of sooting, which further reduces the efficiency of the combustion process.
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Droplet Size Effects Classical theories of droplet combustion characteristics indicate negligible size-dependent effects on the overall burning parameters [34]. For example, the predictions of the vaporization rate and flame standoff ratio are insensitive to the initial droplet diameter. There are dimensional effects related to natural and forced convections that can be used in conjunction with the “d2-law” analysis [35]. These effects are not expected to be important for microgravity experiments with small degrees of negligible forced convection. Recently, Jackson and co-workers [24,29] speculated that the burning behavior is affected by the initial droplet size through variations in the degree of sooting. Although the effects of sooting on droplet burning behavior (including the steady-state burning rate) were established in earlier studies [5,36], Jackson and co-workers were the first to use the initial diameter as a parameter for controlling the degree of sooting under microgravity conditions. The effects of initial droplet diameter on the sooting behavior were investigated under normal gravity by Kitano et al. [37] by measuring soot emitted from the opentipped flame. By varying the initial diameter, they were able to determine the sooting limit, which was the minimum droplet size that produces particulate emission. Under microgravity conditions, the envelope flame is proportional to the droplet size, and therefore, it is expected that longer residence time (which is proportional to the square of the diameter, d2) will be available for larger droplets, during which the fuel molecules undergo pyrolysis reactions leading to soot formation [29]. However, experimental verification of the size-dependent variations in the sooting characteristics for microgravity droplet combustion was not available. For these reasons, experiments were performed by varying the initial diameter of the n-heptane droplet. Experiments were performed using a 1.0- and a 1.75-mm initial diameter droplet burning in atmospheric pressure air for comparisons. (It is to be noted that convection can also affect the soot formation/ accumulation behavior [5]. However, only the experiments with very low degrees of convection were analyzed for this study.) Figure 5 displays the timevarying maximum soot volume fraction, fv,max, and the SSR for the 1.0- and the 1.75-mm droplet plotted as a function of time divided by the square of the initial diameter (which is proportional to the fractional burning time). The measured soot standoff ratios for the 1.0-mm droplet are in good agreement with those reported by Jackson and Avedisian [29]. For the smaller droplet, the available microgravity times are sufficient to observe that the soot volume fractions have achieved a maximum value and begin to decrease as a function of time. As the droplet size decreases due to burning, it is likely that
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Conclusions
Fig. 5. Comparisons of maximum soot volume fraction, fv,max, and soot standoff ratio, SSR, as a function of reduced time, t/d2o for 1.75- and 1.0-mm droplets.
the rate of soot formation at the flame front is also decreasing through combinations of reduced rate of mass vaporization and reductions in maximum temperature due to increased reactant leakage through the flame. Simultaneously, the Stefan flux near the droplet surface increases (assuming that the burning rate is constant), while the thermophoretic force is expected to decrease due to reductions in the temperature gradient, causing soot to be transported toward the flame [23,28]. These combined effects can reduce the amount of soot accumulated within the soot cloud as a function of time. The maximum soot volume fractions measured for the two cases are similar to within the experimental uncertainty. However, quasi-steady conditions were not attained for the larger droplet, and therefore, comparable values of maximum soot volume fractions for the two cases do not indicate that sooting behaviors are similar. A more relevant parameter to compare is the instantaneous soot mass within the region bounded by the droplet and the flame, which is calculated by integrating the product of the soot density and fv distributions with respect to volume. (Soot density of 1.8 g/cc was used.) Since the volume of the flame will scale as d3, soot mass within the envelope flame is likely to be significantly larger for the 1.75 mm droplet. Comparison of the maximum instantaneous soot mass for the 1.75-mm droplet is approximately three times larger than the corresponding value for the smaller droplet. Considering that the instantaneous mass ratios between the two droplets are likely to increase with longer observation times available to analyze the larger droplet, it is evident that there is a strong size-dependent effect on the degree of sooting. However, additional experiments performed for longer observation times are needed to determine the quantitative correlations between sooting and burning characteristics in microgravity droplet combustion.
The transient soot distributions within the region bounded by the droplet surface and the flame were measured using a full-field light-extinction technique and subsequent tomographic inversion using Abel transforms. The soot volume fraction results for n-heptane droplets represent the first quantitative assessment of the degree of sooting for isolated droplets burning under microgravity condition. The absence of buoyancy (which produces longer residence times) and the effects of thermophoresis produce a situation in which a significant concentration of soot is produced and accumulated into a soot cloud. Results indicate that indeed the soot concentrations within the microgravity droplet flames (with maximum soot volume fractions as high as ;60 ppm) are significantly higher than corresponding values that are reported for normal-gravity flames, which are typically on the order of 1 ppm. Experiments were also performed to determine the size-dependent effects on the sooting behavior. Initial experiments using 1.0- and 1.75-mm initial diameter droplets indicate that while the distribution of soot volume fractions are comparable for the two cases (for the observation times that were available), the total mass of soot contained within the fuel-rich region is significantly larger for the 1.75mm droplet. The maximum instantaneous soot mass for the 1.75-mm droplet was nearly three times larger than the maximum mass measured for the 1.0mm droplet. Results analyzed for longer observation times under microgravity conditions will likely produce larger mass ratios and increase the already significant size-dependent effects. Acknowledgments The authors gratefully acknowledge helpful advice regarding the full-field light-extinction technique provided by P. Greenberg, D. Griffin, B. Whiteside, and D. Urban and the experimental apparatus design team led by F. Gati and A. Birchenough of NASA-LeRC. We would also like to thank P. Ferkul, D. Schultz, and R. L. Vander Wal for valuable insights regarding this work. This work was supported by NASA-LeRC through Grant #NAG3-1631 with P. Ferkul and D. Schultz serving as project scientist and project monitor, respectively. REFERENCES 1. Williams, F. A. and Dryer, F. L., Science Requirements Document for Droplet Combustion Experiment, NASA-LeRC, 1994. 2. Law, C. K. and Faeth, G. M., Prog. Energy Combust. Sci. 20:65–113 (1994). 3. Kumagai, S. and Isoda, H., Sixth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1957, pp. 726–731.
INVESTIGATION OF SOOTING IN MICROGRAVITY DROPLET COMBUSTION 4. Kumagai, S., Sakai, T., and Okajima, S., Thirteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1971, pp. 779–785. 5. Choi, M. Y., Dryer, F. L., and Haggard, J. B., TwentyThird Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, pp. 1597– 1605. 6. Saitoh, T., Yamazaki, K., and Viskanta, R., J. Thermophys. Heat Transfer 7:94–100 (1993). 7. Chao, B. H., Law, C. K., and Tien, J. S., Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, pp. 523–531. 8. Chang, K.-C. and Shieh, J.-S., Int. J. Heat Mass Transfer 38:2611–2621 (1995). 9. Randolph, A. L. and Law, C. K., Combust. Flame 64:267–284 (1986). 10. Kadota, T. and Hiroyasu, H., Combust. Flame 55:195– 201 (1984). 11. Vander Wal, R. L., Dietrich, D. L., and Choi, M. Y., “Relative Soot Volume Fractions in Droplet Combustion via Laser-Induced Incandescense,” Presented at the Eastern States Section Meeting of the Combustion Institute, Clearwater Beach, FL, December 1994. 12. Vander Wal, R. L. and Dietrich, D. L., Appl. Opt. 34(6):1103 (1995). 13. Gupta, S. B., Ni, T., and Santoro, R. J., “Spatial and Time Resolved Soot Volume Fraction Measurements in Methanol/Benzene Droplet Flames,” Presented at the Eastern States Section Meeting of the Combustion Institute, Clearwater Beach, FL, December 1994. 14. Vander Wal, R. L., Zhou, Z., and Choi, M. Y., Combust. Flame 105:462 (1996). 15. Lee, K. O., Jensen, K., and Choi, M. Y., Twenty-Sixth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1996, pp. 2397–2404. 16. Griffin, D. W. and Greenberg, P. S., “Selected Microgravity Combustion Diagnostics Techniques,” 2nd Int’l Microgravity Combustion Workshop Proceedings, September 1992. 17. Greenberg, P. S., “Laser Doppler Velocimetry and Full-Field Soot Volume Fraction Measurements in Microgravity,” presented at the Third Int’l Microgravity Combustion Workshop, Cleveland, OH, 1995. 18. Greenberg, P. S. and Ku, J. C., Appl. Opt., 1995, submitted. 19. Shaw, B. D., Dryer, F. L., Williams, F. A., and Haggard, Jr., J. B., Acta Astronaut. 17:1195–1120 (1988).
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20. Jackson, G. S., Avedisian, C. T., and Yang, J. C., Proc. R. Soc. London A 435:359–369 (1991). 21. Knight, B. and Williams, F. A., Combust. Flame 38:111–119 (1980). 22. Green, G. J., Dryer, F. L., and Sangiovanni, J. J., Fall Eastern States Meeting of the Combustion Institute, Gaithersburg, MD, 1984. 23. Choi, M. Y., Ph.D. Thesis, Droplet Combustion Characteristics under Microgravity and Normal-Gravity Conditions, Dept. of Mechanical and Aerospace Engineering, Princeton University, MAE-T1937, 1992. 24. Jackson, G. S., Avedisian, C. T., and Yang, J. C., Int. J. Heat Mass Transfer 35:2017–2033 (1992). 25. Dasch, C. J., Appl. Opt. 31:146 (1992). 26. Choi, M. Y., Mulholland, G. W., Hamins, A., and Kashiwagi, T., Combust. Flame 102:161–169 (1995). 27. Bahadori, M. Y., Stocker, D. P., and Edelman, R. B., AIAA Paper No. 90-0651 presented in Reno, NV, January 1990. 28. Choi, M. Y., Dryer, F. L., Green, G. J., and Sangiovanni, J. J., AIAA Paper 93-0823, 1993. 29. Jackson, G. S. and Avedisian, C. T., Proc. R. Soc. London A 446:255–276 (1994). 30. Hubbard, G. L., Denny, V. E., and Mills, A. F., Int. J. Heat Mass Transfer 18:1003 (1975). 31. Choi, M. Y., Cho. S. Y., Dryer, F. L., and Haggard, Jr., J. B., Some Further Observations On Droplet Combustion Characteristics: NASA-LeRC-Princeton Results, AIAA/IKI Microgravity Science Symposium, Moscow, USSR, 1991. 32. Grosshandler, W. L., RADCAL: A Narrow Band Model for Radiation Calculation in a Combustion Environment, NIST Technical Note 1402, 1992. 33. Glassman, I., Combustion, Academic Press, Orlando, 1987. 34. Spalding, D. B., Some Fundamentals of Combustion, Butterworths, London, 1955. 35. Law, C. K. and Williams, F. A., Combust. Flame 19:393 (1972). 36. Choi, M. Y., Dryer, F. L., Haggard, Jr., J. B., and Borowski, B. A., “Burning Behavior Of Hydrocarbon Droplets In Reduced Pressure Environments,” Eastern States Section Of The Combustion Institute, Orlando, FL, 1990. 37. Kitano, M., Kobayashi, H., and Sugimoto, T., Combust. Sci. Technol. 78:19–31 (1991).