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ABSTRACT
Flame propagation mechanisms in spray and particle clouds of less volatile condensed fuels are discussed, based on microgravity experimental results obtained in tests using the Japan Microgravity Center with a microgravity test duration of 10 seconds and its microgravity quality of 10−5 g.
First, n-decane sprays were tested under microgravity using a flame propagation tube with an inner diameter of 62 mm and a length of 605 mm. A flame front was observed in which there were many envelope flames surrounding droplets. It was suggested that the whole spray flame proceeded by a droplet-to-droplet propagation of envelope flames. The flame propagation speed had a maximum at a certain Sauter mean diameter (SMD) when the total equivalence ratio was kept constant. All experimental flame propagation speeds could be arranged on a curve when the mean droplet spacing divided by the mean radius of envelope flames surrounding droplets was employed as an abscissa.
Second, to examine the fundamental mechanism of spray combustion mentioned above, we analyzed the flame spread rate of droplet arrays under various conditions. The flame spread rate had a maximum at a certain droplet spacing, and so the spread rate decreased when the spacing was large or small. This supports the observation of the above-mentioned flame propagation of spray.
Third, to confirm the mechanism of flame propagation of dispersed less volatile fuels, we rearranged our previous experimental data of Polymethylmethacrylate (PMMA) particle clouds in microgravity. Although we were not able to arrange the data by means of flame radii since the measurements were difficult, the flame propagation speeds had a maximum at a certain mean particle spacing divided by the mean particle diameter.
Acknowledgments
The author would like to express his gratitude to Professor Forman A. Williams for his encouragement to Japanese microgravity programs on combustion and for his frequent, stimulating, and helpful discussion on droplet and spray combustion.
The author also acknowledges the financial support of NEDO (spray and particle cloud) and the Space Forum (droplet array) and the management of JSUP under grants with Professor H. Kobayashi, Dr. H. Hanai, Mr. S. Hasegawa, and Dr. Y. Nunome.