Effect of Thermophoretic Force on Soot Agglomeration Process in Diffusion Flames Under Microgravity

In the previous research the authors reported that soot agglomeration phenomenon in diffusion flames under microgravity are quite different from those under normal gravity. Soot particles in a diffusion flame adhere each other to form very large visible agglomerates. The size of formed agglomerates were more than 100 microns, or more than 500 times usual soot particles formed in normal gravity. Photos of the phenomenon was shown and the conditions where the large soot agglomerates appear were experimentally investigated. The large soot agglomerates formation is a phenomenon occurring under microgravity and the mechanism of the formation in microgravity has not been clarified yet. In the present work the combustion field of a butane diffusion flame, the same as the one used in the previous paper, is investigated to elucidate the mechanism of large soot agglomerates formation under microgravity. The discussion considers the effect of thermophoretic force and it is concluded that the large soot agglomerates formation is due to a the combination of thermophoretic forces and long residence time in a microgravity environment. Soot particle behavior in microgravity has been studied by other researchers. However, the phenomenon of that large soot particle formation has not been reported for a gas get diffusion flame. In the droplet combustion research Avedisian has shown a picture with the large soot agglomerates emission. Some reports have found that soot particles or agglomerates tend to be larger with increases in residence time and then reach to a certain size in a gas jet diffusion flame in normal gravity. However, that large agglomerate large as those observed in microgravity environment have never seen irrespective the flame length. Papers noting thermophoretic effects on combustion matter were published by Dobins and others. They pointed out the importance of thermophoretic forces when a TEM grid was used to sample soot particles. They found that a steep temperature gradient dose to the solid surface attracts soot particles to the grid surface. Some researches reported that the effect of thermophoretic forces on fine seeded particles, such as used for LDV measurements, is not negligible. Hirano has measured the drift speed of fine SiO2 and MgO particles due to thermophoretic force under constant temperature gradient.