Thermophoretic velocity measurements of soot particles under a microgravity condition

Abstract

In a field with a temperature gradient, small particles experience a force toward a colder region. Sootconsists of very small particles and forms near the combustion field, where a very steep temperature gradient exists. Therefore, appropriate understanding of the thermophoretic effect is indispensable in understanding the behavior of soot particles near combustion fields. In this study, accurate measurements of thermophoretic velocities of soot particles were performed under a microgravity condition, which suppresses natural convection and provides an ideal measurement field. The effects of particle morphologies on thermophoretic velocity were also examined. Soot particles used in the experiments were sampled beforehand from pool flames of toluene, benzene, n -heptane, and diethyl ketone. For the soot particles from toluene flames, the measured thermophoretic velocities were almost proportional to the temperature gradient and the highest velocity was 5.5 mm/s at ∇ T =133 K/mm. Morphological analyses of the soot particles and comparisons with previous theoretical research on thermophoresis demonstrate that the thermophoretic velocities scarcely depend on the aggregate sizes (whole size of the soot particle) but are almost constant values, which are nearly the same as the values evaluated by the theory for the free-molecular regime. These results suggest that thermophoretic velocities of soot particles must be governed by primary particle size. Several soot particles from the flames of n -heptane and diethyl ketone showed distinctly lower velocities than the value evaluated by the theory for the free-molecular regime. These discrepancies are considered to arise from the morphological differences: that is, the primary particles aggregate very closely or get together in these particles, whereas the particles from toluene flames are of open structure.