Estimation Method of Thermophoretic Behavior of Soot

Abstract

Soot is a very small particle formed near a combustion field, where a very steep temperature gradient exists. It is important for appropriate control of combustion devices to understand the behavior of soot particles in combustion fields. In this study, the behavior of each soot particle under temperature gradient is observed in order to reveal the thermophoretic behavior of soot particle. The relationship between the morphological character and the measured thermophoretic velocities of soot particles was examined quantitatively. Then, the method to estimate the behavior of soot particle induced by thermophoresis was examined. The dimensionless density, the ratio of the bulk density to the true density, was suggested as one of the morphological character of soot particle. It was found that the relationship between the thermophoretic coefficient, K_th, and the dimensionless density is a first-order negative correlation especially when the size of primary particle is less than 50nm. It was found that the thermophoretic velocity becomes slower as the dimensionless density becomes greater, that is, the aggregated particles are of a closed structure. The thermophoretic coefficient approached an asymptotic value, K_th = 0.53, which is almost same as the value for the particles in free molecule regime, as the dimensionless density becomes smaller. These results enable to estimate the thermophoretic velocity from the dimensionless density of the aggregated particles. This will be useful for the appropriate estimation and control of soot particle near a combustion field and the development of combustion devices.