Experimental studies of soot particle thermophoresis in nonisothermal combustion gases using thermocouple response techniques

Abstract

Using transient size and temperature measurements obtained on a small thermocouple (TC) bead suddenly immersed in the soot-laden combustion products downstream of a water-cooled premixed flat-flame burner, we demonstrate that soot deposition rates are dominated by particle thermophoresis, i.e., soot particle drift down the temperature gradient prevailing in the gas thermal boundary layer (BL) surrounding the TC target. Observed soot deposition mass fluxes are estimated to be up to three orders of magnitude greater than those that would be expected on the basis of only Brownian (concentration) diffusion at the prevailing Reynolds numbers. It is shown that, for thermophoretically dominated soot particle transport, a suitable plot constructed from the transient TC-bead output should yield a straight line whose slope is proportional to the local soot volume fraction in the combustion gases. The latter dependence has been tested at several flame stoichiometries and heights above the burner surface, using the independent experimental technique of line-of-sight laser light (0.63 μm) extinction. Our results suggest that under these conditions small thermocouples can be used not only to estimate local gas temperatures, but also local soot volume fractions, without requiring restrictive assumptions about the prevailing particle size distribution, particle optical properties, or flame gas uniformity/symmetry. We conclude that (a) the dependence of the soot deposition rate on temperature “contrast” is in excellent accord with recently proposed thermophoretic BL-theories, and (b) the behavior of submicron soot particles in nonisothermal combustion products is strongly influenced by thermophoresis, with important implications for understanding rates of soot nucleation, growth, coagulation, burnout, and deposition.