Experimental and numerical investigation of soot deposition in laminar stagnation point boundary layers

Abstract

Soot deposition from a hot gas flow to a cooled solid wall is investigated numerically and experimentally for laminar stagnation point flows. Numerical predictions of soot deposition are made by solving the coupled momentum, energy, and soot transport equations, which include the effects of variable transport properties and thermophoretic transport (i.e. mass transport down a temperature gradient) of soot particles. The results of the numerical computations indicate that the deposition rate, expressed in terms of the deposition velocity, increase with an increasing potential flow velocity gradient and decrease as the wall temperature approaches the freestream temperature. In addition, an insitu laser diagnostic technique is developed which provides simultaneous measurement of the soot deposit thickness and freestream soot concentration. The technique uses two laser beams (488 and 632 nm wavelength) coincident at a point adjacent to the deposition surface to measure the thickness of the deposit and the light extinction by airborne soot particles. Comparison of the experimental results to the numerical model indicates a critical velocity gradient beyond which the “sticking” fraction for the soot significantly decreases. This velocity gradient, which corresponds to maximum deposition, has been found to be approximately 2000 sec −1 for soot produced from fuel rich air/ethylene mixtures.