Photophoretic modification of the transport of absorbing particles across combustion gas boundary layers

Abstract

Since radiation energy fluxes can be comparable to or even dominate ‘convective’ (Fourier) fluxes in large fossil-fuel-fired power stations and furnaces, we examine particle drift (‘phoresis’) induced by the nonuniform photon-produced heating of particles in a ‘host’ gas. Our analysis of the resulting photophoretic particle velocity shows that photophoresis is a significant transport mechanism for micronsized absorbing particles in high radiative transfer combustion environments, with equivalent photophoretic diffusivity ratios (dimensionless photophoretic velocities) being as large as 10% of the better-known thermophoretic diffusivity. Since previous experimental results demonstrated that thermophoresis causes over a 3-decade increase in small particle deposition rates by convective diffusion, clearly, for small, absorbing particles, photophoresis will also be an important contributor to observed deposition rates. Accordingly, we present predicted dimensionless mass transfer coefficients for particle transport across non-isothermal laminar gaseous boundary layers, including the simultaneous effects of both particle thermophoresis and photophoresis. It is also shown that our earlier ‘additive suction velocity’ prediction/correlation scheme successfully anticipates the present numerical (large Schmidt number, laminar boundary layer) results for radiative/conductive flux ratios encountered in practice.