Effect of Radiative Heat Transfer on the Coagulation Dynamics of Combustion-Generated Particles

Abstract

We examine the influences of radiation heat transfer on the size and number density evolution of small coagulating particles. On a microscopic level, radiative emission and/or absorption by the particle will perturb the gas temperature field adjacent to each particle. As a result of thermophoretic particle transport, the nonequilibrium condition can alter the collision rates with neighboring particles. A simplified analysis of the thermophoretic coagulation mechanism suggests that net radiative cooling of the particles can lead to an accelerated growth of μm-sized particles, whereas net radiative heating can act to essentially freeze coagulation rates. On the macroscopic level, the addition or removal of heat in the gas through radiative absorption/emission by the particle cloud can also significantly alter, through thermophoretic transport, the local particle number density. Under certain cases these effects can augment the accelerated coagulation rates that occur under radiative cooling conditions. We also examine the particular situation of equilibrium between particle cloud radiative absorption and emission—which results in no net macroscopic effect on the gas. Under conditions where the spectral distribution of the incident radiation differs from that of the emitted radiation, radiative equilibrium can lead to accelerated growth of certain particle sizes and retarded growth of others. Using numerical solutions to the general dynamic equation for particle growth, we demonstrate the possibility of using incident radiation of controlled intensity and spectral distribution to narrow the particle size distribution function of coagulating aerosols.