Evaporation from a nonspherical aerosol particle situated in an absorbing gas

The problem of an aerosol particle evaporating in an infinite expanse of an absorbing gas is considered. The relevant Helmholtz equation (resulting from the steady-state diffusion equation with an absorption term included) with density jump boundary conditions is converted into a boundary integral equation via the use of the Green’s function. The resulting integral equation is valid for particles of arbitrary shape. Explicit numerical results for the local and average evaporation rates are reported for several axisymmetric particles for a range of values of the dimensionless absorption parameter (λ²), where λ is the ratio of the radius of the particle (a) to the diffusion length (l). Here, the diffusion length is defined as l=[D/(vΣ_a)]^1/2, in which v (cm s^-1) is the average thermal speed of the vapor molecules, Σ_a (cm^-1) is the cross-section for absorption of the vapor by the gas, and D (cm² s^-1) is the diffusion coefficient of the vapor in the gas. Our numerical results for the local and average evaporation rates for a sphere exhibit excellent agreement with the corresponding analytical values (maximum deviation <0.40%). We find that the evaporation rate increases with increasing absorption and that this increase depends on the degree of departure of the particle from a spherical shape. The jump distance has a large impact in that it significantly lowers the evaporation rates as it increases in magnitude. It should be remarked that the results of this paper are also directly applicable to the problem of either neutrons or photons undergoing diffusion from a source situated in an absorbing medium.