Theory of surface deposition from boundary layers containing condensable vapour and particles

Abstract

Heterogeneous condensation of vapours mixed with a carrier gas in the stagnation point boundary layer flow near a cold wall is considered in the presence of solid particles much larger than the mean free path of vapour particles. The supersaturated vapour condenses on the particles by diffusion and particles and droplets are thermophoretically attracted to the wall. Assuming that the heat of vaporization is much larger than the Boltzmann constant times the temperature far from the wall, vapour condensation occurs in a {\em condensation layer} (CL). The CL width and characteristics depend on the parameters of the problem, and a parameter $R$ yielding the rate of vapour scavenging by solid particles is particularly important. Assuming that the CL is so narrow that temperature, particle density and velocity do not change appreciably inside it, an asymptotic theory is found, the $\delta$-CL theory, that approximates very well the vapour and droplet profiles, the dew point shift and the deposition rates at the wall for wide ranges of the wall temperature $\tilde{T}_{w}$ and the scavenging parameter $R$. This theory breaks down for $\tilde{T}_{w}$ very close to the maximum temperature yielding non-zero droplet deposition rate, $\tilde{T}_{w,M}$. For large $R$, we can either assume that the width of the CL is zero (0-CL theory, then the vapour density reaches local equilibrium with the condensate immediately after it enters the dew surface), or use a nonlinear multiple scales theory. The 0-CL theory corrects the $\delta$-CL theory for $\tilde{T}_w$ very close to $\tilde{T}_{w,M}$ and any $R$, whereas the multiple scales theory is appropriate for large and moderate $R$.