Theory of homogeneous vapour condensation and surface deposition from boundary layers

Abstract

Homogeneous condensation of vapours mixed with a carrier gas in the stagnation point boundary layer flow near a cold wall is considered. There is a condensation region near the wall with supersaturated vapour. Assuming that the surface tension times the molecular area is much larger than the thermal energy far from the wall, droplets are nucleated exclusively in a narrow nucleation layer where the Zeldovich flux of clusters surpassing the critical nucleus size is at a maximum. The vapour condenses in the free molecular regime on the droplets, which are thermophoretically attracted to the wall. Unlike the narrow condensation region for heterogeneous condensation on solid particles, in the case of homogeneous condensation the condensation region is wide even when the rate of vapour scavenging by droplets is large. A singular perturbation theory of homogeneous vapour condensation in boundary layer flow approximates very well the vapour and droplet density profiles, the nucleation layer and the deposition rates at the wall for wide ranges of the wall temperature and the scavenging parameter B. A key point in the theory is to select a trial vapour number density profile among a one parameter family of profiles between an upper and a lower bound. The maximum of the Zeldovich flux for supercritical nuclei provides the approximate location of the nucleation layer and an approximate droplet density profile. Then the condensate number of molecules and the vapour density profile are calculated by matched asymptotic expansions that also yield the deposition rates. For sufficiently large wall temperatures, a more precise corrected asymptotic theory is given.

Kinetic Simulation of Fine Particulate Matter Evolution and Deposition in a 25 kW Pulverized Coal Combustor

“…An analogy is the chemical vapor deposition (CVD), where theories have been explored for condensing rates under a variety of conditions. 32,33 However, coagulation was not considered. As for solid fuel combustion, a transport model is developed for alkali chlorine superheater deposition in the biofuel-fired recovery boiler that takes into account multiple interactions.…”

“…Indeed, the deposition propensity is not solely determined by the bulk ash but is also affected by the ultrafine PM formation ability, implying enhanced surface stickiness via condensation and scavenging in the boundary layer. ,,− Considering the possible gas-to-particle route of condensable mineral vapor, it is desirable to quantify the intensity of each pathway and determine a more accurate condensation deposition rate. An analogy is the chemical vapor deposition (CVD), where theories have been explored for condensing rates under a variety of conditions. , However, coagulation was not considered. As for solid fuel combustion, a transport model is developed for alkali chlorine superheater deposition in the biofuel-fired recovery boiler that takes into account multiple interactions .…”