Condensation of subsonic and supersonic gas flows on a flat surface

Intense heat-mass transfer in a gas flow to a condensation surface is studied with the consistent atom-istic and kinetic theory methods. The simple moment method is utilized for solving the Boltzmann kinetic equation (BKE) for the nonequilibrium gas flow and its condensation on the surface, while molecular dy-namics (MD) simulation of a similar flow is used for verification of BKE results and boundary conditions applied at the surface. We demonstrate that BKE can provide the flow profiles close to those obtained from MD simulations in both subsonic and supersonic regimes of steady gas flows. In particular MD con-firms that the surface at a particular temperature condenses completely a steady supersonic flow with a shock front standing in reference to this surface. Hence it can be interpreted as a permeable condensating piston, which compresses and absorbs completely the incoming gas in contrast to a common nonabsorb-ing impermeable piston. The shock front divides the vapor flow on the supersonic and subsonic zones, and condensation of shocked gas happens in a subsonic regime. Conditions required for the complete, partial, and ceased condensation regimes are determined. It is shown that a runaway shock front can stop an inflow gas and condensation is ceased if the surface temperature is above some critical threshold determined by a saturation line of gas and its shock Hugoniot. We also demonstrate that the elementary theory of condensation has a good accuracy in a surprisingly wide range of flow parameters.(c) 2022 Elsevier Ltd. All rights reserved.