A slip model for rarefied gas flows above a moving surface with mass transfer

A slip velocity boundary condition for rarefied gas flows above a moving surface with net mass transfer in-between is derived from kinetic theory. Tangential momentum transfer rate contribution from mass transfer of gas molecules at a moving surface is discovered to introduce an additional gas slip velocity, which is linearly proportional to the mass transfer rate and the average tangential velocity of gas molecules entering/leaving the gas flow domain through the surface. The mass transfer induced gas slip velocity component is very different from the previously studied slip velocity component due to velocity shearing. A generalized lubrication equation for rarefied gas flow is obtained from the derived slip velocity boundary condition. The slip velocity boundary condition and the lubrication equation are suitable for rarefied gas flows at arbitrary Knudsen number with mass transfer on bounding surfaces. Mass transfer such as evaporation/condensation and its induced gas slip velocity at moving surfaces are demonstrated to be able to significantly change gas flow pressure and velocity profiles, and thus have a dominant effect on micro/nanoscale gas flows with non-negligible mass transfer on bounding surfaces. (C) 2014 AIP Publishing LLC.