A thorough study on thermal mass flux of rarefied flow through micro/nanochannels

Abstract

Most of the previous works on mass flux due to thermal creep focused on thermal creep due to wall temperature gradient. In this work, we show that even without wall temperature gradient there exists thermal creep mass flux if one applies heat flux over the wall. This heat flux could induce a temperature gradient inside the flow and subsequently leads to a temperature gradient across the channel. Additionally, we deduce that rarefied gas under thermal non-equilibrium conditions is governed by three separate mechanisms, i.e., thermal creep effect and viscosity and density changes. There is a critical Knudsen number, around unity, where the mass flux remains invariant against the heating/cooling wall heat fluxes. Some analytical expressions for all sorts of thermal mass flux, including that due to the heat flux and that due to the wall temperature gradient are derived. Previous works on mass flux due to the thermal creep only considered linearized flow while this work considers arbitrary temperature gradient effects.