It is well known that in a gas-filled duct or channel along which a temperature gradient is applied, a thermal creep flow is created. Thermal creep flow has attracted the attention of many researchers. For example Akhlaghi et.al. [1] applied a non-uniform wall temperature distribution, studied the effects of thermal creep from the slip to the free molecular regime, and suggested an analytical expression for the normalized thermal mass flow rate. Thermal creep refers to a flow parallel to the temperature gradient, which is often unfavorable from an application point of view, since large differences in temperature are needed. As an alternative, Donkov et al. [2] considered a ratchet channel with the temperature gradient applied between the opposing walls. Mixed diffuse and specular reflection boundary conditions were applied along the ratchet surface. Based on kinetic theory they could show that a significant gas mass flux along the channel can be created that way. Chen et al. [3] also considered two facing isothermal ratchet surfaces with different temperatures, but with diffuse reflection boundary conditions everywhere. They simulated the pumping in the slip flow regime based on the Navier-Stokes equations with appropriate velocity slip and temperature jump boundary conditions. The present study aims at determining optimized ratchet geometries of corresponding Knudsen pumps, using the Direct Simulation Monte Carlo (DSMC) method. The DSMC solver of the OpenFOAM software package, i.e. dsmcFoamStrath, was used. We consider two classes of ratchet surfaces with planar and curved walls and compute the flow and the temperature field of the rarefied gas.

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