Gas separation in a Knudsen pump inspired by a Crookes radiometer

Abstract

In a Knudsen pump, gas flow is induced by thermal gradients along a channel when the mean free path of the gas molecules is comparable to the geometric feature size. By periodically varying both the channel dimension and the reflection properties of gas molecules at the channel walls, a gas flow along the channel can be induced by application of a constant temperature difference between the channel walls. Inspired by the Crookes-Radiometer, one such arrangement consists of placing an array of plates with different reflection properties on their opposite sides along a channel. We investigate the transport of binary gas mixtures along such channels by direct simulation Monte Carlo (DSMC), focusing on the discrimination in transport of individual species due to gradients in temperature, composition and pressure. An exemplary separation column is investigated where a counterflow involving a combination of thermally induced, pressure driven and diffusion flows is established, resulting in an enrichment of the individual species at opposite ends of the column.