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.
Highlights
The Crookes radiometer (Crookes 1876) has fascinated physicists since its inception in 1874 and has played a pivotal role in advancing the understanding of gas dynamics (Reynolds 1879; Maxwell 1879)
When the reflection properties for gas molecules on opposite sides of the plates are different—in our model system one is ideally specularly and the other is purely diffusely reflective at temperature T1 —a periodic temperature profile is induced along the channel when its sidewalls are kept at a different temperature T2
The transport properties of a Helium–Xenon gas mixture in a Knudsen pump inspired by the Crookes radiometer were investigated numerically with particular attention to gas separation
Summary
The Crookes radiometer (Crookes 1876) has fascinated physicists since its inception in 1874 and has played a pivotal role in advancing the understanding of gas dynamics (Reynolds 1879; Maxwell 1879). While in a Crookes radiometer the vanes are allowed to rotate on a spindle, the array of plates considered in the model system drive a flow along the channel. The separation of gas species by mass or size plays an important role for many analytical applications and in chemical engineering. We investigate the transport of gas mixtures in the described Knudsen pump due to gradients in temperature, pressure and composition with special focus on species separation. While thermal diffusion in liquids was observed around the same time (Ludwig 1856; Soret 1879), thermal diffusion in gaseous mixtures was investigated theoretically much later by Enskog (1911) and Chapman (1918), with
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