Thermally induced gas flows in ratchet channels with diffuse and specular boundaries

Vahid Shahabi,Ehsan Roohi,Tobias Baier,Steffen Hardt

doi:10.1038/srep41412

Vahid Shahabi, Ehsan Roohi + Show 2 more

Open Access

https://doi.org/10.1038/srep41412

Copy DOI

Abstract

A net gas flow can be induced in the gap between periodically structured surfaces held at fixed but different temperatures when the reflection symmetry along the channel axis is broken. Such a situation arises when one surface features a ratchet structure and can be augmented by altering the boundary conditions on different parts of this surface, with some regions reflecting specularly and others diffusely. In order to investigate the physical mechanisms inducing the flow in this configuration at various Knudsen numbers and geometric configurations, direct simulation Monte Carlo (DSMC) simulations are employed using transient adaptive subcells for collision partner selection. At large Knudsen numbers the results compare favorably with analytical expressions, while for small Knudsen numbers a qualitative explanation for the flow in the strong temperature inhomogeneity at the tips of the ratchet is provided. A detailed investigation of the performance for various ratchet geometries suggests optimum working conditions for a Knudsen pump based on this mechanism.

Highlights

A net gas flow can be induced in the gap between periodically structured surfaces held at fixed but different temperatures when the reflection symmetry along the channel axis is broken
Thermal creep flow along a temperature gradient on a diffusely reflecting wall can be understood from the momentum in wall direction carried towards a patch of wall by gas molecules originating from regions of different temperature in the gas
Fast molecules coming from the hot regions are reflected predominantly specularly, while slow molecules coming from the cold regions predominantly thermalize, reflect diffusively and impart a net tangential momentum to the wall[8,9]

Summary

Introduction

A net gas flow can be induced in the gap between periodically structured surfaces held at fixed but different temperatures when the reflection symmetry along the channel axis is broken. Such a situation arises when one surface features a ratchet structure and can be augmented by altering the boundary conditions on different parts of this surface, with some regions reflecting specularly and others diffusely. Thermal creep flow along a temperature gradient on a diffusely reflecting wall can be understood from the momentum in wall direction carried towards a patch of wall by gas molecules originating from regions of different temperature in the gas. Its magnitude is of second order in temperature derivatives and when the wall is not isothermal it is usually overshadowed by thermal creep, which is proportional to the thermal gradient

Methods

Results

Conclusion