Molecular heat transfer in rarefied gas

Experimental determination of thermal accommodation coefficients

Molecular heat transport in rarefied gas can be a significant factor in the thermal system performance of vacuum applications. An experimental setup is realized to measure the relation between gas pressure and heat transfer coefficient, which is determined by the thermal accommodation coefficient. The influence of surface contamination on the thermal accommodation coefficient is investigated and found to be of significant impact on the heat transfer performance.

This article was presented at the Special Interest Group Meeting on Thermal Issues, Laboratory for Machine Tools and Production Engineering (WZL) of RWTH Aachen, Germany, February 2020, www.euspen.eu

Introduction

Thermal management under vacuum conditions can be crucial in the performance of high-precision equipment. Two heat transfer mechanisms play a role in heat transport through a vacuum: thermal radiation and molecular heat transport. Especially at ambient temperatures (or lower) and when there is a considerable amount of gas (few Pascal and higher), the molecular heat transport will dominate the overall heat transfer rate.

Unlike in continuum conditions, the heat transfer coefficient in the free molecular regime is dependent on the gas pressure as well as on various gas and surface properties. This heat transfer process is illustrated in Figure 1: an incident gas molecule with temperature T_i interacts with a solid surface with (constant) temperature T_s, thereby exchanging energy such that the temperature of the reflecting molecule has changed to T_r.

Figure 1. Illustration of molecular heat transfer process

The efficiency of this molecular heat transport is expressed by means of the thermal accommodation coefficient. Knowledge on the thermal accommodation coefficient is essential to predict the relation between heat transfer coefficient and gas pressure. However, this is dependent on both gas and surface characteristics such as surface material, surface roughness, gas composition, cleanliness of the surface and much more.

The current work describes the development of an experimental setup at Philips Engineering Solutions to determine the thermal accommodation coefficient for different solid surfaces under a controlled gas composition and gas pressure. The focus is to achieve high accuracy on reproducibility to be able to perform comparative measurements.

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