An experimental assembly for precise measurement of thermal accommodation coefficients

Abstract

An experimental apparatus has been developed to determine thermal accommodation coefficients for a variety of gas-surface combinations. Results are obtained primarily through measurement of the pressure dependence of the conductive heat flux between parallel plates separated by a gas-filled gap. Measured heat-flux data are used in a formula based on Direct Simulation Monte Carlo (DSMC) simulations to determine the coefficients. The assembly also features a complementary capability for measuring the variation in gas density between the plates using electron-beam fluorescence. Surface materials examined include 304 stainless steel, gold, aluminum, platinum, silicon, silicon nitride, and polysilicon. Effects of gas composition, surface roughness, and surface contamination have been investigated with this system; the behavior of gas mixtures has also been explored. Without special cleaning procedures, thermal accommodation coefficients for most materials and surface finishes were determined to be near 0.95, 0.85, and 0.45 for argon, nitrogen, and helium, respectively. Surface cleaning by in situ argon-plasma treatment reduced coefficient values by up to 0.10 for helium and by ∼0.05 for nitrogen and argon. Results for both single-species and gas-mixture experiments compare favorably to DSMC simulations.

A Comprehensive Cold Gas Performance Study of the Pocket Rocket Radiofrequency Electrothermal Microthruster

“…While α T is roughly constant for each gas across different surface materials and roughnesses, it appears to decrease as the temperature difference between the two walls increases [54]. However, due to the lack of extensive studies [55] on this behavior, the mean value from Vargaftik [48], Porodnov and Kulev [49], Song and Yovanovich [50], Rader et al [51], Ganta et al [52], and Trott et al [53] is used for each gas in the simulations.…”

“…In general, α T is not the same as α u for each gas species. α T is measured experimentally [48][49][50][51][52][53] using a variety of techniques often involving two walls held at different temperatures. While α T is roughly constant for each gas across different surface materials and roughnesses, it appears to decrease as the temperature difference between the two walls increases [54].…”

Diffusive-to-ballistic transition of the modulated heat transport in a rarefied air chamber

“…1(c)) and therefore the heat transport is in ballistic (free molecular) regime, which usually occurs at very low pressures and is dominated by non-Fourier effects. 27 Theoretical studies based on general heat transport laws have been performed to analyze the propagation of thermal waves in the transitional and ballistic regimes in micro-and nanostructures using phonons, [28][29][30][31][32][33][34][35][36] photons, 35,36 electrons 36 and molecules 36 as heat carriers. However, for gases, a detailed study of modulated heat propagation taking into accounts both molecular and radiation contributions deserved further analyses.…”

Conductive heat transfer in a gas confined between two concentric spheres: From free-molecular to continuum flow regime

“…(2), is independent of the geometrical configuration of the system, see Refs. [8,3,9]. The accommodation coefficient α can be obtained by fitting heat fluxes measured as a function of pressure by Eq.…”

“…This coefficient is useful in the analysis and the management of heat transfer in micro-and nano-devices, where gas flow should be treated as rarefied, even if the operating pressure is an atmospheric pressure, due to their small characteristic dimension. Additionally, the surface-to-volume ratio of the fluid for microand nano-devices becomes much larger than that for the conventional devices, therefore, the gas-surface interaction plays an essential role [3,4,5,6].…”