Extraction of the translational Eucken factor from light scattering by molecular gas

TL;DR

This paper introduces a new method to determine the translational Eucken factor of molecular gases using Rayleigh-Brillouin scattering data, enabling separate analysis of translational and internal contributions to thermal conductivity.

Contribution

It presents the first experimental extraction of translational Eucken factors and bulk viscosities of gases from RBS spectra, highlighting the sensitivity of RBS to these parameters.

Findings

Translational Eucken factor can be extracted from RBS spectra.

RBS spectra are sensitive to translational Eucken factor even with fixed total thermal conductivity.

Simultaneous measurement of bulk viscosity and translational Eucken factor for N2, CO2, and SF6.

Abstract

Although the thermal conductivity of molecular gases can be measured straightforwardly and accurately, it is difficult to experimentally determine its separate contributions from the translational and internal motions of gas molecules. Yet this information is critical in rarefied gas dynamics as the rarefaction effects corresponding to these motions are different. In this paper, we propose a novel methodology to extract the translational thermal conductivity (or equivalently, the translational Eucken factor) of molecular gases from the Rayleigh-Brillouin scattering (RBS) experimental data. From the numerical simulation of the \cite{LeiJFM2015} model we find that, in the kinetic regime, in addition to bulk viscosity, the RBS spectrum is sensitive to the translational Eucken factor, even when the total thermal conductivity is fixed. Thus it is not only possible to extract the bulk viscosity, but also the translational Eucken factor of molecular gases from RBS light scattering spectra measurements. Such experiments bear the additional advantage that gas-surface interactions do not affect the measurements. For the first time, bulk viscosities (due to the rotational relaxation of gas molecules only) and translational Eucken factors of N2, CO2 and SF6 are simultaneously extracted from RBS experiments.