Uncertainties in the Transport Properties of Helium Gas at Cryogenic Temperatures Determined Using Molecular Dynamics Simulation

TL;DR

This paper investigates the transport properties of helium gas at cryogenic temperatures using molecular dynamics simulations, addressing classical and quantum effects to improve accuracy in a scarcely studied temperature range.

Contribution

It introduces methods to account for quantum effects and reduce uncertainties in classical simulations of helium's transport properties at cryogenic temperatures.

Findings

Transport properties data for helium from 10 K to 150 K.

Quantum effects significantly influence properties below 40 K.

Efficient autocorrelation averaging reduces simulation uncertainties.

Abstract

In this study, the transport properties, such as diffusivity, viscosity, and thermal conductivity, of 4He at the gaseous phase are computed for state points in the temperature range of 10 K to 150 K and pressure range of 0.10 MPa (1 atm) to 0.21 MPa using classical and quantum frameworks. Within the classical molecular dynamics simulation (MDS), the Green-Kubo (GK) approach is used. The GK method has an inherent uncertainty associated with it due to the random fluctuations in the flux autocorrelation functions. Moreover, in the temperature range of 10 K to 40 K, the quantum nature of the helium gas particles becomes prominent. The classical MDS performed does not include these effects and hence introduces uncertainties in the calculated results. We discuss efficient ways of time averaging the autocorrelation function to reduce statistical fluctuations and perform the quantum scattering phase shift calculations within the Chapman-Cowling theory to study the quantum effects. Furthermore, this study also provides the transport properties data in the cryogenic temperature limit of 10 K to 150 K, which is scarcely studied in the literature and can be applied in complex systems.