Collisional cooling in confined gases at cryogenic temperatures: Time dependent near-wall accumulation and gas dynamics

Abstract

The heat transfer characteristics of a confined gas are often quantified by anisotropic thermal conductivity and are affected by near-wall accumulation. However, in non-equilibrium conditions, where gas and walls are at different temperatures, transport coefficients, and state variables become dynamic, thus offering an interesting aspect of heat transfer, which is investigated in the present work. Here, we focus on the cooling of confined He by collisions with cold metal walls at cryogenic temperatures (30 - 150 K) in pristine and mixed states with a secondary gas at atmospheric pressure. The effects of wall temperature (Tw), type, and mass (m⁎) of the secondary gas on the cooling rate constant (k) and subsequently on the near-wall accumulation of He are investigated through non-equilibrium molecular dynamics simulations. The cooling curves of the confined gases are found to obey Newton's law of cooling. To probe the cooling process further, the temporal evolution of the near-wall accumulation is analyzed and found to be significant for lower Tw, which, in turn, promotes heat transfer at the interface through gas-wall and gas-gas collisions. In a gaseous mixture, such accumulations are found to be smaller for a heavier secondary gas, which leads to the inhibition of gas-wall collisions. Furthermore, the kinetic energy transfer between He and a considerably heavier gas particle is inefficient, resulting in the slower cooling of He, as observed in He-Ne and He-Ar mixtures. The quantity k is additionally affected by the interaction strength and thermal conductivity of the secondary gas, including the initial temperature difference between the wall and the gas (ΔT). The results of this study find their applications in the fields of microelectronics and microplasmas.