Influence of rarefaction on the flow dynamics of a stationary supersonic hot-gas expansion

Abstract

The gas dynamics of a stationary hot-gas jet supersonically expanding into a low pressure environment is studied through numerical simulations. A hybrid coupled continuum-molecular approach is used to model the flow field. Due to the low pressure and high thermodynamic gradients, continuum mechanics results are doubtful, while, because of its excessive time expenses, a full molecular method is not feasible. The results of the hybrid coupled continuum-molecular approach proposed have been successfully validated against experimental data by R. Engeln et al. ͓Plasma Sources Sci. Technol. 10, 595 ͑2001͔͒ obtained by means of laser induced fluorescence. Two main questions are addressed: the necessity of applying a molecular approach where rarefaction effects are present in order to correctly model the flow and the demonstration of an invasion of the supersonic part of the flow by background particles. A comparison between the hybrid method and full continuum simulations demonstrates the inadequacy of the latter, due to the influence of rarefaction effects on both velocity and temperature fields. An analysis of the particle velocity distribution in the expansion-shock region shows clear departure from thermodynamic equilibrium and confirms the invasion of the supersonic part of the flow by background particles. A study made through particles and collisions tracking in the supersonic region further proves the presence of background particles in this region and explains how they cause thermodynamic nonequilibrium by colliding and interacting with the local particles.

Hybrid simulations of rarefied supersonic gas flows in micro-nozzles

“…This can be determined dynamically, e.g. based on the local value of the Knudsen number [28,36,37], or statically, based on some a priori knowledge about the flow characteristics [15,19,35].…”

Improved size distribution control of silicon nanocrystals in a spatially confined remote plasma

“…Due to the remote character of the ETP, the effect of electric field forces on the transport of nanoparticles is negligible, and particle transport is dominated by the convective flow and the diffusional processes [1]. In the ETP, the plasma expands in the form of a central beam, which is surrounded by background regions, or recirculation cells [26][27][28]. The flow in the central beam draws a linear trajectory for the growth of nanoparticles under convective transport, particularly with a beam residence time of τ beam ∼ 10 ms leading to growth of small Si-NCs in the size range 2-10 nm.…”

Nonequilibrium Processes in Supersonic Jets of N2, H2, and N2+ H2Mixtures: (II) Shock Waves

“…Other experiments on the invasion of supersonic rarefied plasma jets by background gas , concluded that part of the molecules in the zone of silence come indeed from the background gas, crossing the barrel shock wave. Numerical simulations of those jets yielded values of the local Knudsen number (eq ) greater than 0.1 in an extensive region of the zone of silence. With such large Knudsen numbers it may be expected that a significant fraction of the molecules that cross the barrel shock could reach the jet axis.…”