Crystal cluster forming process under rarefied gas and low temperature environment

Abstract

Aerospace engine plume flow under high altitude environment generates crystal due to low atmosphere temperature, thus causing variation on flow field and radiation properties. In order to gain better understanding on crystal generation and growth process, this article fundamentally establishes thermal motion phase change (TMPC) model based on molecular thermal motion. Direct calculation on molecular motion in phase change process takes influence of large scaled molecular free path caused by rarefied environment into consideration. The effect of temperature, H2O vapor number density, and characteristic length on phase change process is studied. According to the results, characteristic length determines non-continuum degree quantitatively under rarefied gas condition. Increasing characteristic length decreases phase change time before reaching steady state. Crystal number density decreases when working condition is near saturation condition, while crystal radius shows contrary trend. The effect of characteristic length on crystal cluster quantity and radius variation is prominent under low vapor number density condition, for increasing vacuum effect caused by decreasing gas molecules quantity. Existence of other gas components contamination obstructs H2O vapor molecules thermal motion, thus decreasing non-continuum effect on phase change process, where variation of characteristic length shows relatively limited influence on crystal cluster properties and phase change time.