H2O saturated phase change threshold under molecular free path scale*,**

The distribution of droplet and crystal is a key point in space engine plume flow research. Plume flow in vacuum may have great gas molecular free path, thus resulting in non-continuum and LTE (local thermal equilibrium). To obtain understanding on more general H2O phase change threshold, this article establishes a Gibbs phase calculation method. Internal pressure is proposed as a new concept to be distinguished from static pressure. The effect of vapor transitional temperature Tg_tran, vapor rotational temperature Tg_ rot, phase change surface temperature Tneu and space characteristic length L on H2O phase change threshold is investigated. According to the results, phase change is more hard for non-continuum vapor than continuum one. Under TE (thermal equilibrium), the saturated phase change boundary line for H2O deflect toward low temperature side when the vapor number density is low. Under LTE, vapor can transform to water under the vapor number density below three phase point in LTE circumstances, which is impossible under TE situation. Tg_tran, Tg_ rot and Tov shows different influence degree on the H2O phase change threshold, whose degree order from large to low is: vapor thermodynamics temperature Tg_ov, vapor transitional temperature Tg_tran and vapor rotational temperature Tg_ rot.

Automated summary

This article investigates the distribution of droplet and crystal in space engine plume flow research. The study establishes a Gibbs phase calculation method to understand the H2O phase change threshold. It explores the effects of various factors, including vapor transitional temperature, vapor rotational temperature, phase change surface temperature, and space characteristic length, on the threshold. The research finds that phase change is more difficult for non-continuum vapor compared to continuum vapor. Additionally, under thermal equilibrium, the saturated phase change boundary for H2O shifts towards lower temperatures with lower vapor number density.