H2O saturated phase change threshold under molecular free path scale

• Published in: Vacuum 2022-09, Vol.203, p.111167, Article 111167
• Main Authors: Ming, Ze-peng, Liu, You-hong, Wu, Zhen-yuan, Zhao, Xiao-Wang, Li, Zhi-Hui
• Format: Article
• Language: English
• Subjects: Local thermal equilibrium; Molecular free path scale; Non-continuum; Phase change; Saturation
• ISSN: 0042-207X; 1879-2715
• DOI: 10.1016/j.vacuum.2022.111167

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. •A mathematical model is built to obtain the aerospace engine plume flow water phase transition condition.•The noncontinuous environment in high altitude shows significant influence on water three phase diagram.•Obvious difference is found between phase transition condition of impure water and pure water.•Small plume flow characteristic length causes structural variation on pure water three phase diagram.