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 $T_{g-tran}$, vapor rotational temperature $T_{g-rot}$, phase change surface temperature $T_{neu}$ 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. $T_{g-tran}$, $T_{g-rot}$ and $T_{ov}$ shows different influence degree on the H2O phase change threshold, whose degree order from large to low is: vapor thermodynamics temperature $T_{g-ov}$, vapor transitional temperature $T_{g-tran}$ and vapor rotational temperature $T_{g-rot}$.

液滴和晶体的分布是航天发动机羽流研究的重点。真空中的羽流可能具有很大的气体分子自由程，从而导致非连续体和 LTE（局部热平衡）。为了了解更一般的H2O相变阈值，本文建立了一种吉布斯相计算方法。内压被提出作为与静压区分开来的新概念。蒸气转变温度的影响${吨}_{G-吨r\mathrm{一个}n}$, 蒸汽旋转温度${吨}_{G-r○吨}$, 相变表面温度${吨}_{ne你}$研究了H2O相变阈值的空间特征长度L。根据结果​​，非连续蒸汽的相变比连续蒸汽更难。在 TE（热平衡）下，当蒸汽数密度较低时，H2O 的饱和相变边界线会偏向低温侧。在LTE下，在LTE情况下，蒸气数密度低于三相点的情况下，蒸气可以转化为水，这在TE情况下是不可能的。${吨}_{G-吨r\mathrm{一个}n}$,${吨}_{G-r○吨}$和${吨}_{○v}$对H2O相变阈值的影响程度不同，影响程度从大到小依次为：蒸气热力学温度${吨}_{G-○v}$, 蒸气转变温度${吨}_{G-吨r\mathrm{一个}n}$和蒸汽旋转温度${吨}_{G-r○吨}$.