Theory and Analysis of Liquid-Oxygen-Hydrogen Interface Dynamics in Liquid Rockets at Supercritical Pressures

摘要

Liquid injection in systems such as liquid rockets where the working fluid exceeds the thermo-dynamic critical condition of the liquid phase is not well understood. Under some conditions when operating pressures exceed the liquid phase critical pressure, surface tension forces become diminished when the classical low-pressure gas-liquid interface is replaced by a diffusion-dominated mixing layer. Modern theory, however, still lacks a physically-based model to explain the conditions under which this transition occurs. In this paper, we derive a coupled model to obtain a theoretical analysis that quantifies these conditions for general multicomponent liquid injection processes. Our model applies a modified 32-term Benedict-Webb-Rubin equation of state along with corresponding combining and mixing rules that accounts for the relevant thermodynamic non-ideal multicomponent mixture states in the system. This framework is combined with Linear Gradient Theory, which facilitates the calculation of the vapor-liquid molecular structure. Dependent on oxygen and hydrogen injection temperatures, our model shows interfaces with substantially increased thicknesses in comparison to interfaces resulting from lower injection temperatures. Contrary to conventional wisdom, our analysis reveals that LOX-H2 molecular interfaces break down not necessarily because of vanishing surface tension forces, but because of the combination of broadened interfaces and a reduction in mean free molecular path at high pressures. Then, these interfaces enter the continuum length scale regime where, instead of inter-molecular forces, transport processes dominate. Based on this theory, a regime diagram for LOX-H2 mixtures is introduced that quantifies the conditions under which classical sprays transition to dense-fluid jets.