Abstract: The non-contact hydrodynamic mechanical seal is a key core component of the high-speed turbopump of the liquid rocket engine, and the liquid film between seal pairs can effectively prevent the abrasion and significantly improve the seal’s life. But the film is easily induced to form a liquid-gas two-phase flow because of the extreme working conditions such as high speed, high pressure, and easy vaporization of low viscosity fluid, etc, which may threaten the safe operation of the whole turbopump. Moreover, the formation mechanism on this kind of microchannel two-phase flow in such sealed space is unclear, and there is no available theoretical model and experimental results. In this paper, the formation and evolution of boiling two-phase flow in micro-gaps are studied by combining the numerical simulation and experimental test. A gap boiling two-phase flow model based on the VOF model is proposed, and a set of boiling visualization experiment platforms based on a high-speed camera system is built. The bubble formation law of the water vaporization process in the gap and the influence of working parameters (temperature, flow rate, velocity, etc.) on gas-liquid phase transition behavior characterization parameters (gas phase proportion, heat transfer coefficient, initial saturated boiling position, etc.) are explored. The results show that when the inlet temperature is high and the flow rate is low, with the increase of heat flux, the gas phase proportion increases, and the two-phase flow pattern is more inclined to bubble and slug flow. With the inlet temperature increases and the volume flow rate decreases, the average heat transfer coefficient increases, and the initial saturated boiling position is closer to the inlet. The comparison of multiple sets of experimental and simulation results shows that the theoretical model has certain rationality, and the minimum and average relative errors of the average heat transfer coefficient and the initial saturation boiling point at different temperatures and flow rates are 7.5% and 2.6%, 20.3% and 15.1%, respectively. The results will provide the important theoretical and experimental value for obtaining the mechanism on the formation of the two-phase flow in the microchannel gap of the mechanical seal and for ensuring the safe operation of the whole turbopump.

Key words: boiling two-phase flow, mechanical seal, liquid-gas phase transition, VOF model, visualization experiment