Natural circulation precooling of cryogenic liquid rocket engine is a spontaneous high velocity reflooding heat transfer process. In the present study, the flow regimes and heat transfer characteristics of this precooling process have been analyzed, and one set of heat transfer models, which couples the heat release equation of the pipeline wall and the flow and heat transfer equations of cryogenic liquid, has been established and validated to investigate the heat transfer characteristics of the unsteady precooling process. The results indicate that the circulation flow rate of the precooling process fluctuates due to the unstable heat released to the cryogenic liquid. Moreover, it is found that the optimal reflux location is at 4 ∼ 6 meters above the bottom header, and the precooling effects can be enhanced by decreasing the gas pillow pressure.
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