As a direct source of power, the water vapor with high temperature and high pressure affects the thermal efficiency of an underwater vehicle power system. In this study, the subcooled flow boiling heat transfer performances in straight and helical tubes are studied at underwater vehicles conditions. A numerical model for the subcooled flow boiling in the tubes is established and verified by the existing experiment data. The local heat transfer capability, and content and distribution of the vapor are analyzed under various thermal conditions and helical angles. The results show that inlet mass flux, heat flux and inlet subcooling have obvious effects on the local heat transfer capacity, vapor content as well as wall temperature. It is found that the maximum local heat transfer coefficient in a helical tube increases by 20.75 compared with that in the straight one. The analyses of vapor volume fraction and secondary flow show that the vapor mainly distributes in the inside and top of tubes, and the secondary flow intensity increases along the axial direction, but it is little influenced by helical angles.
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