Energy loss assessment of a two-stage pump under natural flow in the marine cooling system based on entropy production theory

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Natural flow conditions are commonly used in the cooling systems of modern high-speed vessels and nuclear submarines. When the vessel's speed satisfies the requirements, it relies on its kinetic energy to supply seawater to the condenser in the cooling system. Under natural flow conditions, the pump is not driven, and the rotor will be “stuck” or rotated passively by the inlet flow shock. The local energy loss caused by the blade tip leakage vortex, surface separation flow, and wake flow significantly impact the system’s performance. Entropy production is an irreversible dissipation effect caused by the energy transfer process. According to the second law of thermodynamics, the entropy production theory can effectively characterize the intensity of local energy loss under natural flow conditions. In this study, the variation law of hydraulic loss and passive rotational speed under different flow rates of a two-stage pump were obtained by natural flow experiments. Moreover, the generation mechanism of intense energy loss under different natural flow conditions was investigated based on computational fluid dynamics (CFD) and entropy production theory. The results show that the entropy production induced by turbulent kinetic energy (TEP) is dominant, averaging 71.8%, followed by the wall entropy production (WEP), about 26.1%. The contribution of viscous dissipation entropy production (VDEP) can be negligible. When the impeller is “stuck” at low flow rates, entropy production is mainly from the flow separation near the blade and the flow shock region close to the guide vane back. The total entropy production of the guide vane accounts for 78%, higher than the impeller. However, when the impeller rotates passively at high flow rates, the energy loss inside the impeller is dominant, about 62%. The energy loss occurs around the blade wake region and the high velocity-gradient region near the guide vane. The flow field patterns between the two stages are similar, but the energy loss in the impeller and guide vane of stage Ⅱ is much higher. The TEP and WEP of the two impeller stages differed by 14% and 10%, respectively, and the guide vane of the two stages differed by 8% and 3%, at a flow rate of 196.8 m3/h.