Research on the Influence of Impeller Tip Clearance on the Internal Flow Loss of Axial Circulating Pump Under Unpowered Driven Condition

An axial-flow pump (AFP) a key hydraulic component in the circulating water system of large ships. When the speed of a large ship meets certain requirements, the AFP operates in the unpowered driven condition and has the characteristics of low speed, positive rotation and driven rotation, thereby achieving energy recovery efficiency without investing in the driving device. The unsteady internal flow characteristics of the AFP under the unpowered driven condition differ from those of conventional conditions. The blade tip clearance (TC) is an essential basis for the comprehensive technical indicators of the lift pump and the safe and stable operation of the circulating water system. In this study, the performance of the AFP with different TCs under the unpowered driven condition is investigated based on experimental tests and numerical simulations. Based on the entropy production theory, the energy loss characteristics of the AFP with different TCs are studied, and it is concluded that increasing the TC increases the strength of tip leakage flow. Particularly at large TCs, the energy loss of the pump increases significantly, causing flow separation on the blade surface and the formation of large-area vortex structures on the blade. In addition, under the large spacing TC, the instantaneous entropy production of the pump is unstable, and the entropy production frequency and amplitude are much larger than those of the conventional TC.