Study on the energy conversion characteristics in the impeller of USSPAT based on velocity triangle space decomposition

Abstract

With the development of micro-hydropower generation, traditional pump as turbine (PAT) is difficult to meet the needs of micro-hydropower for developing dispersed and small-flow water resources. Ultra-low specific speed pump turbine (USSPAT) has broad prospects in micro-hydropower generation in remote mountainous areas due to its advantages of larger output power at a small flow rate. In this paper, the dimensionless number ξ to measure the power output capacity of the fluid particle ξ is proposed based on the velocity triangle space decomposition. The impeller is the only energy conversion component in (USSPAT). The study of ξ in the impeller of USSPAT found that the area with the largest output power of the impeller is mainly concentrated in the area with a larger impeller radius, and the overall power output of the impeller increases with the increase of flow rate. The local high-speed jet formed by the rotor-stator interaction (RSI) between the impeller and the volute at the inlet of the region S is the fundamental reason for the NP and jet-wake phenomenon in the impeller. W* = 0.9 is the critical value of NP in region S. The jet wake formed in region A makes the critical value of the region decrease along the flow direction. Therefore, the region A is the core region of the NP inside the impeller. In addition, the secondary flow at the impeller outlet is the main reason for the NP in this region. In addition to 0.6Qd, the secondary flow intensity increases with the increase in flow rate, and WLF inhibits the secondary flow intensity in the wear ring area. However, at a small flow rate (0.6Qd), the WLF gradually enters the mainstream, which weakens the inhibition of the secondary flow in the wear ring region and promotes the secondary flow intensity in the downstream.