Tip clearance on pressure fluctuation intensity and vortex characteristic of a mixed flow pump as turbine at pump mode

The present work investigates the pressure fluctuation intensity and vortex characteristic of a mixed flow pump as turbine at pump mode with a tip clearance. The tip clearance between the blade tip and shroud can induce tip leakage flow and interact with main flow, which causes the unstable flow structure and complex vortex in the passage. The external characteristics of experimental results and numerical simulations are in agreement. With tip clearance increasing, the head and efficiency of pump decrease by 10.8% and 6.26%, respectively. The distribution of pressure fluctuation intensity is presented as a triangular shape under design flow rate. Results show that the tip leakage vortex (TLV) can be divided into four categories, namely, primary TLV, secondary TLV, entangled TLV, and dispersed TLV. The flow rate has a significant influence on the TLV structure and trajectory, and the starting point of the primary TLV shifts to approximately 20% of the blade chord at large flow rate. The relative vorticity transport equation is introduced to analyze the vortex derivation by using the relative vortex stretching, Coriolis force and viscous diffusion.