Numerical Simulation of Internal Flow in a Contra-rotating Axial Flow Pump

An application of contra-rotating rotors, in which a rear rotor is in tandem with a front one and these rotors rotate in the opposite direction each other, has been proposed against a demand for developing higher specific speed axial flow pump. In our previous studies, we have designed prototype rotors and observed a positive slope of pressure performance curve especially in the rear rotor. In order to clarify the reason, we have carried out the unsteady numerical simulations using a commercial CFD code for the internal flow of contra-rotating axial flow pump at a partial flow rate as well as at design flow rate. It was found that the "unsteady" simulation for whole front and rear rotors was necessary to well predict the pump performance even at the design flow rate. The ensemble averaged flow distributions obtained by the unsteady simulation could well reproduce the complex flow structures observed by the experiments. The fluid near the leading edge of rear rotor blade smoothly flows along the suction surface even at the partial flow rate, forming the strong tip leakage vortex from near the leading edge. However, the flow separation occurs downstream of the mid-chord on the suction surface in the region from the hub to the mid-span, whose blockage with that due to the tip leakage vortex seems to be responsible for the degradation of pressure performance of rear rotor at partial flow rate range.