Internal Flow Phenomena of Two-Way Contra-Rotating Axial Flow Pump-Turbine in Pump Mode under Variable Speed

This paper investigated the variable speed operation of a two-way contra-rotating axial flow pump–turbine in pump mode. When counter-rotating impellers operate at the same speed, a significant difference exists in the shaft power of the two impellers, thereby causing difficulties in motor selection. However, the same motor is required for both impellers in two-way pump–turbines. To solve this problem, this paper aims to determine the appropriate speed at which the two impellers operate with a similar shaft power. Accordingly, improving the performance of the tidal unit while enabling the basic functions of the pump–turbine is of considerable engineering and academic significance. An analysis of performance variations of the front and rear impellers in the pump mode reveals the law of power variation for each impeller at different speeds of the rear impeller. Under different flow conditions, shaft power of the rear impeller is found to be at least 31% higher compared with that of the front impeller. This result provides practical reference for further research on the operation of counter-rotating axial flow pump–turbine.