Numerical study on loss mechanism in rear rotor of contra-rotating axial flow pump

It is known that higher efficiency can be achieved with reduced specific speed design of rear rotor in contra-rotating axial flow pump. To investigate the mechanism of increased efficiency with the reduced specific speed of rear rotor, three models with different specific-speed rear rotors are designed with the conventional method, and the flow fields are simulated by unsteady RANS simulation. To analyze the loss generation mechanism, two loss evaluation methods based on the entropy production rate and the material-derivative of rothalpy are employed. It is found that, although the both methods qualitatively estimate the total loss through the rear rotor, the derivative of rothalpy can give much better quantitative prediction of the losses. Two distinct flow features are observed in the rear rotor, the corner separation at the hub corner of blades and the tip leakage vortex, both of which are responsible for the loss generation. With the evaluation of local loss generation based on the material derivative of rothalpy, the loss contribution of corner separation is found to be very small compared with that due to the tip leakage vortex. The tip leakage vortex structure in high specific speed rear rotor shows the strong interaction with the leading edge of adjacent blade, which seems to strengthen the blockage effect in the tip region. This is relieved in the lower specific speed rear rotor, resulting in the achievement of higher efficiency with it.