Experimental Evaluation of the Flow Inside an Open Faced Impeller

The pressure distributions and forces presented in a thesis by Hossain [1] for a centrifugal pump illustrated a somewhat complex inter-relationship between various geometric and operating parameters of the pump studied. The pump had an open faced impeller of 33.65 cm diameter with 5 blades of backswept design. It was felt that the best way to resolve some of the questions related to Hossain’s results was to determine the fluid velocity field inside the pump. The flow field through the impeller passages was measured using a 1-D Laser Doppler Velocimetry (LDV) system. The LDV was used to measure the radial and tangential velocity components over the region accessible through the two optical windows in the front of the pump housing. Five axial planes were investigated by recording measurements along two radial lines at azimuthal angles of 45° and 315° (with respect to the horizontal axis of the pump) for design operating conditions. A once per revolution signal was used to supply the LDV system with a reference for the rotor position. It was found out that a leakage flow existed near the front wall of the impeller at z/h = 0.11, which was generated by the pressure difference between the impeller exit and inlet. It was also concluded that the velocity field was not fully two-dimensional in nature. This was believed to be a result of the 90° turn that the fluid endures as it enters the impeller inlet from the suction pipe.