Investigation of Internal Flow in Centrifugal Pump Diffuser using Laser Doppler Velocimetry (LDV) and Computational Fluid Dynamics

An unshrouded impeller is being developed for high head pumps to reduce costs and disk friction losses. On the other hand, research of internal flow in a diffuser did not clearly reveal flow structure. In this experiment, we measured the velocity distribution at the diffuser inlet and outlet plane using laser doppler velocimetry (LDV) to capture the flow from the unshrouded impeller. The relation between the total pressure and velocity distribution was evaluated. The circumferential velocity and meridian velocity were measured by short-focus LDV (Diode Laser, 74mW) about the circumferential and the height direction of the vane direction. Operating conditions in this steady measurement are at the design point flow rate. The result was compared with computational fluid dynamics (CFD) simulations carried out in steady conditions at the previously defined operation points. In this experiment, a phenomenon that the streamline moved toward the shroud side was confirmed. There was also a region where the static pressure increased on the shroud side at the diffuser inlet. This phenomenon was caused by the influence of the tip leakage flow of the unshrouded impeller downstream and the gap of the impeller's main plate. Furthermore, two high velocity regions on the hub and shroud side at the diffuser outlet were observed because of the secondary flow in the diffuser. From the above studies, it was clarified that the ununiform flow in the diffuser was caused by the influence of the secondary flow in the unshrouded impeller.