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.

The unsteady internal flow in a low specific speed centrifugal pump was experimentally and numerically investigated. Unshrouded impellers enable high head designs but on the other hand, they exhibit complicated internal flow and an efficiency decline compared to shrouded impellers. Furthermore, the complicated impeller outlet flow induces unsteady internal flow in the vaned diffuser. Therefore, a detailed investigation of the internal flow is required in order to increase the efficiency of these low specific speed centrifugal pumps. The aims of this paper are to clarify the loss mechanisms in the impeller and to investigate the effect of impeller outlet flow to the diffuser internal flow at the design point. The detailed pump internal flow is investigated using unsteady computation, which enables the computation of the 3D flow pattern. The reversed flow and the secondary flow are induced by the tip leakage flow, and this creates a high loss region in the blade-to-blade region. On the other hand, the mixing effect is remarkable at the impeller outlet, and this affects the creation of the wake. This flow behavior makes the internal flow of the diffuser unsteady and the diffuser performance fluctuates due to the impeller wake at the design point.

In recent years, an unshrouded impeller is being developed for rocket turbopumps to reduce production costs and disk friction losses. However, the internal flow structure in a diffuser influenced by an impeller has not been clarified yet. In this study, we focused on the investigation of unsteady flow in turbopumps with unshrouded impellers and vaned diffusers by experiments and CFD. Furthermore, we investigated the effect of changing the number of blades of the impeller and diffuser on the unsteady losses in the diffuser. First, we measured the static pressure at the shroud side in the impeller and the velocity at the diffuser inlet and outlet. Second, we confirmed the accuracy of the CFD by comparing CFD results with experimental results. Third, we investigated unsteady losses in diffusers by CFD. We could confirm increased entropy at the suction surface and shroud side because of the tip leakage and the vortex. Finally, we changed the number of impeller blades or diffuser blades. In conclusion, the loss region at the suction surface and the high-pressure region at pressure surface in the unshrouded impeller were mixed and caused unsteady losses through the diffuser. And diffuser efficiency decreased because channel width in diffuser became narrower by the number of diffuser blades increasing.