Investigation of Flow Structure in a Narrow Clearance of a Low Specific Speed Centrifugal Impeller

The disk friction loss is remarkably large in low specific speed centrifugal pumps, and an effective reduction method has not been established. Therefore, to develop such a method, the loss mechanism was investigated. To grasp the internal flow structure in the narrow clearance, both experimental and computational approaches were used. An experimental apparatus that imitates clearance between a rotating impeller disk and a stationary casing disk was used, and the static pressure distribution in the radial direction was measured. The internal flow where the disk friction loss occurs was investigated. In the case of outward flow, the static pressure decreased because the influence of the centrifugal force lessened toward the outer diameter side of the disk, as the flow rate surged. For this reason, the pressure gradient became steep. According to the CFD analysis, there was a vortex in the cross-section of the clearance. This vortex encouraged flow recirculation and promoted the increased of the circumferential velocity in the potential core. When the flow rate grew, the vortex diminished. The circumferential velocity gradient and the shear stress intensified. As a result, the disk friction escalated. In the case of inward flow, the pressure gradient became steep as the flow rate increase. There was a vortex in the clearance, the size of which lessened when the flow rate surged. The disk friction had a minimum value at the flow rate was 6e-4 m3/s. This research clarified that the vortex in the clearance has a remarkable effect on reducing the disk friction.