Influence of clearance flow on disk friction loss in low specific speed hydraulic machines

Abstract

Generally, in industrial applications, multi-stage pumps are used to obtain high working pressure. However, utilizing these pumps requires larger body size and longer shafts which will lead to substantial costs. As a result, a low specific speed machine is needed to minimize costs. In such case, energy can be saved and manufacturing costs can be reduced by increasing the efficiency of these low specific speed machines disk friction loss is one important cause of friction in low specific speed machines. It is caused by fluid friction occurring in the clearances between the shroud and back-shroud of the impeller and the casing. This loss is not directly related to the energy conversion in turbo machines, however it is related to the main characteristics of a machine. It will be increased proportionally with the impeller diameter and decreased with its specific speed. In this research, a test apparatus for measuring friction torque on a rotating disk was established, including a pressure resistant tank and boost pump connected in a closed loop using water as the working fluid. The sample rotating disk simulated the leakage flow path between the back-shroud and the casing of a centrifugal pump. Disks with different clearances were prepared, and fluctuations of torque were studied as a function of flow rate and Reynolds number. As a result, an optimum value for disk clearance with the minimum torque was observed to exist under each flow condition. CFD analysis was performed using a model simulating the disk clearance to understand the internal flow structure. For purpose of validation, the radial distribution of static pressure on the stationary wall was simulated by CFD and compared to experimental results. The CFD analysis agreed with the experimental distribution of wall static pressure in an acceptable manner, therefore it is proved that CFD model can reliably be used in evaluation of disk friction loss. Moreover, disk friction loss variation as a function of flow rate was investigated with inward and outward flow directions. It is seen that for both directions, disk friction loss decreases with flow rate. However, a vortex structure was observed at the inlet of the disk clearance for inward flow direction. Size of the vortex structure changes as a function of flow rate and has an influence on disk friction loss. The occurrence of this vortex structure may increase the efficiency of actual low specific speed machinery.