Investigation of a mixed flow immpeller.

Abstract

Investigations have been conducted on a mixed?flow impeller to provide basic information for use in the design of high?specific?speed pumps. Such pumps are widely used for cooling?water duties in thermal power stations in the country. Extensive flow traverses were carried out upstream and downstream of the impeller using a novel four?hole probe and a T?probe for a range of flow coefficients and for different tip clearances. The overall head杅low characteristics showed a narrow hysteresis, with the curve obtained during throttling being slightly, but distinctly, different from the curves obtained during opening the throttle. This hysteresis extended through almost the entire operating range, unlike the usual hysteresis associated with stall in axial impellers. A method has been developed by which blade losses could be rapidly estimated using a hot?wire probe. The results of this method are in good agreement with the losses measured using conventional techniques. The local efficiencies have been measured and integrated to give an overall hydraulic efficiency of the impeller. The pressure rise across the impeller, as well as the efficiency, was found to vary linearly with tip clearance for small clearances. The measured head?rise coefficient, when estimated near mid?span and corrected for the measured efficiency and slip, agreed well with the design objective. The pressure rise measured at the wall differed considerably from that within the main flow and must be taken into account when analysing the performance of high?specific?speed impellers. The slip factor has also been modified to include the effect of the pump cone angle. The decrement in efficiency and pressure rise due to tip clearance is higher than the values normally accepted for pumps.