Interaction Between Impeller and Volute of Pumps at Off-Design Conditions

Abstract

In a centrifugal pump of volute type, the respective characteristics of the impeller and the volute are such that at only one operating point can the flow parameters be constant along the length of the volute. At off-design conditions the mismatching of characteristics causes variations of velocity and pressure along the periphery of the impeller. This in turn forces cyclic variation of the flow in the impeller channels, introduces variations of the inlet incidence and contributes significantly to the direction and the magnitude of the radial thrust. Furthermore, below a certain pump output, a complete flow reversal occurs over a part of the impeller periphery, thus explaining the onset of recirculation. The paper describes the calculation approach used to derive this aspect of the flow behavior. Because of difficulties in obtaining a closed analytical solution, a step by step computation is employed. Beginning with arbitrarily chosen conditions at the volute tongue, the program computes the flow parameters for following segments, using the continuity and the momentum equations, until the exit from the last segment is reached. The inherent unsteadiness of the relative flow in the impeller is explicitly accounted for. Since the inflow and the velocity in the first segment depend upon the exit conditions of the last, the initial input must be modified, and the computation repeated, until the values are compatible with the exit conditions. In spite of several simplifying assumptions, the results of the calculations show very good agreement with published test results.