Investigation of the Flow Instabilities of a Low Specific Speed Pump Turbine Part 2: Flow Control With Fluid Injection

Abstract

This is the second part of a two-part paper focusing on the flow instabilities of low-specific speed pump turbines. In this part, results of the flow control application with fluid injection in the vaneless space for suppressing the flow instabilities of a model pump turbine in turbine mode operation at HSLU (Lucerne University of Applied Sciences) Switzerland are presented. Using the experimental data and CFD results, the onset and development of flow instabilities are explored in the first part of this paper. Based on these analyses, a flow control technology using injection of fluid in the vaneless space of the model pump turbine is implemented in order to suppress the flow instabilities and, thereby, to extend the operating range. Through use of an external energy source, air and water injection are applied with discrete nozzles that are circumferentially distributed in the vaneless space. The S-shaped pump turbine characteristics in turbine operating mode are modified so that the slope at speed no load conditions is no more positive meaning an improvement in the stability behavior. To the best of our knowledge, this is the first successful application of flow control with fluid injection in the vaneless space of pump turbines. Fluid injection is applied at two different guide vane openings. The analysis of the unsteady pressure data indicates that fluid injection in the vaneless space suppresses the flow instability such as rotating stall. Moreover, the water injection turns out to be more effective than the air injection for modifying the slope of the pump turbine characteristics.