Modification of Pump Turbine Characteristics With Fluid Injection in Pump Operating Mode

Abstract

Abstract The requirements of a stable and reliable pump turbine operation under continuously expanding operating ranges, challenges the design of pump turbines and demand new developments. This paper presents the results of experimental (model pump turbine at the test rig) and numerical (CFD) investigations of the instabilities of a low specific speed (nq = 25) pump turbine in pump operating mode. During the operation at off-design conditions, not only performance penalties but also flow instabilities may occur, limiting the operating range of the pump turbines. Although unstable behavior can be observed during the part load operation in turbine mode (i.e., draft tube vortex rope) and during the turbine mode operation near runaway speed (speed no load conditions), the focus of this paper is on the pump operating mode, the so-called hump instability at part load operation, when the head vs. flow rate characteristics of the pump has a positive slope. The pump mode dimensionless head vs. flow rate characteristics are measured for different diffuser opening angles, DA. Using the data of the unsteady pressure measurements, flow instabilities in the low-specific speed model pump turbine are analyzed in pump operating mode. Flow control with fluid injection in the vaneless space (between the runner and the diffuser) on a model pump turbine during pump mode operation was applied for different diffuser angles at HSLU (Lucerne University of Applied Sciences, Switzerland) in order to modify the pump mode, head vs. flow rate characteristics and to eliminate the regions with positive slopes in the characteristics. Water-injection is first applied using an external energy source (pump) with discrete nozzles that are circumferentially distributed in the vaneless space. Later water injection is also implemented without an external energy source using a bypass, which is connecting the discharge of the pump with the vaneless space. With fluid injection, the pump head vs. flow rate characteristics are modified so that there are no more positive slope regions, meaning an improvement in the stability behavior and extension of the operating range of the pump mode operation. After the implementation of flow control in the laboratory, CFD simulations were carried out to calculate the pump head vs. flow rate characteristics with and without fluid injection. CFD simulations with the BSL EARSM (Baseline Explicit Algebraic Reynolds Stress) turbulence model are able to predict the experimentally determined pump head vs. flow rate characteristics with- and without water injection.