A stable and reliable pump-turbine operation under continuously expanding operating range requirement often imposes challenges on the hydraulic design of the pump-turbines and requires new developments. During a previous study carried out at the HSLU (Lucerne University of Applied Sciences, Switzerland), the flow instabilities of a low specific speed (i.e., nq = 25) pump-turbine were analyzed while a CFD methodology was developed through taking different numerical approaches and applying several turbulence models. The goal was to predict the turbine-mode characteristics of the reversible pump-turbines in the S-shaped region (at speed no load conditions) accurately as well as analyzing the flow features especially at off-design conditions. This CFD model was validated against the experimental data at different guide vane openings in turbine operating mode.

Based on the analysis of the experimental data, flow visualization, and CFD results focusing especially on the flow features in the vaneless space and at the runner inlet, the onset and development of the flow instabilities were explored. Furthermore, a flow control technology that entailed injecting air and water in the vaneless space of a model pump-turbine was implemented for suppressing the flow instabilities and thus extending the operating range of the pump-turbine. Both air- and water-injection were applied by using an external energy source (compressor and pump) and discrete nozzles that are distributed in the vaneless space circumferentially. The S-shaped pump-turbine characteristics in turbine operating mode were modified so that the slope at speed no load conditions was no more positive indicating an improvement in the stability behavior. The analysis of the unsteady pressure data indicates the suppression of flow instability such as rotating stall with fluid injection in the vaneless space. The positive effect of fluid injection on the pump-turbine characteristics was also demonstrated in the CFD calculations. CFD was able to predict the pump-turbine dimensionless discharge, Kcm1, - speed, Ku1, characteristic curve with water injection correctly.

After the CFD tool is validated for the prediction of the pump-turbine characteristics with fluid injection, further CFD simulations were carried out in order to improve the effectiveness of flow control and if possible, using less amount of injected fluid in the vaneless space. The goal was to optimize the fluid injection so that the instabilities can be suppressed with the lowest possible water/energy consumption. Parameters such as number of injection nozzles, nozzle position, nozzle diameter, and injection direction are varied. Several configurations of water injection system i.e., changing the number, location, and distribution of injection nozzles circumferentially and radially, direction of flow injection with respect to the main flow in the vaneless space, symmetrical and asymmetrical circumferential distribution of the nozzles in the vaneless space were analyzed using the CFD simulations. In addition to the flow injection in the vaneless space from the hub wall, fluid injection through the guide vanes was also investigated. The results of the fluid injection modifications were compared with the results of the baseline flow injection case. Using a parameter study, optimal nozzle configurations were found, that resulted in stable pump-turbine behavior in turbine operating mode with fewer injection nozzles and lower injected flow rate in comparison to the baseline case.

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