The pump–turbine unit used in pumped-storage power generation plays a role in backing up and adjusting the power system in a flexible power generation facility, providing stability in response to ...renewable energy power grids with intermittent power generation. Owing to changes in the operating conditions of flexible power generation facilities, pump–turbine units have been operated continuously in off-design conditions. At the high flow rates in off-design conditions, undesirable flow with vortex rope is caused in the draft tube, which leads to operating system instability. Meanwhile, when operating at high flow rates in the turbine mode of the pump–turbine unit, the Thoma number must be considered for relatively stable operations due to cavitation phenomena. Thus, to expand stable operating ranges through the understanding of high flow rates, unsteady internal flow and pressure phenomena were investigated depending on the Thoma number in the turbine mode of the pump–turbine model through analyses with laboratory-scale pump–turbine model experiments and unsteady-state numerical analyses via systematic quantitative and qualitative comparisons. There was no difference in flow characteristics at the runner and draft tube areas based on the Thoma number at the best efficiency point. Nevertheless, torch-shaped visible vortex ropes developed and differences in flow characteristics at the draft tube appeared based on Thoma number under high flow rates.
In the low-flow rates of off-design conditions in the pump–turbines, complicated flows can develop with vortices in the flow passages, which significantly increase the vibrations and noise, resulting ...in destabilizing the pump–turbine system. Under low-flow-rate conditions, an appropriate Thoma number is the most essential requirement for stable operation in turbine mode of the pump–turbine. Thus, this study elucidated to examine the internal flow and unsteady pressure phenomena in a laboratory-scale pump–turbine model operated in turbine mode with different Thoma numbers. The experimental investigations were conducted under 70% of the flow rate at the best efficiency point with different Thoma number to observe the unsteady pressures and vortex rope behavior. Then, the numerical validations were conducted by three-dimensional unsteady-state RANS analyses in a SAS–SST model. The Thoma number did not significantly affect the performance of the pump–turbine model or the unsteady flow phenomena in the vaneless and runner regions. However, the Thoma number affected the draft tube region at low flow rate condition. At low Thoma numbers, the irregular flow phenomena along the flow direction enhanced the unsteady pressure characteristics.
•Internal flow phenomena by Thoma number of a pump-turbine model were presented.•Experimental and numerical investigations were conducted by different Thoma number.•The Thoma number effects were investigated under low-flow-rate condition.•The performance was independent of the Thoma number under low-flow-rate condition.•The Thoma number affected the draft tube region with a visible vortex rope.
En el presente trabajo, se reporta la caracterización experimental de la formación y desarrollo de la nube de cavitación en diferentes temperaturas de líquido 20, 30, 40 y 50 °C. Para ello, se ...construyó una instalación hidráulica que tiene como elemento principal un tubo Venturi de sección rectangular, con ella, se generaron las condiciones de flujo necesarias para formar la cavitación y medir las propiedades termodinámicas para el cálculo de los números adimensionales de Thoma y Reynolds. El error promedio de sesgo de las mediciones no superó el 1%, por tal razón, se aseguró la buena calidad del cálculo en los números adimensionales de Thoma y Reynolds. Con los números de Thoma “σ” y Re se caracterizaron las diferentes fases de la nube de cavitación, desde incipiente, cuasi, desarrollada y super cavitación en el rango de temperaturas del experimento, encontrando que el tránsito de la cavitación incipiente a desarrollada es más fácil a temperatura ambiente, ya que, el régimen de flujo aumentará solo 15.82% en comparación con las otras temperaturas. El inicio de la nube de cavitación depende de la viscosidad del fluido, en el intervalo de prueba la variación de la viscosidad fue de 55% por lo que el régimen de flujo será afectado por la viscosidad y por consiguiente en el inicio de la nube de gas. Finalmente, la densidad del fluido en el rango de temperatura de la prueba permaneció constante, ya que no vario más de 1%, por lo que no afectó al régimen de flujo.
In this paper, the influence of Thoma number and model testing head on pressure fluctuation of Francis turbines was studied through experimental method. Firstly, the influence of model testing head ...on pressure fluctuation in the draft tube was carried out by varying model testing head at 6 typical operating conditions including no load, deep part load, part load, optimum, rated and overload points. It is found that model testing head has little influence on amplitudes of the pressure fluctuation in the draft tube of Francis turbine within the test range, which represented the influence of similitude number such as Reynolds number, Froude number, Weber number and so on. Then, analysis of the influence of Thoma number on pressure fluctuation amplitudes in the draft tube as well as frequency was performed at the part load and rated load conditions. It shows that the Thoma number not only influences pressure fluctuation amplitude but also the distribution of the frequency components in the draft tube. Finally, comparison of pressure fluctuation with two different cavitation levels was carried out. It is reasonable that selection of guide vane centerline is as cavitation reference level in the pressure fluctuation tests for France turbines. Hence, when pressure fluctuation similarity is studied, apart from load condition, the influence of the difference of Thoma number and the selection of cavitation reference level should be considered.
Water hammer phenomena are important issues for the design and the operation of hydro power plants. Especially, if several reversible pump-turbines are coupled hydraulically there may be strong unit ...interactions. The precise prediction of all relevant transients is challenging. Regarding a recent pump-storage project, dynamic measurements motivate an improved turbine modeling approach making use of a Thoma number dependency. The proposed method is validated for several transient scenarios and turns out to improve correlation between measurement and simulation results significantly. Starting from simple scenarios, this allows better prediction of more complex transients. By applying a fully automated simulation procedure broad operating ranges of the highly nonlinear system can be covered providing a consistent insight into the plant dynamics. This finally allows the optimization of the closing strategy and hence the overall power plant performance.
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