Abstract
The unsteady flow field in a reversible pump-turbine is investigated during the continuous load rejection using a 3D computational fluid dynamic analysis. Numerical calculations are carried out using the detached eddy simulation (DES) turbulence model and a new approach involving automatic mesh motion. In this way, the instability of the flow field is analyzed by continuously changing the guide vane openings from the best efficiency point (BEP). Unsteady flow characteristics are described by post-processing signals for several monitoring points including mass flow, torque, head and pressure in the frequency and time-frequency domains. The formation of vortices of different scales is observed from the origin to further enlargement and stabilization; the effect of the rotating structures on the flow passage is analyzed, and the influence of unsteady flow development on the performance of the turbine is investigated. Finally, the evolution during the period of load rejection is characterized in order to determine the hydrodynamic conditions causing the vibrations in the machine.
Highlights
In the last few decades, significant and unpredictable developments in the research of new energy sources have made electrical energy storage become a key issue
The unsteady phenomena in the reversible pump-turbine have been analyzed through the use of automatic mesh motion where the guide vanes’ movements are realized through the adoption of dynamic mesh based on a given closure law
The results demonstrate the good accuracy of the model in the description of the internal flow field, and the unsteady flow evolution in each domain can be continuously observed for different openings
Summary
In the last few decades, significant and unpredictable developments in the research of new energy sources have made electrical energy storage become a key issue. The change in load, together with frequent starts and stops, represents one of the most important characteristics of a pump-turbine: it allows one to counter the increased need for leveling the peak demand of electrical energy and to better balance the system For these reasons, pump-turbines’ working conditions are suitable to be investigated using unsteady analyses [4,5]. 3D numerical simulations are used to investigate the off-design conditions of the reversible turbine, and an automatic moving mesh technique has been implemented in order to continuously assess the unsteady flow characteristics during guide vanes’ closure; the procedure guarantees a high mesh quality during the whole process of load rejection. Achieving stability of the unit under no-load condition is difficult using the existing closing laws; the definition of an improved method is extremely important
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