Mechanism of runner high-pressure side on stall characteristics at typical unsteady operating points in both modes of a pump turbine

Pressure fluctuations in the draft tube mainly arise from the rotational motion of the vortex rope. The formation and development of the vortex rope are significantly influenced by cavitation. However, the precise mechanism underlying the progression of cavitation from its initial weak state to a more severe level remains unclear. The objective of this study is to investigate cavitation in a pump-turbine operating under part load turbine conditions. Numerical simulations were conducted to analyze the impact of the cavitation coefficient on various parameters, including vapor volume, efficiency, and head. With increasing degree of cavitation, the vortex rope's pattern in the draft tube undergoes a transition from a helical type to a torch-like type. This transition consequently leads to a shift in the dominant frequency of pressure fluctuations within the draft tube. Furthermore, the helical vortex rope exhibits a higher rotational speed compared to the torch-like vortex rope, thereby causing a more pronounced effect on the pressure field. The factors contributing to the morphological transition of the vortex rope were explored, with particular emphasis on the accelerating reverse axial flow and the reduction in the draft tube's circumferential velocity. Additionally, this study examined the effect of the cavitation coefficient on the swirl number and flow pattern, while also assessing its impact on entropy production. These findings provide valuable insight into the control of cavitation flow in pump-turbines operating under part load conditions. Moreover, they carry significant implications for the design and optimization of pump-turbine systems.

Study on cavitating vortex rope characteristics of reversible pump-turbine under part load turbine condition

Pang,

Zhu,

Shen

et al. 2023

A comprehensive assessment on detached eddy simulation and delayed detached eddy simulation approaches in large-scale rotating separated flow in a centrifugal impeller under small flow rate

Pan,

Zhang

2023

This work performed a comprehensive comparative assessment on the capabilities and performances of detached-eddy simulation (DES) and delayed detached-eddy simulation (DDES) approaches in revealing the flow physics in a six-blade centrifugal impeller under a small flow rate. The numerical results are well validated by the experimental data of a similar configuration and are compared with the benchmark data generated by an additional large-eddy simulation investigation. The results are presented and analyzed in terms of the time-averaged flow field, the transient flow, and momentum transport characteristics. The capabilities of DES and DDES approaches in predicting the rotating turbulent flow are compared and assessed. The numerical results show that DES performs better in the pressure distribution, while DDES is superior in resolving the transient evolution of velocity field, especially the fluctuation quantities and turbulent statistical quantities. It is also emphasized that both approaches perform distinctly in quantifying the patterns of internal flow in different regions within the centrifugal impeller.

Rotor–stator interaction investigations in variable speed reversible pump-turbine at higher head

Rode,

Kumar

2024

Efficiency and grid stability can be improved by variable speed operation using doubly fed induction machine technology for pumped storage plants experiencing significant head variations. With the higher penetration of intermittent and variable renewable energy sources, viz., solar and wind, the grid may be stabilized by operating the reversible pump-turbines (RPTs) in off-design conditions. In a turbine mode, the RPT is more susceptible to fatigue and vibrations when operating at a higher head as a result of hydraulic instability generated by rotor–stator interaction (RSI); therefore, its performance becomes even more critical. The powerhouse structural components, including floors and columns, could experience intense vibrations because of this instability. Therefore, it is essential to investigate the RSI in the variable speed RPTs at the higher head. These investigations present the results of a numerical analysis of RSI and its associated pressure fluctuations in the variable speed RPTs at the higher head. The high-head scaled model of variable speed RPT was used, and the numerical simulations were executed by utilizing the shear stress transport k-ω turbulence model. The numerical analysis was performed at the best efficiency point and high-head operating conditions having optimized rotational speed. The results show that the main source of pressure fluctuations in the variable speed RPT at all operating conditions is RSI, where the dominant frequencies are blade passing frequency (9fn) and its harmonics. It is also found that the variable speed operation lowers the pressure fluctuations in the RPT.