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

Abstract

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 the Unsteady Flow Characteristics of a Pump Turbine in Pump Mode

“…The equations of SST are shown in Equations ( 5) and ( 6). This model provides better predictions of flow separation under reverse pressure gradients and is widely used in the calculation of pump turbines [26][27][28][29][30]. In this study, the SST turbulence model is selected for the CFD calculation.…”

Numerical Investigation of Inner Flow Characteristics of a Prototype Pump Turbine with a Single Pier in Draft Tube at Part Load Conditions

“…The study by Lin et al [15] showed that the draft tube vortex rope is also significantly affected by leakage flow and the blade vortex. Pang et al studied the influence of the cavitation number on the draft tube vortex rope [16]. Numerical simulation revealed that, under different cavitation numbers, the vortex rope presents a spiral shape or a torch-like shape, and the torch-like vortex rope under a lower cavitation number has a higher swirling intensity and a lower dominant frequency.…”

Analysis of Cavitation-Induced Unsteady Flow Conditions in Francis Turbines under High-Load Conditions

“…Yu et al [14] studied the energy characteristics of the pump turbine, analyzing the hydraulic losses of different internal structures quantitatively under various load conditions. Pang et al [15] summarized the variations in the energy characteristics of the vortex rope's evolution and examined the hydraulic characteristics of the pump turbine. Abu Shahzer et al [16] investigated the pump turbine operating at different cavitation numbers and proposed a method to effectively suppress the generation of the vortex rope.…”