Analysis of flow characteristics and cavitation in the vanes of a reversible pump-turbine in pump mode

Abstract

In reversible pump-turbines, cavitation erosion can be found at the leading edge of stay vanes (SVs), which is generally considered related to the undesirable flow regime in the double radial vanes in pump mode. In this study, the flow characteristics in the vanes were studied at small guide vane (GV) openings in pump mode to evaluate the easily-ignored stay vane (SV) cavitation. Numerical simulations for three-dimensional (3D) flow passages of pump-turbine revealed that a large pressure drop was induced at SV leading edge in the situation of large velocity magnitude in the GV-SV vaneless space with a backflow flow pattern in the SV channel. Since a quasi-two-dimensional characteristic was confirmed for the flow regime in the vanes, refined simulations for GV-SV region with a two-dimensional (2D) approach were conducted to analyze the evolution of flow regime and the influence of flow rate coefficient. A low-frequency pressure fluctuation was found at SV leading edge, in which the minimum pressure appeared when the flow separation occurred near SV inlet in the situation with backflow in SV channel. This special flow pattern transferred to the adjacent SV channel in the direction of runner rotation. Moreover, the increase of flow rate coefficient resulted in a larger pressure drop at SV leading edge and accelerated the evolution of the flow regime in the vanes. This implies that a high cavitation risk at SV leading edge appears when operating at small GV opening angles with large flow rate coefficients. The outcomes of this study may help researchers to focus on the SV cavitation and improve the start-up and shutdown processes in pump mode.