Unstable flow analysis of transient process in the pump as turbine during turbine mode caused by pump power failure

Mixed-flow pump as turbines (PATs) serve as pivotal components within energy micro-grids, facilitating energy conversion and storage. However, the emergence of pressure pulsations in these systems can markedly affect their stability and efficiency, especially in pump power failure. To simulate the power-off transition accurately, the commercial computational fluid dynamics code ANSYS CFX (R) is integrated by a Fortran program through ANSYS parametric design language in the transient simulation to enable real-time iterative calculations of angular momentum equations for mixed-flow PAT at varying speeds. This study integrates the analysis of radial forces, vortices, and flow lines to elucidate the sudden changes in pressure pulsations observed during the transition stages. Specifically, significant fluctuations in the amplitude of pressure pulsations at the volute tongue were found for various initial flow rates, which correlated closely with changes in radial forces. The sudden increase and nonuniform distribution of radial forces emerged as the main factors of these fluctuations. In addition, the study reveals that the intensity of pressure fluctuations evidenced by wavelet time-frequency analysis on the pressure surface of the blade significantly exceeds that on the back surface of the blade. Furthermore, in the flow characteristics inside the draft tube, the pressure pulsation signals are mainly concentrated in the low-frequency region and are accompanied by the presence of a double-helix structure. These results provide an important reference for further understanding of the operating mechanism of the mixed-flow pump as a turbine, which helps to optimize the design and improve the performance.