Transient simulation on internal flow characteristics and pressure pulsation of variable speed Francis turbines during acceleration process

Variable-speed Francis turbines offer the advantages of a broader operational range and enhanced stability. To investigate the internal flow dynamics and pressure pulsation characteristics during the variable-speed operation of the Francis-99 turbine, this paper employs numerical simulations to examine the internal pressure, flow behavior, and energy loss variations over time during the acceleration process. The Hilbert–Huang Transform method is utilized to generate the corresponding time-frequency diagram of the pressure pulsation signal, thereby enhancing the accuracy and reliability of feature extraction. The pressure pulsation signal was decomposed into an intrinsic mode function (IMF) using variational modal decomposition, and fast Fourier transform was applied to the IMF to identify the components contributing most significantly to the pressure fluctuations. The final results demonstrate that the accelerated operation of the Francis-99 turbine positively affects the reduction of hydraulic losses and enhances operational stability. Regarding hydraulic stability, the rotor−stator interference effects must be considered. Following a change in rotor speed, the flow within the runner becomes more complex. However, total entropy production within the Francis-99 turbine decreased by 9.1% following accelerated operation. During acceleration, the dominant frequencies of pressure pulsations at the guide vane outlet and impeller inlet remained at 30 fn and 28 fn, respectively, as speed increased. Furthermore, the peak pressure pulsations at these dominant frequencies were reduced by 83% and 76%, respectively, compared to those observed during fixed-speed operation. This study offers a valuable reference for assessing the instability characteristics of the turbine during variable-speed operation and enhancing operational efficiency.