Prediction of runner eccentricity and Alford force of a Kaplan turbine based on variational mode decomposition

The rotor blades of an axial-flow turbine are cantilever structures, and there is inevitably a gap between them and the casing. Due to factors such as rotor wear and unit vibration, the eccentricity of the impeller will change during the operation of the turbine, resulting in the impeller being affected by additional radial forces, which can even lead to rubbing or biting between the impeller and the casing. To monitor the eccentricity of the impeller and the additional radial forces in real time during the operation of the turbine, this study conducted numerical simulations of the internal flow of the turbine under different eccentricities of the impeller, and analyzed the characteristics of pressure pulsation and impeller radial force in the turbine using the variational mode decomposition method. The results showed that there was a good linear relationship between the eccentricity of the impeller and the amplitude of the frequency corresponding to the rotor in pressure pulsation at the monitoring point and the Alford force acting on the impeller. Based on this finding, we established mathematical formulas for the relationship between the pressure pulsation at the monitoring point and the eccentricity of the impeller, as well as the eccentricity of the impeller and the Alford force acting on it. According to these formulas, we only need to monitor the pressure pulsation during the operation of the turbine to realize the real-time monitoring of the eccentricity of the impeller and the Alford force, which is of great significance for ensuring the safe and stable operation of the turbine. This article conducts in-depth research on the eccentricity problem of axial-flow runners. First, we obtained pressure pulsation data of the impeller under different eccentricities through numerical simulation. Then, we used the variational mode decomposition method to find the relationship between pressure pulsation characteristic values and eccentricity, as well as eccentricity and Alford force. Finally, a method was proposed to predict the eccentricity and Alford force of the axial flow impeller using pressure pulsation data. Through this method, real-time monitoring of impeller eccentricity and Alfred force can be achieved to ensure safe and stable operation of hydraulic turbines. image