Study on runaway performance of pump-turbine based on Finite-Time Lyapunov Exponent and Proper Orthogonal Decomposition method

When the water diversion mechanism fails to operate normally during the frequent mode transitions in a pumped storage power station, the unit may enter into a runaway condition, posing a significant threat to the normal operation of the power station. The coherent flow structures inside the unit under runaway conditions are complex, requiring a new perspective for extraction and analysis. Addressing this practical engineering issue, this study established a numerical model for a high-head pump-turbine, validated it through numerical simulations and experimental results, and conducted a visualization analysis and interpretation of the internal flow patterns under runaway conditions with different guide vane openings. The results indicate that, under the large-opening runaway condition, the moving guide vanes and impeller experience greater radial force fluctuations. Specifically, the axial force on the impeller under the large-opening condition is approximately twice as high as that under the small-opening condition, while the radial force is more than six times higher. Additionally, using the Finite-Time Lyapunov Exponent (FTLE) and Proper Orthogonal Decomposition (POD) methods, the coherent structures of the complex flow field in the runner area of the pump-turbine were extracted and analyzed. The findings suggest that both methods effectively capture the main coherent structural features of the flow field in the runner area, indicating a potential correlation between the velocity features of the complex flow field in the runner area and the distance between the guide vanes and the stay vanes of the spiral case. Furthermore, the LCS method provides a clear representation of the flow patterns of vortex structures in the complex flow field. This study reveals the variations in the internal flow patterns of the pump-turbine under runaway conditions, offering new perspectives and tools for analyzing the turbulent structures in complex flow fields in pump-turbines.