Analysis of hydraulic stability of a Francis turbine under partial load conditions based on Liutex method and entropy production theory

Hydropower turbines, as crucial components for grid regulation, must operate under frequently changing partial load conditions to accommodate the integrated generation of wind, solar, and hydro power in new hybrid grids. Ensuring their safety and stability is of paramount importance. This study focuses on the HLA551-LJ-43 turbine, using the Liutex method and entropy production theory to comprehensively analyze the influence of vortex structures on cavitation, pressure pulsations, and hydraulic losses under five representative operating conditions, with validation against experimental efficiency. The conclusions are as follows: (1) Increasing the rotational speed enhances the helical nature of the draft tube vortex, whereas reducing the speed effectively eliminates this helicity, favoring the stability of the unit. (2) There is no direct correlation between the occurrence of cavitation and vortex structures; however, increasing the head can induce extensive cavitation phenomena. (3) Due to the propagation of pressure waves, the frequency of the draft tube vortex generated at the most downstream point appears in the spiral casing domain, significantly impacting the hydraulic stability of the entire unit. (4) A comparative analysis of vortex structures and entropy production reveals that, in the runner region, blade passage vortices and blade tip leakage vortices are the main sources of hydraulic losses. This pattern holds true under high head, low-speed, and high-speed conditions. This research provides robust numerical analysis for optimizing new hybrid grid generation and offers effective engineering guidance.