Unraveling spatial multi-scale energy loss mechanisms in a large Francis turbine: a mean kinetic energy perspective

Clearance flow in a Francis turbine significantly affects energy characteristics and losses, involving complex flow patterns and energy exchange. This study uses numerical simulations to examine the effect of micro-scale clearance flow on energy losses in a large Francis turbine, utilizing the mean kinetic energy method. Results show that the mean kinetic energy method is better at identifying turbulent dissipation, while the entropy production method tends to overestimate energy losses due to wake vortices. The analysis reveals that accounting for clearance flow increases total hydraulic losses, with the draft tube being most affected. Under a low flow rate condition, increased swirl intensity in the draft tube enhances the spiral vortex rope, leading to higher energy losses. At high flow rates, the increased swirl intensity reduces flow separation caused by reverse pressure gradients, resulting in lower energy losses. Among all clearance regions, the band clearance contributes the most to energy loss. Localized rigid vortices caused by reverse pressure gradients and intense shear motion induced by high velocity gradients near stationary walls are the primary factors contributing to energy loss in the clearance region, with shear-induced energy losses being more significant. As the flow rate increases from 0.68Qr to 1.00Qr, shear vorticity near the crown clearance cavity inlet increases, raising energy losses locally, while inside the cavity, both rigid and shear vorticity decrease, resulting in an overall reduction in energy loss. This study provides a foundation for accurately predicting energy characteristics and optimizing clearance design in Francis turbines.