The rapid increase in the penetration of renewable energy has imposed more stringent requirements on the regulation capacity and response speed of Francis turbines in modern power grids. Vortex-induced energy loss significantly constrains the energy performance and hydraulic stability of giant Francis turbines. However, the formation mechanisms of vortex-induced hydraulic loss near the operating boundary remain insufficiently understood. Based on numerical simulations and parameter validation under 30 representative operating conditions, three 50% rated load conditions located near the operating boundary were strategically selected for detailed investigation. By integrating rigid vorticity analysis with entropy production theory, the vortex dynamics and hydraulic loss characteristics were systematically quantified and visualized. The results indicate that entropy production rates caused by turbulent dissipation and wall shear constitute the primary components of hydraulic loss, among which entropy production rate caused by turbulent dissipation (EPRT) is more sensitive to variations in external operating conditions and dominates both the magnitude and spatial distribution of energy dissipation. Distinct loss evolution patterns are observed in the runner and the draft tube. Recirculation and separation flows along the blade surfaces alter the normal blade loading distribution in the runner. In the draft tube, hydraulic loss is mainly governed by the energy dissipation associated with the interaction between the main flow region and the reverse flow region, while the intensity of hydraulic loss is not directly related to the specific vortex morphology. Overall, shear vorticity remains the key mechanism responsible for the increase in EPRT. This study provides theoretical insights and practical evidence for understanding the mechanisms of vortex-induced energy loss in giant Francis turbines and for quantitatively evaluating the distribution and evolution of hydraulic loss.
Zhang et al. (Thu,) studied this question.