Effects of Vortex Structure on Hydraulic Loss in a Low Head Francis Turbine under Overall Operating Conditions Base on Entropy Production Method

Multi-energy complementary power generation mode has become the theme of modern power grid construction. In response to the requirements of the national multi-energy strategy, the operation mode of hydropower units will inevitably change frequently, resulting in unpredictable hydraulic unstable flow phenomena (such as the blade passage vortex, the vortex rope in the draft tube, etc.) which will reduce the hydraulic performance of units and may lead to strong vibration of units, and even threaten the safe operation of units and even power stations in severe cases. In order to study the effect of the blade passage vortex and the vortex rope in the draft tube on the hydraulic losses under the part load conditions, the sliding mesh method and SST k-omega turbulence model are adopted. The blade passage vortex and the vortex rope in the draft tube captured by the latest Liutex vortex identification method undergo a contrastive analysis with the hydraulic loss of internal flow based on the entropy production theory. The results indicate that compared with the results in the references, the results of the research are verified accurate; under different upstream flow conditions, the proportion of the hydraulic losses of the spiral casing is small and has no obvious changes; the shock and stall phenomena, the dynamic-static interference, the relatively high flow velocity gradient, and the streamwise vortices in the guide vane domain are the main factors that cause the increase of entropy production; a large number of the blade passage vortices, the fluid-impact blades, and the tip leakage vortices are the main causing factors of the hydraulic losses in the runner; and the various shapes of the vortex rope in the draft tube can lead to the dramatic increase of the hydraulic losses of the draft tube as the opening decreases, and the hydraulic losses of the draft tube account for the largest proportion, followed by that of the runner. At the designed flow rate, the hydraulic loss of draft tube is 0.58 m, accounting for 50% of the total channel hydraulic loss, and that of runner is 0.38 m, accounting for 32.6% of the total channel hydraulic loss. When the flow rate drops to 55% of the designed flow rate, the hy-draulic loss of draft tube is 1.95 m, accounting for 68% of the whole channel, and that of runner is 0.70 m, accounting for 24.4% of the whole channel.