Large Eddy Simulations (LES) were conducted to characterize the turbulent flow structures in a Francis turbine unit operating at an ultra-low flow rate, 40% of the rate of the 2.5 /s design rate. The runner rotates with a constant rotation speed of 1,000 rpm that corresponds to Reynolds number 2×107 based on the runner reference diameter. A second-order discretization scheme is used to solve the filtered LES governing equations with the WALE subgrid scheme.The runner domain consists of 15 blades with a diameter of D = 0.62m. It is connected to a diffuser-shaped draft tube length of L = 5D with a 5° angle of expansion. The effect of water injection on pressure field and corresponding fluctuations in the draft tube will be investigated. Tangential water injection will be introduced through the draft tube wall in the same and opposite direction of runner rotation. The injection angle is varied (, &nbsp;, &nbsp;and ). The amount of water injection is also varied between 4 and 8 % of the total flow rate at the optimal design point regimes. Flow images and pressure signals will be acquired to characterize the velocity and pressure field with and without water injection.A no-slip boundary condition is applied at the surfaces of the runner and the draft tube, a zero gauge pressure boundary condition is imposed at the outlet of the draft tube, and a mass flow inlet with fixed turbulence intensity is imposed at the runner. 50 million elements were used to discretize the domain with a corresponding y+ value of 1.4, deemed acceptable for LES simulations.Contours of vorticity, velocity, and iso-surfaces of Q-criterion will be presented to characterize the flow field in the draft tube. Probes located at the center and wall of the draft tube will also be used to assess the level of pressure fluctuations related to the vortical activities inside the draft tube. The frequency and standard deviation (similar to amplitude) of the pressure fluctuations will be analyzed to determine the effect of water injection, and power of the turbine will be presented. Iso-surface of Q criterion help determine the vortex rope structure for different injection conditions. High vortical activity can lead to low-frequency, high amplitude, pressure fluctuations inside/on the walls of the draft tube, harming system operation. There could also be high-frequency pressure fluctuations typically caused by the interactions of small- and large-scale eddies generated from the runner region. Examining the impact of water injection on the pressure fluctuation coupled with the turbine performance are amongst the goals of the present study.The purpose of this study is to assess the impact of tangential water injection to reduce the magnitude of pressure fluctuations on a Francis turbine operating at ultra-low loads. These pressure fluctuations have been proved to damage the system. By mitigating the fluctuations, hydropower becomes more adaptable to alternate load conditions, making hydroelectricity an even more attractive energy source.

Flow Characterization of an Industrial Size Francis Turbine Operating at Ultra-Low Load – The Effect of Water Injection

Paper Proceedings: https://asmedigitalcollection.asme.org/FEDSM/FEDSM2021/volume/85284

ASME 2021 Fluids Engineering Division Summer Meeting (FEDSM2021)

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