Large Eddy Simulations (LES) are conducted to characterize the turbulent flow structures and pressure fluctuations in the Francis turbine unit operating at an excess load condition. The effect of water injection on the velocity and pressure field in the draft tube will be investigated. Flow images and pressure signals will be acquired to characterize the velocity and pressure field with and without water injection from the runner cone. The turbine consists of a runner and a draft tube. 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 incoming flow rate, 3 /s, is 120% of the optimal design point rate, 2.5 /s. The water injection will be introduced through the turbine runner cone in the axial direction of the flow into the center of the draft tube. The amount of water injection will be varied: 4 and 8 % of the total flow rate of the operational regimes.  Investigating the influence of water injection on the pressure fluctuation and the turbine performance are among the goals of the present study.

Simulations will be performed using the commercial CFD software ANSYS Fluent to predict the spatial and temporal characteristics of the turbulent flow fields. Second-order discretization schemes will be used to solve the filtered LES governing equations with WALE subgrid approximation. The spatial resolution of the mesh inside the runner and the draft tube will be discussed in terms of yplus value and the first layer thickness. In order to monitor the pressure fluctuations inside the draft tube, several probes will be spatially distributed on the wall and inside the draft tube. The influence of water injection on the vorticity field inside the draft tube will be evaluated by comparing the contours of pressure, velocity, vorticity, and the iso-surfaces of -criterion with and without the water injection. The pressure signals at various locations of the draft tube will be presented. The pressure signals and the power characteristics of the turbine will be presented with and without the water injection to assess the influence of the injection on the flow-induced pressure fluctuations. This study aims to determine whether mitigation is necessary at excess load and at what injection rate should be considered.

Francis hydraulic turbines are commonly used in hydropower plants and operate at a variety of different load conditions. As the demand from the electrical grid fluctuates, turbomachinery might operate at excessive flow rates. At excess load conditions, flow instabilities can lead to high-pressure fluctuations in the unit. Flow-induced pressure fluctuations are a structural threat and have undesirable effects on the operational stability of the power generation unit. Air and water injection from the draft tube are among the well-known passive control tools to mitigate the pressure fluctuation in these systems. Air or water injections could suppress, or delay flow instabilities encountered in these systems, and thus aid in mitigating flow-induced pressure fluctuations.


Francis Turbine Operation at Excess Flow Rate Using Large Eddy Simulation: The Effect of Water Injection

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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