A Numerical Study of Francis Turbine Operation at No-Load Condition

This paper presents a numerical methodology to study Francis turbines at no-load condition, an important operating condition regarding static and dynamic stresses. The proposed methodology uses unsteady Reynolds-averaged Navier-Stokes (RANS) simulations that have been integrated with a user subroutine to compute and return the value of runner speed, time step, and friction torque. The modeling tool is the commercial software ANSYS-CFX 14. The research compares the simulations that were performed using transient rotor-stator (TRS) and stage interface models and validate the results through experiments over the full range of admissible guide vane angles (GVAs). Both TRS and stage interface models yielded similar trends for all turbine runner parameters during the no-load process. Results show sizable differences in the average and maximum pressure on the blades between TRS and stage simulations. Analysis of the flow behavior in TRS simulation demonstrates complex flow phenomena involving a vortex breakdown within the draft tube, and strong vortices blocking the runner inlet, which dissipate the input energy into the turbine and yield a near zero-torque at no-load condition.