Time-dependent inception of vortex rings in a Francis turbine during load variation: large eddy simulation and experimental validation

The present study investigates a transient condition, i.e. load variation from the best efficiency point to the part load, of a Francis turbine. A complete turbine is considered for the study. The dynamic mesh approach is implemented to close the guide vanes and to perform load variation numerically. The primary goal is to investigate the inception, propagation and interaction of vortex rings as load on the turbine changes. The secondary goal is to determine characteristic frequencies. The continuous change in flow angle, due to the closing of the guide vanes, induces strong boundary layer separation in the runner. Flow decelerates along the blade and establishes an adverse pressure gradient due to steep curvature of the blades. Separated flow from the boundary layer creates two swirling regions attached to the blade suction side. Fully developed vortex rings exiting from the adjacent blade channels interact and induce high-amplitude stochastic fluctuations at the runner outlet.