Impact of runner crown shape modifications on the onset of the precessing vortex core

Abstract

In this experimental study, a passive flow control concept is provided to extend the stable part load operation regime by shifting the rise of the precessing vortex core (PVC), also known as vortex rope, toward lower mass flows respectively deeper part loads. A parametrized runner crown design working as a passive flow control device is derived. This control device aims for shifting the bifurcation point of the PVC to lower flow rates. To determine the most influential design parameters and derive an optimized runner crown design, a design of experiments (DoE) approach is used. This DoE approach is based on data obtained from differential pressure sensors inside the draft tube wall of a generic hydro turbine test rig using air as working fluid. By means of stochastic modeling, the growth rate of the PVC mode is derived from the statistics of the measured pressure signals. The growth rate is used to estimate the bifurcation point of the PVC characterized by a certain normalized flow rate. It is shown that the stable part load operation regime is extended by up to 25% due to the passive impact of the modified runner crown. Moreover, the operational range featuring considerable PVC-induced pressure oscillations is diminished and the pressure recovery of the draft tube is improved compared to the baseline case.