Dynamic Characterization of Transient Cavitation in a Kaplan Turbine Model during Large Grid Frequency Disturbances

Tip leakage vortices are known to plague Kaplan turbines resulting in flow instabilities, noise, and cavitation erosion. This study investigates the tip leakage vortex cavitation (TLVC) experimentally on an emulated grid-coupled lab-scale Kaplan turbine during transient power stepping operations under two cavitation numbers, namely \((\sigma = 1.0)\) and \((\sigma = 0.6)\). The two turbine power-stepping modes consist of runner rotational speed and blade angle variations. This is to characterize the behavior of the TLVC during turbine transient operation and to uncover underlying fluid-structure interactions with the blade. It is found that TLVC oscillates in two primary modes, namely 'breathing' and 'whipping', where 'breathing' takes place during any runner blade angle variation, while 'whipping' is induced by rotational speed variations. TLVC’s low frequency ‘breathing’ , under atmospheric conditions, plays a major role in inducing torque peaks on the blade, hence increasing the dynamic load on the blades. Additionally, 'whipping' generally appears to facilitate excessive blade torque signal jittering, which reflects stochastic loading on the blade. A suggested attribution to this is touchpoint formations along the blade surface. Based on their fluctuation analysis, turbines operating at \((\sigma = 1.0)\) seem to induce the most complex, unstructured blade loading, regardless of the power stepping direction. However, downward power stepping led to a stochastic blade torquing under \((\sigma = 0.6)\) flow regime.