Cavitation in a high specific speed Kaplan pit-type turbine – Two-phase CFD-simulations and experimental verification

Due to the growing exploitation of the worldwide small-scale hydro power potential high specific speed hydraulic turbines – typically used for combinations of rather high flow rate and low head - are gaining increasing importance. An unfavourable characteristic of such turbines is their high susceptibility to cavitation.
The subject of the investigations presented in this paper is a horizontal axis Kaplan pit-type turbine with three runner blades and a specific speed of nq = 263.5 rpm. Using a commercial software package steady-state and unsteady simulation methods were tested in a preliminary study in order to define the most promising approach for a reliable efficiency prediction. In the next step a statistical evaluation of the pressure distribution on the runner blades was performed. By using the so-called “Histogram-Method” the Thoma number  was calculated for various operation points of the turbine. The results gave a first impression of the expected cavitation performance. Additionally two-phase simulations were performed to determine the extent of cavitation and its impact on the turbine performance. Furthermore measurement results obtained from an experimental investigation on a model test rig served as basis for the evaluation of the results achieved by means of Computational Fluid Dynamics (CFD). In course of the measurement campaign the efficiency alteration due to the onset of cavitation was measured and recorded and pictures and videos were taken to capture the cavitation conditions.
It turned out that the location of the cavitation zones in the CFD simulation agrees well with the observations made in course of the model test. Even the influence of cavitation on the efficiency can be calculated with satisfying accuracy. The only downside is that the numerical simulation predicts the incipient cavitation at much lower pressure values than experimentally observed.