Velocity profiles in a water jet of a Pelton nozzle: CFD simulations on both 2D and 3D geometries

Hydropower is an important energy sector deployed all over the World with a significant share in the global electricity production. Due to the ongoing climate changes, the lack of water in summer periods compromises the performance of hydraulic machines, whose efficiency quickly decreases when they operate at strong part-load conditions. Among the most known hydraulic turbines, Pelton is suitable to be used with high geodetic heights and relatively low flow rates. When dealing with flow rates 30% lower than the Best Efficiency Point (BEP), the efficiency drastically decreases mainly due to the non-uniform velocity profiles of the water jets coming out from the nozzles. The quality of the jet is affected by the distributor geometry, bending pipes and spear valves with the respective supports located upstream. The aim of this work is to investigate different velocity profiles of the water jet coming out from a Pelton nozzle geometry by means of Computational Fluid Dynamics (CFD) simulations performed by ANSYSR Fluent solver. Four different spear valve angles were simulated on a 2D-axisymmetric geometry, starting from a reference case of β = 50◦ with increasing steps of 5◦, maintaining a fixed positioning of the spear valve that corresponds to a partial load operation of the machine. In addition, also a 3D simulation has been carried out and the streamlines along a symmetry plane are shown. The results of the 3D simulations were compared to the 2D-axisymmetric ones taking into account the aforementioned reference case, showing how this last simplification of the computational domain leads to almost the same macroscopic results.