EFFICIENCY SCALE EFFECTS IN PELTON TURBINES.

Comparisons of measured efficiency on model turbines and homologous prototypes may show considerable differences, which are explained as scale effects. In contrast to reaction turbines, where scaling is based on the application of resistance coefficients for pipes and plates, efficiency scaling for Pelton machines in considerably more difficult. Here, the flow pattern consists of pipe flow, free-jet flow, unsteady flow with free surface in the buckets and two-phase flow in the casing. From dimensional analysis it has become clear that, for the scaling of Pelton turbines, the specific flow rate and Reynolds, Froude and Weber numbers have to be considered. Based on comparisons of models and prototypes, a set-up procedure is presented which is in good agreement with full-scale test results.