Analysis and Quantification of the Jet Flow Distortion Inside and Outside of the Pelton Turbine Nozzle

Abstract Experimental investigations and theoretical simulations have highlighted how the jet discharged at the outlet of a Pelton turbine nozzle is influenced by the presence of secondary flows generated by bends (or bifurcations) and guide struts present inside and outside the turbine nozzle itself. This causes two types of problems: on one hand, the presence of these secondary flows negatively affects the overall kinetic energy available to the jet and transmitted to the bucket on the Pelton wheel, in the sense that these secondary flows (especially circumferential flows) represent lost kinetic energy that reduces the total kinetic energy available to the buckets. On the other hand, these circumferential velocities also affect the shape of the jet, causing a displacement of the jet axis with respect to the axis of the nozzle. Quantifying these secondary flows and the jet displacement are important indications for the design of turbine buckets, so that they can exploit the maximum input energy. This work, starting from the available literature, aims to analyze the effect of the curvature of the flow supply pipe on the shape and intensity of the jet. However, the methodology is of a general nature and can be applied whenever there is a variation in the layout upstream of the buckets and concerning both the inside of the nozzle (i.e. type of guide struts and their arrangement) and the upstream duct. Specifically, using Cadence’s multiphase Fidelity Fine Marine code, the flow behavior inside and outside a nozzle used by Troyer S.p.A. in a hydroelectric plant with a Pelton turbine and subjected to a given geodetic head was simulated. An index expressing the amount of circumferential kinetic energy compared to axial energy was calculated for different cross-sections of the jet (inside and outside the nozzle) and for different load conditions (position of the doble needle). Moreover, two other indexes, representing jet section deformation and axis displacement were introduced and evaluated. It may be said that this work aims to provide a contribution to support manufacturer with quantitative information related to the jet distortion deriving from the choice of certain layout (upstream curvature of the pipe, bifurcations, type of guide struts inside the nozzle) during the design phase.