Investigation on the Impact of Air Admission in a Prototype Francis Turbine at Low-Load Operation

Abstract

Due to significant changes in the energy system, hydraulic turbines are required to operate over a wide power range. In particular, older turbines which are not designed for these environments will suffer under off-design conditions. In order to evaluate whether or not such a turbine could fulfill the new requirements of the energy market, a study about the behavior of a prototype plant in low-load operation is presented. Therefore, prototype site measurements are performed to determine the most damaging operating point by means of acceleration sensors and pressure transducers. Moreover, unsteady computational fluid dynamics (CFD) simulations considering two-phase flow and two hybrid turbulence models are used to analyze the flow conditions inside the turbine. The resulting pressure pulsations are mapped onto the runner blade to obtain stress and further calculate damage factors. Accordingly, the stresses are compared to those obtained by the strain gauge measurement. Moreover, the influence of active flow control by means of air injection on plant behavior and runner lifetime is discussed as well.