No abstract

With the need for flexible operation Francis runners are exposed to various operating conditions outside the traditional operating range. These machines have to be designed for long time Part Load Operation to meet hydraulic and structural requirements. Deep Part Load Operation mainly consists of stochastic loads on the blades. Calculation methods for the dynamic stress at Full Load Operation are well established, whereas the calculation of the dynamics during low load operation are in the focus of the current research to increase the predication accuracy. Through a model test of a Francis runner with multiple sensors in the rotating frame, knowledge is gained about low load operating conditions. The sensors consist of both pressure transducers and strain gauges. On the basis of the results for multiple operating conditions a comparison between pressure sensors and strain gauges is made and connected to flow phenomena. Prototype strain gauge data is utilized to prove the feasibility of the data gained in model size.

For a certain operating point of a horizontal shaft bulb turbine (i.e. volume flow, net head, blade angle, guide vane angle) the efficiency for different pressure levels (i.e. different Thoma-coefficient σ) is calculated using a commercial Computational Fluid Dynamics (CFD-)-code including two-phase flow and a cavitation model. The results are compared with experimental results achieved at a closed loop test rig for model turbines.The comparison of the experimentally and numerically obtained efficiency and the visual impression of the cavitation show a good agreement. Especially the drop in efficiency is calculated with satisfying accuracy. This drop in efficiency in combination with the visual impression is of high practical importance since it contributes to determine the admissible cavitation in a bulb-turbine. It is seen that the incipient cavitation in Kaplan type turbines has no major importance in determing this admissible amount of cavitation.