Progress in load prediction for speed-no-load operation in Francis turbines

Francis turbines are increasingly required to operate from 0-100% of power output. For the design of these turbines, a sound understanding of formerly "off-design" operating points, e.g. speed-no-load, is necessary. One way to assess loads at these operating points is to apply scale resolving CFD methods in order to account for the broad-band turbulence spectrum. This results in stochastic load patterns, which are characteristic for these off-design points. In this paper, two CFD approaches for scale resolving simulations are applied to a Francis model turbine to resolve the larger anisotropic scales in turbine flow at speed-no-load operation. The first uses a hybrid RANS-LES model presented by Menter & Egorov [2]. Here the resolved scales are adapted continuously by RANS-LES blending based on the actual flow condition. The second approach is a LES with a dynamic model based on Germano et al. [3]. For both approaches, spectra of resulting pressure fluctuations at the draft tube cone wall are presented. Additionally, pressure loads from LES are applied to calculate mean and dynamic stresses. The dynamic stresses are finally compared with measurements in the corresponding prototype turbine.