Surface Roughness Impact on Francis Turbine Performances and Prediction of Efficiency Step Up

Abstract

In the process of turbine modernizations, the investigation of theinfluences of water passage roughness on radial flow machine performanceis crucial and validates the efficiency step up between reduced scalemodel and prototype. This study presents the specific losses percomponent of a Francis turbine, which are estimated by CFD simulation.Simulations are performed for different water passage surface roughnessheights, which represents the equivalent sand grain roughness height.As a result, the boundary layer logarithmic velocity profile stillexists for rough walls, but moves closer to the wall. Consequently,the wall friction depends not only on roughness height but also onits shape and distribution. The specific losses are determined byCFD numerical simulations for every component of the prototype, takinginto account its own specific sand grain roughness height. The modelefficiency step up between reduced scale model and prototype valueis finally computed by the assessment of specific losses on prototypeand by evaluating specific losses for a reduced scale model withsmooth walls. Furthermore, surveys of rough walls of each componentwere performed during the geometry recovery on the prototype andcomparisons are made with experimental data from the EPFL Laboratoryfor Hydraulic Machines reduced scale model measurements. This studyunderlines that if rough walls are considered, the CFD approach estimateswell the local friction loss coefficient. It is clear that by consideringsand grain roughness heights in CFD simulations, its forms a significantpart of the global performance estimation. The availability of theefficiency field measurements provides a unique opportunity to assessthe CFD method in view of a systematic approach for turbine modernizationstep up evaluation. Moreover, this paper states that CFD is a verypromising tool for future evaluation of turbine performance transpositionfrom the model scale to the prototype.