CFD Investigation for Surface Roughness Effects on the Hydrodynamics of Cavitating Turbulent Flow Through a Low Head Prototype Francis Turbine

Surface characteristics have an important role in defining hydrodynamics of the flow through hydraulic machines. Surface roughness is a critical parameter that contributes to altering near-wall flow features and promotes frictional losses in various components of a water turbine. The nature of flow through the Francis turbine runner is highly complex, especially at cavitating regimes, and the surface roughness effects add to the flow complexity. The present work is aimed at evaluating surface roughness effects on the cavitation performance of a low head prototype Francis turbine computationally. Complete cavitation characteristic of the turbine is derived with the consideration of smooth and rough boundaries by implementing SST k-ω turbulence model and cavitation model based on the Rayleigh-Plesset equation, and a comparative study is carried out comprehensively. For the analysis, the complete flow domain of the turbine is considered, and the simulations are conducted for four different operating conditions from the part load of 60% to overload of 120%. Different values of equivalent sand grain roughness, ks, are assigned to different components of the turbine by following the International Electro-Technical Commission standard IEC 62097 Edition-2. It is concluded that the surface roughness effects on the performance of the turbine in the absence of cavitation are not significant for operation at BEP but for the part load of 60% and overload operations, it has considerable hydrodynamic effects. However, these effects become more detrimental at developed cavitation regimes. The obtained computational results are found in a fair agreement with the available experimental results and are quite consistent with the previous research.