Numerical Investigation of the “Tandem Cascade” Effects for the Flow Through Stay and Guide Vanes of a Francis Turbine

In the open literature and recent publications, many explorations were done on the flow in axial two-dimensional tandem cascades whereby examining each cascade parameter separately it has been shown that the performance strongly depends on the interference effects of the front and aft blade profiles. Governed by the fact that very limited research is done and published on the influence and the effects of separate parameters in a radial tandem cascade for water turbines, this paper presents an initial research in that area. In order to achieve high efficiency for water turbines, the working medium has to be pre-conditioned for entering the runner of the turbine. Despite the role of the spiral casing of the turbine, the stay and guide vanes also play a huge role in these requirements. The flow in the zone of the stay and guide vanes of an existing Francis turbine has been numerically analyzed, emphasizing the effects of the relative angular position between the stay and guide vanes forming a tandem cascade. By varying the relative angular position as a parameter, the total loss, acting forces and the flow distribution of the configurations have been examined. The specialized grid generation software GAMBIT and the solver FLUENT by ANSYS, employing the finite volume method, are used to solve the governing equations for turbulent flows, and conclusions and insights are being made on the basis of the obtained results.