Secondary Flow and Sediment Erosion in Francis Turbines
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Sediment erosion of the hydropower turbine components is one of the key challenges due to the constituent of hard particles in the rivers of Himalayas and Andes. In these regions, Quartz is found as a main constituent (more than 50%), along with feldspar and other hard minerals. These particles have hardness more than 5 Moh’s scale, which is capable to erode turbine components. This has not only caused maintenance problems of the turbines, but has also decreased the efficiency of the plant during operation. In the case of Francis turbines, erosion is mostly observed around stay vanes, guide vanes and runner blades. The quantity and pattern of erosion depends upon the operating conditions and type of flow phenomena in particular regions. The flow phenomena in Francis turbines are highly unsteady, especially around guide vanes and runner. The flow instability arises in the form of leakage through clearance gap, horseshoe vortex, rotor-stator interaction and turbulences supported by high velocity and acceleration. The erosion on the other hand, deteriorates the surface morphology, aggravating the flow. This study focuses on the leakage flow through the clearance gap of guide vanes of Francis turbines by using both numerical and experimental techniques. The clearance gap is identified as a simultaneous effect of secondary flow and erosion inside guide vanes of sediment affected power plants. A cascade rig containing a single guide vane (GV) was developed in a previous study, which gives a close estimation of the flow field around one GV compared to that in the real turbine. The walls covering the rig were designed such that a sufficient swirl component of the flow is developed at the inlet of the guide vane. In this study, the velocity field around the GV containing clearance gap of size 2 mm at one end is captured using Particle Image Velocimetry (PIV) technique. Pressure sensors are used to estimate the GV loading at designed GV opening angle. A numerical model of the same rig is made, and the results from the CFD are validated with the experiments. The leakage flow is investigated further using CFD, and it has been found that this flow leads to the formation of a vortex filament, which travels downstream striking the runner blade at inlet. Since the one GV cascade rig is unable to predict the velocity field at different GV opening angles, a three GV cascade rig has been considered in this work. The three GV cascade rig overcomes the limitations of the one GV rig by causing minimum influence from the neighboring walls. On comparing asymmetrical GV profiles with the reference GV profile, it is found from both one GV and three GV cascade rig that asymmetrical GV profiles are more suitable for turbines affected by erosion. This is due to reduced pressure difference between the two GV sides, which consequently reduces the extent of the leakage flow and vortices originating from it. A numerical analysis has also been performed in a complete turbine passage, including GV and runner to investigate the effect of the leakage flow on the performances of the turbine. The results are compared with experiment conducted in one GV cascade rig, developed for the same turbine. Simulations are performed for 3 GV profiles with each at 11 operating conditions. It is found that the symmetrical guide vane profile, which is the reference profile in the plant, is not suitable for best efficiency and part load conditions. Such a profile could wear the runner blade by both erosion and cavitation. It is also found that asymmetrical profiles could increase the performance of the turbine at all operating conditions. However, in the case of asymmetrical profiles, some negative leakage flow could appear at full load conditions, which have a tendency to hit neighboring GV causing erosion. This thesis gives an indication of the flow behavior through the clearance gap of GV for different GV profiles. The results from this thesis can be used to conduct rigorous optimization technique such that the most optimized profile suitable for all the operating conditions can be chosen.