Secondary flows in Francis turbines are induced by the presence of a gap between guide vanes and top–bottom covers and rotating–stationary geometries. The secondary flow developed in the clearance gap of guide vanes induces a leakage vortex that travels toward the turbine downstream, affecting the runner. Likewise, secondary flows from the gap between rotor–stator components enter the upper and lower labyrinth regions. When Francis turbines are operated with sediment-laden water, sediment-containing flows affect these gaps, increasing the size of the gap and increasing the leakage flow. This work examines the secondary flows developing at these locations in a Francis turbine and the consequent sediment erosion effects. A reference Francis turbine at Bhilangana III Hydropower Plant (HPP), India, with a specific speed (Ns = 85.4) severely affected by a sediment erosion problem, was selected for this study. All the components of the turbine were modeled, and a reference numerical model was developed. This numerical model was validated with numerical uncertainty measurement and experimental results. Different locations in the turbine with complex secondary flows and the consequent sediment erosion effects were examined separately. The erosion effects at the guide vanes were due to the development of leakage flow inside the guide vane clearance gaps. At the runner inlet, erosion was mainly due to a leakage vortex from the clearance gap and leakage flow from rotor–stator gaps. Toward the upper and bottom labyrinth regions, erosion was mainly due to the formation of secondary vortical rolls. The simultaneous effects of secondary flows and sediment erosion at all these locations were found to affect the overall performance of the turbine.

It is difficult to define a general relationship for discharge characteristics of a Pelton injector due to high degree of variability in geometry of injector amongst different Pelton turbine designs. This study has been aimed to determine discharge characteristics of injector of Pelton rig at Waterpower Laboratory, NTNU. Three different approaches have been undertaken, viz. calculation based on empirical relations, numerical simulations, and experimental validation. For empirical calculation, the injector has been simplified into series of flow geometries with equivalent cross-sectional area and axial length for multiple stroke to constant aperture diameter ratio ( s/D 0 ). Discharge coefficient has been calculated for a constant pressure difference between inlet and atmospheric outlet for each case of s/D 0 , based on head loss estimated using empirical resistance coefficient ( ζ - values) available in engineering handbooks. Discharge has been determined through numerical simulations for same s/D 0 values. Results from numerical and empirical calculations are compared and correction factor (CF) has been determined. It is found that the CF is almost constant, at value of 0.85, for all values of s/D 0 . The discharge coefficient calculated by numerical simulation is around 0.02 lower than the values obtained by experimental observation.

Sediment erosion is one of the major factors affecting the overall efficiency of hydro-turbines working under sediment laden conditions. The erosion not only decreases the efficiency but also changes the flow characteristics. However, the research regarding the changes in flow characteristics due to erosion is limited. The objective of this study is to identify the changes in flow characteristics of a Pelton nozzle due to erosion. Particularly, in a Pelton turbine system, the nozzle and the bucket are the components that are mostly affected by the erosion. The flow in the nozzle is unaffected by the erosion in the bucket. Nonetheless, the overall flow characteristics in the bucket can be affected by nozzle erosion. The erosion in the nozzle decreases the jet velocity and make it bit more dispersed which eventually decreases its impact force on the bucket and eventually leads to the decrement in overall efficiency of the turbine. A numerical study was conducted to compare the pressure loss, jet diameter, velocity, and dispersion angle between eroded and uneroded nozzle. Some wavy perturbations were induced on the needle and nozzle casing of the reference uneroded case to create the eroded nozzle case. The CFD simulation was then performed in OpenFOAM using its inbuilt multiphase solver, incompressibleVoF (interFoam) . It was observed that, with erosion the total pressure loss, jet diameter, and dispersion angle increases but the velocity of the flow decreases. If these changes in flow get detected during operation, it can be used to develop real-time condition monitoring system.

The hydro-turbines working in sediment-laden conditions often experience some material loss from their surface, which modifies the surface design. This material loss alters the mechanical strength of the turbine component as well as the corresponding flow phenomenon. These changes in the flow pattern can lead to efficiency losses, but they can also be exploited to determine the actual condition of the component in real time. In fact, if detected, they can be useful for developing maintenance strategies to predict the turbine conditions in real time. To compare the flow parameters, a numerical simulation has been performed for a 2D stationary Pelton bucket under both eroded and uneroded scenarios considering clean water as the working fluid. The 2D middle section of a reference Pelton bucket has been considered in two configurations: the uneroded bucket case and eroded, whose uneven surface patterns were created taking inspiration from real erosion patterns inside Pelton buckets observed in sediment-laden power plants. The CFD simulations have been carried out using OpenFOAM multiphase solver interFoam. The two cases were then compared based on several flow parameters such as water film thickness, velocity, and exit flow angle. The result analysis confirmed flow disturbances due to erosion causes the decrement in the overall flow velocity and the increment in water film thickness. The velocity reduction effect, as well as the water film thickness increment effect, strengthens approaching the outlet of the bucket. The results also showed a significant alteration of exit flow angle due to erosion. Specifically, for the erosion pattern considered in this study, the exit flow angle was observed to be larger compared to the uneroded bucket case.