Design and analysis of francis turbine for sediment load

Abstract

Hydraulic turbines of several hydropower projects have been facing severe problem due to sediment erosion. In fact, the problem is much worst in case of run-off-the river hydropower plants as the sediment particles dont get enough settling time in contrast to reservoir hydropower plants. The particles eventually find their way to- wards the turbine and causes the subsequent change in flow pattern inducing several operational and maintenance problems in such plants. Several erosions inducing factors like particle velocity, sediment shape, size, concentration, operating condi- tions, impingement angle, hardness of substrate etc. are responsible for the loss of efficiency and working capability of hydraulic turbines operating in sand laden con- ditions.

In order to overcome these challenges, several approaches were put forward. Among them, usage of coated materials and expansion of settling basin have drawn the in- terest of some of the researchers. The application of the coating has been practiced in few hydropower plants in Nepal, but the results were not found economically fa- vorable. In addition, maintaining uniformity in coating thickness at all surfaces in case of Francis turbine runner blade is quite difficult task. Due to the countrys ge- ographical condition, it is always not possible to construct large settling basin. In such circumstances, hydraulic design changes may be only available last resort for this case.

This research was carried out to address these issues and assist in hydro sectors to achieve the high performance Francis turbine with less prone to sand erosion. First of all computational tools were implemented in order to study the effects of erosion inducing factors at different operating conditions. Further, encompassing the previ- ously conducted experiments, the nature of effects due to different combinations of input factors like shape, size, concentration, and impingement angles on the turbine material were analyzed. The analysis includes digital image processing to extract shape and size of sediment particles from the erosion sensitive power plants and its experimental studies carried out at Kathmandu University (KU) using two different

methods High velocity impact and Rotating Disc Apparatus (RDA).

In this PhD study, Computational Fluid Dynamics (CFD), Finite Element Method (FEM) and Fluid Structure Interaction (FSI) were employed in order to minimize the sediment effects on turbine material. The application of CFD, FEM and FSI at different design stages innovated new way for Francis turbine design with less ero- sion impact as compared to reference design. Comparative study was carried out among the five different shapes of runner varying the guide vane and stay vane an- gles. The effect of erosion was studied in terms of average erosion density rate on optimized runner design with Lagrangian particle tracking method in CFD and com- parison was done using two turbulence models. The simulation analysis results are compared with the measurements carried out in the real turbine. The results showed that runner material is more susceptible to erosion at part load conditions rather than at best efficiency point (BEP), however erosion in guide vanes is dominant at full load conditions. The comparison of five different shapes demonstrated that Shape 5 provides an optimum performance with high efficiency and lesser erosion on the studied operating conditions. To conform structural integrity, multidisciplinary opti- mization was performed on shape 5 in which energy extractions occur with high and low energy distribution from inlet to mid span in first half and then from mid span to the outlet of the runner in the latter half.

Although the problems of sediment can't be eliminated completely from its root, the findings of this research will definitely assist in minimizing its consequences. Further, the outcomes are believed to aid to lessen the erosion problem of turbine components and increase the lifetime.