Optimizing runner blade profile of Francis turbine to minimize sediment erosion

Hard sediment particles as quartz are present in high amount in the rivers across Asian and Andes mountain ranges. This cause the run-off-river hydropower plants in these regions to suffer from erosion wear. The hydro turbine components erode severely during the monsoon periods. Due to high relative velocity the turbine runners are more vulnerable. Loss of turbine efficiency and high cost of repair and maintenance are the major consequences of the erosion. Several attempts including surface coatings to control the sediment erosion in Francis runners have not shown satisfactory results. One of the emerging solutions is to reduce the relative velocity inside the runner by improving the hydraulic design. This includes optimization of the runner blade profile to reduce sediment erosion, while avoiding cavitation and still maintaining the highest possible efficiency. This study has been conducted to identify the alternative blade profiles of high head Francis runners and estimate the effects of sediment erosion on each new profile. A new design program named as "Khoj" has been developed to facilitate this study. The program can generate the profiles of Francis runner based on the traditional equations. It is also capable to export the designs for CFD and FEM analysis. Erosion factor has been defined as a means to compare the relative change in sediment erosion due to the variation of the runner design. A reference turbine has been established and alternative blade profiles have been designed. CFD analysis has been conducted to evaluate the performance of the alternative designs relative to the erosive conditions of the reference turbine. It has been observed that the shape of runner blade has a significant effect on velocity distribution and hence on the sediment erosion of the runner. Results of CFD analysis validates prediction from the design program that the blade profile with higher blade loading at outlet can reduce the sediment erosion in runner up to 33%. It was also observed that this condition improves the runner efficiency without any change in the runner main dimensions. Results of this study can be useful to design Francis turbines operating in sediment conditions.