Numerical Modelling of Deterioration of Efficiency of Pelton Turbine due to Bucket Erosion

Abstract

Most of the hydropower plants built in the Himalayan originating rivers contain an enormous amount of sediment in them with the dominance of hard minerals. These hard minerals in the sediment is the major reason for hydro-abrasive erosion of hydro- mechanical and turbine components Erosion depends on many factors and main factors that causes erosion in hydraulic turbines are particle velocity, impact angle, physical and chemical properties and operating conditions. Pelton turbine is used for high head and low flow rate hydropower plants. It is found that in Pelton turbine effect of sediment erosion mainly occurs on the buckets depth and splitter, injectors and needle. The erosion causes mass loss of turbine materials which leads to loss of efficiency and eventually mechanical failure. In this study base on the numerical modelling of Pelton turbine, using a commercial computational fluid dynamics (CFD) code, to analyze erosion in a micro Pelton turbine (2 kW) is analyzed. The validation and comparison of this thesis is done with previously carried out research work in Mechanical Department of Pulchowk campus. The flow modelling has been done with Shear Stress Transport (SST) turbulence model with interphase transfer method as a free surface and mixture model and erosion modelling were done with Tabakoff and Grand erosion model in ANSYS CFX. From the numerical analysis it was found that the erosion rate density due to sediment particle is maximum at bucket splitter and depth. The back side of bucket and tip are also erosion prone area. The result obtained from numerical analysis was compared with experiment to validate the findings, mass loss has been evaluated from the computed erosion rate density. The flow pattern in the Pelton turbine bucket has been numerically modeled for both transient and steady-state case and pressure contour and power has been calculated. The maximum pressure has been found to be 1.34*105 Pa. The sand concentration and the flow rate are the strong parameters for mass loss of the Pelton buckets, its variation has been found to be almost linear. From the numerical analysis, mass loss in each bucket due to sediment particle has been found to be 82 mg for 4 l/s and particle concentration of 261 ppm by considering 72 operating hours. The deviation from the experimental value has been below 20%. From the numerical modelling, the efficiency of Pelton Turbine has been found 69% and this value decreases by 6% and 4% in analytical and experimental results respectively due to the erosion which elucidates the error of 3% as compared to the experimental results.