International Conference on Small Hydropower - Hydro Sri Lanka, 22-24 October 2007 1 Alternative Design of a Francis Turbine for Sand Laden Water Hari Prasad Neopane 1) , Ole Gunnar Dahlhaug 1) and Bhola Thapa 2) 1) Department of Energy and Process Engineering, Norwegian University of Science & Technology, Norway 2) Department of Mechanical Engineering, Katmandu University, Nepal E-mail: hari.p.neopane@ntnu.no , ole.g.dahlhaug@ntnu.no , bhola@ku.edu.np ABSTRACT The main objective of this paper is to propose an alternative design of a Francis turbine for minimizing the sand erosion effect in sand laden river. To achieve this objective, one erosion model for hydraulic machinery has been selected and all technical and managerial aspects suppose to be considered in this study. Hydraulic turbine components operating in sand-laden water subject to erosive wear. Erosion reduces efficiency and life of turbine. It also causes problem in operation & maintenance. Himalayan rivers carry large amount of hard abrasive particles. Withdrawal of the clean water from the river for power production is expensive due to design, construction and operation of sediment settling basins. Even with the settling basins, 100% removal of fine sediments is impossible and uneconomical. The design of the Francis turbine can be done in two main stages. The first step is the initial design, based on empirical data and the Euler’s turbine equation. The second step is a three dimensional CFD analysis for fine tuning of the design, normally based on the assumption of uniform flow field from the guide vanes cascade towards the runner blade inlets. The hydraulic design strategy for the Francis turbine has played the major role in the design. The main objectives in the design of the medium head Francis turbines are to reduce the pressure pulsation, avoid cavitations, to reduce sand erosion and to gain maximum efficiency. The ideal goal is to design a runner that has the widest possible operating range for head variations beyond the normal design head, and that would require the minimal maintenance. The initial step is the most important. The basic hydraulic design of the runner is done during this stage, and the balancing of the pressure distribution on the blades by controlled parameters gives the designer an important feeling, which assists in evaluating the result of the CFD analysis. This feeling is very useful in helping to make the right changes in the geometry and to improve the performance of the runner. The CFD analysis is also important in the study of the model turbine behavior and forms the basis for final tuning of the prototype Francis runners. 1 CONVENTIONAL DESIGN OF FRANCIS TURBINE 1.1 The importance of the Runner The hydraulic energy of the water is converted into mechanical shaft energy in the turbine runner at the expense of the interaction between flow and the runner blades constituting a rotating cascade of profiles. Runner is one of the most important components of the turbines. Francis turbine is a reaction type of turbine where the specific energy in front of the runner consists of partly pressure energy and partly kinetic energy.