DESIGN AND EVALUATION OF PERFORMANCE OF CONICAL TYPE DRAFT TUBE WITH VARIATION IN LENGTH TO DIAMETER RATIO

Reaction turbines are widely used in medium and low head hydro power plants. Draft tube is one of the major components for reaction turbines. Water after passing through the runner has very high kinetic energy but lags in pressure energy. To recover pressure head and utilize kinetic energy of water, draft tubes are installed at outlet of turbine. Draft tube can be of gradually increasing cross-sectional type (commonly called conical diffuser type), simple elbow type or elbow with varying cross- section. Conical diffuser type draft tubes are simple in construction, simple to install, simple in designing and easy to maintain. The diffuser angle is less than Ten Degree to prevent flow separation and cavitation. The length to inlet diameter ratio is one of the most important parameter in designing of draft tube. For variation in the ratio, inlet diameter of draft tube cannot be altered as variation in diameter may not fit site conditions. So the only length of draft tube can be varied for finding the best optimum length. In present work, three conical diffuser type draft tube with length (22.45 m, 30.4 m and 38.4 m) have been modeled with a diffuser half angle of (4°) The velocity of water at inlet of draft tube has been kept as 9.17 m/s in axial direction, in radial direction as 0.14215 m/s and in tangential direction as 4.35 m/sec. The outlet of draft tube has been defined as outlet to atmosphere. The performance of all three draft tubes have been evaluated using ANSYS CFD. Velocity and pressure contours have been obtained for all the cases. It has been found that with the help of CFD, fluid flow problems can be easily handled. For present considered cases, the draft tube with length 30.4 m is best suited. The efficiency is found out to be 22.7% for both 30.4 m and 38.4 m draft tube