Numerical analysis of the head covers deflection and the leakage flow in the guide vanes of high head Francis turbines
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This thesis includes a brief presentation of the different losses developed in high head Francis turbines, where special attention is paid to the losses in the guide vanes due to the head covers deflection. The different factors determining the extent of the leakage flow are thoroughly examined. Also, since the guide vane facing plate and the head covers are exposed to serious wear due to the presence of sand particles in a high accelerating flow, efficiency measurements of sand eroded and repaired turbines are presented. In addition, results from efficiency measurements of Francis turbines with different clearance gaps are included to underline the severity of the clearance gap between the covers and the guide vanes. The thesis presents a numerical approach to calculate the head covers deflection. And the scope of this work is to develop a simplified 2-D model, which calculates the head covers deflection within a certain order of accuracy. Thus a 3-D modelling of the head covers deflection is also included with the purpose of validating the 2-D modelling. Furthermore, a discussion of the efficiency with respect to the design of the head covers is performed to describe the potential of increasing the hydraulic efficiency by altering the design method. Also, a numerical computation of the leakage flow in the guide vanes due to the head covers deflections, is presented to better visualize the effect from the clearance gap between the head cover and guide vane facing plate. Also, to underline the severity of large clearance gaps, the guide vane has been modeled with four different sized clearance gaps. Finally, an analytical interpretation of the leakage flow is included to evaluate the numerical calculation of the leakage flow.