Numerical and Experimental Analysis of Draft Tubes for Francis Turbine

Objectives: Draft tube is an important component of a hydro turbine (reaction) because it permits a negative head thereby increasing the total head on the turbine. It is located below the runner and allows deceleration of flow at exit. Since, the efficiency of the turbine can be increased by increasing the overall-efficiency of the draft tube, it's important to identify and optimize the design parameters which affect the efficiency of draft tube. Methods/ Statistical Analysis: In the present study, numerical and experimental analysis was done on different draft tubes in Francis turbine. In experimental study, different types of draft tubes were designed, fabricated and tested on a Francis turbine test rig. For conical draft tube, cone angle and cross section (outlet) was varied to understand the flow dynamics. Further, elbow draft tube (rectangular section) was also designed and tested to determine Best Efficiency Point (BEP). Numerical analysis was done using ANSYS CFX for various draft tubes which were designed in ANSYS Design Modeler. Findings/Results: The results of numerical analysis were validated experimentally. High pressure zones were identified in conical and elbow draft tube, although, overall pressure was less than atmospheric pressure. The efficiency of Francis turbine using 8 degree cone angle gives higher efficiency. For angles greater than 8 degree, backflow was observed. For variable discharge conditions (part-load), elbow draft tube can be used because of lesser variations in efficiency at different flow rates. Applications/Improvements: Further, conical draft tube with square section was also analyzed. Higher values of efficiency were obtained at higher speed of the turbine. Hence, it is advisable to further study this type of draft tube.