Experimental and Numerical Simulation Study on the Flow Characteristics of the Draft Tube in Francis Turbine

The flow characteristics of the draft tube of a Francis turbine have a significant influence on turbine stability. Numerical simulations were performed for a Francis turbine under three different output conditions of 20%, 100%, and 120% at the rated and maximum heads. Laser Doppler velocimetry (LDV) tests were conducted to test the flow characteristics of the draft tube of the Francis turbine model. The flow characteristics in the draft tube, the mechanism of the flow characteristics change, and the effect of the opening on the vortex rope were analyzed. The results showed that the large and invisible vortex in the conical cross-section at the inlet of the draft tube gradually changed to a tangible vortex rope as the guide vane opening (GVO) increased. The pressure and velocity are significantly influenced by the GVO, and the flow characteristics in the draft tube improve as the GVO increases. Simultaneously, the influence range of the vortex rope increased as the head increased.