Numerical study of peripheral air injection in draft tube of a Francis turbine at upper and normal part load operations

Francis turbines are adversely affected when operated under part load operating conditions. The increase in pressure pulsations and dynamic stresses results in the less efficient operation of turbine components. Flow instabilities in the draft tube, particularly the rotating vortex rope (RVR), are responsible for such adverse effects. Air injection is a widely used mechanism for attenuating pressure pulsations due to RVR formation and has been employed with varied degrees of success. However, the exact mechanism by which the air influences the turbine flow field is still not fully understood. The influence of peripheral air injection in the draft tube is analysed in the present work using numerical simulations. The effect of injected air on the draft tube flow field at the upper and normal part load is investigated qualitatively and quantitatively. The results elucidate the reasons for air to be beneficial only at low injection rates. The simulations show the manner in which the unsteadiness in the flow increased at high injection rates, leading to additional vortices and secondary flow structures. The results further elucidate the effect of injected air on the changes in the effective area and pressure recovery in the draft tube.