Optimal condition of simultaneous water and air injection in a Francis turbine in order to reduce vortices using experimental and numerical methods

Optimal operation of hydro turbine as one of the most important sources of renewable energy is of great importance. In this article, using a combination of experimental and numerical methods, the effect of different water and air injection on the vortex reduction in a Francis turbine draft tube was studied and the most optimal condition is selected in order to minimize pressure fluctuations and maximize effective efficiency. For this purpose, pressure fluctuations of the water turbine of a real hydropower plant were experimentally measured under different loading conditions to find the worst condition need to be improved. It was found with injecting water at 1.5% and 3% of the total volume of inlet flow rate, the performance index (& gamma;) improves by 38% and 10% respectively, compared to the case without injection. By injecting air at 1% and 2%, the performance index shows 36% and 38% improvement respectively. Optimization was done with the help of Nondominated Sorting Genetic Algorithm II. The results showed that the most optimal condition of injections are 1.63% and 0.65% air and water volume flow rates, respectively, using 0.019-meter air nozzle diameter at -2.80-meter injection height, which reduce pressure fluctuations by 33.44%.