Experimental-numerical investigation of the air and water injection effect on francis turbine vortex fluctuations

In this article, the effects of the vortex rope in the Francis turbine draft tube in one of Iran hydropower plant are studied experimentally and numerically. First, the experimental results of the unit at four operational loads of 22.52, 45.74, 68.73 and 85.60 MW was extracted by a pressure gauge which is located at a distance of 3.199 meters below the runner. According to the fast fourier transform, the load with a power of 45.74 MW had the most pressure fluctuations. Then numerical simulation of the whole turbine at different condition was performed using transient solution and SAS-SST turbulence model. At five injection condition namely 1.5% and 3% only water, 1% and 2% only air, and the combined injection of water and air (3% water and 2% air) order of vortex fluctuations were investigated. Also, an algorithm to optimize the parameter of efficiency multiplication in the pressure recovery coefficient over the peak-to-peak amplitude of fluctuations at different air and water injection was presented. The results showed that injection with 1.58% of water and 0.02% of air, cause the mentioned ratio to be maximum, which is the most optimal condition of operation.