Numerical Investigation of Pressure Fluctuation and Cavitation inside a Francis Turbine Draft Tube with Air Admission through a Fin

Abstract

The stabilizer installation and air admission have been proved to be two effective ways to alleviate the pressure fluctuation in the Francis turbine draft tube. In this research, a combined method of these two ways, i.e. air admission through a fin (one type of the stabilizer) is applied, and its effectiveness, especially air admission’s effects, on pressure fluctuation suppression has been investigated through numerical calculation. Two cases of different air amount (Q *=0.01 and 0.02) are investigated. Results show that cavitation inside the draft tube is suppressed and the size of vortex rope is also slightly decreased after air is admitted. As a result, pressure fluctuation inside the draft tube is generally alleviated compared with no air admission case. The maximum amplitude of pressure fluctuation decreases by 9.2% with 0.01 air admission, and by 26.3% with 0.02 air admission. In addition, the position where the maximum amplitude of pressure fluctuation occurs also moves downstream, which should be due to the more violent interaction between the vortex rope in the main stream and a small vortex rope attached to the fin. Consequently, a slight increase of pressure fluctuation downstream the fin can be observed. Although air admission through the fin can suppress pressure fluctuation in the draft tube, it would also lead to turbine efficiency decrease. Therefore, the application of both fin and air admission should be carefully considered to achieve better performance for Francis turbines.