Numerical Simulation of Pressure Vibrations in a Francis Turbine Draft Tube With Air Admission

The pressure vibrations in a draft tube are harmful for the stable operation for a Francis turbine at part load conditions. In this paper, air admission is proposed to depress those pressure vibrations. The unsteady flow in a Francis turbine, whose hydraulic performance has been tested experimentally, is simulated at a part load operation condition. The flow simulation is conducted using RANS methods coupling with SST k-ω turbulence model. The results indicate the pressure vibrations in the turbine are reasonably predicted by the present numerical method. Based on the calculations, the dominant pressure vibration component for a hydro turbine operated at part-load condition is caused by the vortex rope in draft tube, and its frequency is near 0.2 times of the runner rotation frequency. The frequencies of pressure vibration do not change by air admission, and the pressure vibration amplitude decreases with the air admission. Further, the depression effect for pressure vibration would be improved if air admission is from the crown holes instead of the spindle hole. The results also indicate that the turbine hydraulic efficiency changes periodically with the pressure vibration induced by vortex rope in turbine draft tube, would be degraded with air admission from the spindle hole, and improved with air admission from the crown holes. With the increase of air admission, the turbine hydraulic efficiency would improve. The present research will be helpful for the safe operation of Francis turbines.