Numerical Investigation of Unsteady Flow in Draft Tube with Ribs

Unsteady flow in the draft tube of hydraulic machines is a dangerous phenomenon that can lead to destruction of the structure. The most serious is precession of vortex rope, which usually occurs at partial load modes, and significantly limits the operating range. In order to determine the level of such pulsations and the influence of presence of ribs inside the draft tube, a numerical simulation of unsteady flow in the model of the draft tube of the Francis pump-turbine of the Dniester HPP in turbine mode was carried out. Selection of the most suitable turbulence model was made based on a comparison of calculation data and data obtained as a result of experimental studies. The comparison was made both for values of total pulsation amplitude on surface of the tube diffuser, and for the frequency-amplitude characteristics, which show a satisfactory correspondence in frequency, but a significant difference in amplitude. Using flow data in experimentally determining modes, a simulation of flow in the draft tube with ribs installed in the vertical diffuser and designed to reduce the level of pressure pulsations at partial loads, was carried out. As a result of the analysis of received data, the optimal size of additional elements was determined.