Draft Tube Vortex Rope Analysis of a Pump-turbine Unit under Different Operating Conditions

Pumped-storage power stations which can be flexibly adjusted for power generation and energy storage, are widely constructed worldwide. Pumped-storage units work under different operating conditions according to the demand of the power grid, and vortex ropes in the draft tube will appear and show different shapes under the partial load conditions of the unit in generation mode. The unstable pressure pulsation caused by the eccentric vortex ropes inside the draft tube may cause the vibration of the unit during operation. So it is important to study the characteristics of draft tube vortex ropes for flow and vibration analysis. In this paper, a three-dimensional model of the unit is fully established, and the unsteady calculations of 60 %, 86.7%, and 100 % load conditions in turbine mode are carried out by numerical calculations to obtain the flow characteristics of the full flow field. The pressure and velocity distributions, shapes of vortex rope in the draft tube under different load conditions are extracted from the CFD analyses, respectively. The pressure pulsations of different monitoring points set up inside the draft tube are evaluated in the time domain and frequency domain. The results show that the vortex rope in the draft tube is more obvious under small flow discharge, and the vortex rope itself is not sensitive to the selection of Q criterion threshold, but the vortex on the draft tube wall is greatly affected by the selection of the threshold. The influence of draft tube vortex ropes on the vibration of the pumped-storage unit is also discussed.