Study On Characteristics And Improvement Methods Of Draft Tube Vortex Rope In A Pump Turbine

In recent years especially the “13th Five-Year” period,the construction speed of pumped storage power stations was significantly accelerated in China,and the pump turbine plays an important role as the most important part of the pumped storage power station.Because the current technology of turbine is well developed,the efficiency is no longer the most important factor that restricts the operation of pump turbine.But the cavitation and instability are gradually attracting more and more attention with the development of pump turbine to higher water head,higher specific speed and larger capacity.The vortex rope in draft tube is the main cause of hydraulic vibration of pump turbine,and it will produce a visible vortex rope inside the draft tube when the pump turbine deviates from the optimal operating conditions of the turbine.The rotation of the vortex rope will result in a strong pressure fluctuation in the draft tube,and the pressure pulse will propagate upstream and cause vibration of the unit.In this paper,the CFD method with RNG k-? turbulence model were used to simulate the vortex rope of draft tube in pump turbine,and the main objects are as follows:(1)For the same cavitation factor,the transient simulations were conducted at the guide vane opening of 13 mm,15mm and 31 mm for the given turbine,in which the flow was considered to be single phase,and the effects of flow rate and runner rotational speed variation on the vortex rope of draft tube were studied respectively.(2)In order to make the simulation more close to the actu al developing process of vortex rope,the ZGB cavitation model is chosen to calculate the unsteady flow in the whole passage at the guide vane opening of 15 mm with ? =0.065? ? =0.045.The effect of cavitation variation on the vortex rope of draft tube was investigated,and the results of calculation under the conditions of single-phase flow and cavitating flow were compared and analyzed.(3)The way of air make-up in the center hole of spindle is selected,unsteady calculation of the ? =0.045 operating point under the above 15 mm opening is carried out by using the mixture gas-liquid model(without using the cavitation model),in addition,unsteady calculations of single-phase water jet are performed at the same location.The purpose of doing so is to study the effect of air supplement and water jet on the improvement of the vortex rope and its effect on improving the overall performance of the pump turbine respectively.Research indicates that the vortex rope of draft tube is affected by the flow rate,the rotational speed of the runner and the cavitation number greatly,and the vortex rope is closely related to the vortex flow of the straight cone.The vortex rope does not have a positive or negative correlation with the flow rate,but it reaches maximum at a smaller flow rate,and the decrease of runner rotational speed and cavitation number will all weaken the intensity of vortex rope.Both air supplement and water jet in the central air hole can all improve the vortex rope,and the more flow rate of air or water will make the improvement effect on the vortex rope become more obvious.But air supplement can reduce the volu me of vortex rope and even make it disappear,but the water jet can only change the vortex rope from helix to cylinder,and the water jet will reduce the efficiency of the pump turbine,so the effect of air supplement is better than water jet synthetically.