Analytical Model And Control Method Of Hydraulic Turbine Tail Vortex Based On Burgers Vortex Method

Centrifugal pump reversal as a hydraulic turbine as a liquid residual pressure energy recovery device,with the advantages of simple structure,low installation and operation and maintenance costs,is widely used in petrochemical,steel smelting,seawater desalination and other fields.However,in engineering applications,it is found that this type of hydraulic turbine has problems such as low efficiency,unstable operation and difficult selection.The reason is that the centrifugal pump is not designed with the flow characteristics of the reversing operation in mind,and the flow phenomena such as velocity slip and tail vortex make the efficiency and operation stability of the hydraulic turbine reduced.However,the flow mechanism of the wake vortex and the influencing factors are not clear yet,and the control method of the wake vortex is not available.Therefore,in this study,the flow mechanism,analytical model and vortex control of hydraulic turbine wake are investigated.The details are as follows:(1)The flow mechanism of the tail vortex was analyzed in a centrifugal pump with a specific speed of 48.Constant numerical simulations revealed that the tail vortex of a hydraulic turbine is usually represented by a large scale spiral main vortex and a small scale vortex group distributed around it.The results of non-constant numerical simulations show that the tail vortex can be divided into a vortex core area where the circumferential velocity increases approximately linearly along the radial distance and a vortex decay area where the circumferential velocity decreases more slowly.Under the high flow condition,the vortex breaks up,resulting in a backflow in the center of the vortex core,the radius of which is about 50%of the radius of the vortex core,which increases the mixing loss in the center of the pipe.Under high flow conditions,the pressure pulsation of the tail pipe is mainly lowfrequency pulsation,which is mainly caused by the evolution of the tail vortex motion.The energy loss in the tail pipe is 8.73%of the total energy loss,of which the turbulent energy dissipation is mainly caused by the vortex and the friction between the fluid and the pipe wall.The vortex core area of the tail vortex under optimal conditions is dominated by the hydraulic loss caused by counterflow and vortex rotation,and the vortex decay area is dominated by the hydraulic loss caused by axial flow,and this part of the hydraulic loss is unavoidable.(2)Through theoretical analysis of the tail flow field at the outlet of the hydraulic turbine impeller,it is found that the generation of the tail vortex is related to the structure of the impeller grid,velocity slip and the constraint of the tail pipe.Based on the flow characteristics of the hydraulic turbine,an analytical model of the hydraulic turbine wake vortex based on the Burgers vortex method is proposed.It is found that the velocity slip has a reinforcing effect on the wake vortex in small flow conditions and a controlling effect on the wake vortex in large flow conditions,and the effect of both decreases with the increase of flow rate.Based on this,the analytical model of the wake vortex is modified by considering the velocity slip,and the validation results show that the modified analytical model of the wake vortex is very similar to the wake vortex flow structure.(3)Based on the hydraulic turbine wake vortex analytical model,the influence mechanism of the blade grille structure on the wake vortex is analyzed,and the wake vortex control method of changing the blade exit angle is proposed.The intensity and range of wake vortex is the smallest when the blade exit angle is 27°,and the impeller exit flow field distribution is the smoothest,the peak circumferential velocity is 76.5%of the original model,the radial range of counterflow is 50%of the original model,and the amplitude of counterflow velocity is 35.67%of the original model.The pressure pulsation phenomenon under this scheme is also greatly improved,and its pressure coefficient fluctuates around 0,and the amplitude of pressure pulsation is only 31%of that of the original model.