Investigation On The Effect Of The Draft-tube On The Flow Characteristics In The Surface-aerated Stirred Tank

In this paper,a Surface-aerated stirred tank with a draft-tube is taken as the research object.The methods of numerical simulation and experiment have been used to investigate the length of the drafttube,the angle of the short blade inclination,and the rotation direction of the stirring paddle(when rotating,it corresponds to up-pumping mode,and when rotating counterclockwise,it corresponds to down-pumping mode)on the flow field in the stirred tank.Firstly,particle image velocimetry(PIV)technology was used to measure the velocity field in a stirred tank with a draft-tube.The time-average speed field,time-average flow line,vorticity,pulsation and the turbulent kinetic energy was studied,and the optimal length of the draft-tube,the angle of inclination of the short blade and the rotation direction of the stirring blade were analyzed and compared.Then the experimental conditions were numerically simulated by two different methods,Reynolds time average(RNG K-ε)and large eddy simulation(LES).Finally,the simulated results were analyzed by the time-averaged velocity distribution in the horizontal plane and the 3D vorticity.The research results are as follows: the optimal mixing effect of the fluid can be obtained when the short blade inclination angle is 60 ° at the same length of the draft-tube and the same rotation mode of the mixing blade;the mixing of the fluid is the best with the draft-tube having a length of170 mm at the same inclination angle of the short blade and the same rotation mode of the mixing blade;as to the up-pumping mode stirring paddle kettle,the best mixing effect is achieved when both the length of the draft-tube and the inclination angle of the short blades are same.It shows that the LES model is in better agreement with the experimental result and is more accurate than the RNG K-ε model.At the same time,the simulation results of the LES model indicates that the presence of the draft-tube will obviously affect the axial and radial velocity of the fluid,limit the flow path of the fluid and improve the axial flow of fluid in the stirred tank,which will help to improve the mixing effect.As shown in the simulation results,the radial pumping will be different due to the change of short blade inclination angles and it gives a greater load at a larger angle.For the two different rotation modes of the paddle,the down-pumping mode helps to reduce the volume of the dead zone at the bottom of the tank,but the up-pumping one has a better overall mixing effect on the fluid in the stirred tank.