Study On Flow Characteristics Of H-like Fractal Impeller Based On Fractal Theory In A Stirred Tank

Stirred tank is the main unit equipment for hydrogenation,oxidation and other reactions in the chemical industry,which plays an important role in the chemical industry.Among them,the impeller is the most critical component.Through the rotation of the impeller,the rapid mixing and heat transfer process of the fluid in a stirred tank can be realized.The selection and design of the impellers are the most important factor affecting the efficiency of the stirred tank.However,for most types of impellers,a prominent problem is that the distribution of hydrodynamic characteristics such as velocity and turbulent kinetic energy(TKE)in a tank is uneven,especially for the gas dispersion process.On the other hand,for the impeller,the hydrodynamic performance will decline significantly in the amplification process.Therefore,the design of a new structure of the impeller and the introduction of a new impeller structure design theory are urgent problems to be solved in the chemical industry.Fractal theory provides a new strategy for equipment design and industrial amplification.In view of the problems existing in the impeller,fractal theory is introduced to optimize the structure of the impeller.The structure of the impeller has self-similarity,so that the distribution of hydrodynamics parameters in the stirred tank also has similarity,so as to strengthen the mixing process in the tank and break through the technical bottleneck.A self similar H-like fractal impeller with fractal structure is designed.Using the method of LES and TR-PIV,the flow pattern,flow rates,vortices,turbulent kinetic energy distribution and power number of the fractal impellers are studied,and the flow characteristics of the designed fractal impeller are compared with the Rushton turbine(RT)impeller which has been widely used in industry.The results show that the HLF impellers with self similar fractal structure can realize the uniform arrangement of small blades in the whole stirred tank,produce uniform distribution of velocity,vortices,TKE,and reduce the area where velocity,vortices and turbulent kinetic energy are concentrated.At the same time,the dispersion range of blades is large enough to reduce the dead zone.The radial flow rates of HLF impellers are stronger,and the axial flow rates are more evenly distributed in the axial direction.A series of vortices arranged in the axial direction with equal size collide and produce fluid exchange,which indicates that the mass exchange in the axial and radial directions is stronger.The distribution of velocity and TKE of the designed HLF impellers are more uniform than that of the traditional RT impeller,and the power number is also lower than that of the RT impeller.It can be seen that the fractal impellers can produce better flow characteristics in a stirred tank when the energy consumption is lower than that of the RT impellers,and has higher energy efficiency.Secondly,the structure of the HLF-2 impellers is improved.the blades are shifted vertically and rotated horizontally,respectively,so that the small blades are dislocated(HLF-2C)or cross distributed(HLF-2X).Their flow patterns,flow rates,vortices distribution,TKE distribution and power number are studied by LES.LES results show that the distributions of velocity,vortices and TKE of HLF-2C impeller become more uniform but there are large cycles near the liquid surface and the bottom of the tank respectively.The distance between the two cycles is relatively far,and the fluid exchange in the axial direction is weak.The uneven degree of the distribution of velocity and TKE of HLF-2X impeller increases but the flow pattern doesn’t change a lot.The radial flow rates distribution of HLF-2X impeller is similar to that of HLF-2 impellers,but the value of radial flow rates obviously higher than that of HLF-2C impeller.At the same time,the axial flow rates of the three impellers are close near the upper part and lower part.but the axial flow rates of HLF-2C impeller in the axial middle area of the tank are low,indicating that there is less fluid exchange in the axial direction,resulting in a certain degree of isolation.The power number of HLF-2C and HLF-2X impellers is lower than that of the HLF-2 impeller,but the power number of HLF-2X impellers is higher than that of HLF-2C impeller.