Experimental Investigation And Numerical Simulation Of Flow Characteristics In Stirred Tank By Multi-Blade Combined Agitator

In this paper,experimental research and numerical simulation are used to analyze the flow characteristics of the stirred tank equipped by a new design of the multi-blade combined(MBC)agitator.Firstly,the time-resolved PIV and LES approach were used to investigated the flow characteristics in a tank of H/T=1.5 stirred by a novel multi-blade combined agitator(MBC).The LES prediction accuracy using the dynamic kinetic energy sub-grid scale is first assessed by the analysis of Y~+values,the Kolmogorov length scale,the Taylor microscale and the time-dependent power spectrum,as well as the validations of the velocity and its fluctuations obtained by the TR-PIV technique.On above foundation,the flow pattern,flow rate,vorticity,pumping capacity and other characteristics were investigated by LES,and turbulent characteristics were compared with the multi-stage impeller systems,e.g.dual RT impellers and dual pitched blade turbines,and the Maxblend impeller reported in the literatures.In addition,LES was used to explore the influence of rotation direction on the flow characteristics of 45°MBC agitator.Results demonstrate that the MBC agitator can load the energy into the system effectively with a power number of 12.5 in a turbulent regime,resulting in improved axial and radial mass exchange.The upper and lower short blades produce an axial down-flow in the top half and an axial up-flow in the bottom half,respectively.Part of axial flows change to radial flows by the radial pumping of the long blades,meanwhile,the impingement of two axial flows improves the axial mass exchange.These flow characteristics lead to an obvious improvement in the turbulent kinetic energy distribution uniformity with higher turbulent intensity.Compared with the multi-stage impeller system,there are an obvious improvement of the turbulence intensity(TI),the TKE distribution uniformity,as well as the increase of the TKE magnitude.At 90 rpm,the pumping number of MBC agitator can reach 2.28,the axial and radial flow number are similar,and its shear characteristics are also larger than those impellers reported in the literature.In addition,for 45°MBC agitator,the turbulent kinetic energy in the stirred tank is opposite to the time-averaged velocity,and the rotation direction has almost no effect on the flow characteristics in the tank.Secondly,the hydrodynamic characteristics of standard MBC agitator were predicted by LES in laminar region and transition region.Results demonstrate that:in the laminar region,the Reynolds number has little influence on the flow pattern in the agitator,but the flow velocity increases with the increase of Re,and a large dead zone is created near the tank wall.In the transition region,the fluid near the connecting blade has a tendency to flow in the upper syncline.The flow rate of the upper part of the stirred tank is larger than that of the lower part,and a stagnation zone is formed near the lower long blade.When Re>486,the profiles of time-averaged velocity and the velocity components at different radial positions in the stirred tank almost coincide with the turbulent region,but the TKE is lower than that in the turbulence,and the difference in TKE between the upper and lower parts of the stirred tank decreases significantly with the increase of Re.When the complete turbulence is present in the stirred tank,the difference completely disappears.