PIV Experiment And Numerical Simulation Study Of Non-Newtonian Fluid Flow Field In A Mechanical Agitation Tank

Agitation is a basic operating unit for mixing,and has been widely used in industrial production.In particular,mechanical agitation has many advantages in the current stirring mode such as easy management,flexible operation,convenient adjustment of mixing strength,and high mixing efficiency.The mixed fluids of mechanical agitation often involve non-Newtonian fluids,whose rheological properties are different from those of Newtonian fluids such as elastic effects,shear thinning,and shear thickening,which make the study of mixing more difficult.Currently there are many numerical calculations and relatively few experimental studies on non-Newtonian fluid agitation.In order to fill the gap in the experimental study of the non-Newtonian fluid flowing field in mechanical agitation tanks,this paper conducts a more in-depth study on the flowing characteristics of non-Newtonian fluid in the agitating tank with axial flow impellers based on the PIV(Particle Image Velocimetry).In this paper,four kinds of axial flow agitating paddles with similar flow numbers but different structures are used:(1)four pitch blades turbine impeller(4PBT/45°);(2)disc six pitch blades turbine impeller(6PBT/45°);(3)Lightnin-A300 impeller(Lin-A300);(4)Lightning-A315 impeller(Lin-A315).The PIV technique was used to test the flow characteristics of nonNewtonian fluid with four kinds of agitating paddles in an agitating tank.The experiment analyzed the flow field distribution generated by different agitating blades,the influence of the rotational speed on the flow field distribution and the rheological properties of the mixing paddles with different structures from different aspects such as the time-average flow field distribution,the dimensionless flow velocity distribution,the vorticity distribution and the turbulent kinetic energy distribution.The experimental result shows that the structure of the agitating paddle affects the distribution of time-averaged flow field,the distribution of the vorticity and the change of flow velocity,and also affects the position and flowing scope of the main circulating flow,the scope of the reverse circulating flow in the lower part of the impeller.Comparing experimental result of the flow field four kinds of impellers,it is found that the 4PBT/45° impeller has the best mixing effect.The change of agitating velocity not only changes the flow pattern of the flow field,but also changes the velocity distribution,the turbulent kinetic energy distribution,and the position of the vorticity distribution and its size.The increase of the concentration of the xanthan gum solution or the decrease of the rheological index,affects the flowing scope of the main circulating flow,and changes the flow intensity in the lower part of the agitating paddles.Therefore,a high agitating speed is very necessary to improve the efficiency of mixing.The rheological properties of xanthan gum solution with different mass fractions were tested and the relationship between apparent viscosity and shear rate was obtained and it was consistent with the Herschel-Bulkley rheological model,in order to define of non-Newtonian fluid for numerical simulation.The research used the commercial software Fluent,basing on the Realizable k-? turbulence model and large eddy simulation to numerically simulate the flow characteristics of non-Newtonian fluid in mechanical agitating tanks.Then the research employed sliding grid method to treat the motion region of agitating paddles and used the Herschel-Bulkley Rheology Model to define properties of non-Newtonian fluid.The numerical simulation of the flow field distribution calculated by the Realizable k-? Turbulence Model and the Large Eddy Simulation is roughly similar to that of the experiment.By comparing and analyzing the flow pattern of the lower part of the agitating paddles and the rotational center of the main circulating flow and the distribution of turbulent energy,it is found that the calculating result of the Large Eddy Simulation is closer to the testing result.The Dimensionless flow velocity distribution is compared and analyzed,and calculating result of the large eddy simulation is more consistent with the experimental results than the result calculated by the Realizable k-? Model,but there is still a difference in flow velocity distribution in the local area.