Flow Field Characteristics In A Transitional Flow Stirred Tank

Flow field characteristics in a transitional flow stirred tank has important guiding significance for the scale-up and optimization of the stirred tank.It has always been a mainstream research idea to observe the flow field characteristics by experimental methods and then evaluate the performance of numerical simulation methods or models.In this paper,a standard Rushton turbine stirred tank was used as the stirring device,and the flow characteristics of Newtonian fluid and Non-Newtonian fluid near the impeller region under transition flow conditions were observed by PIV experiments,respectively.Based on the experimental results,the performances of various numerical simulation methods or models under transition flow conditions were evaluated.This article have significant reference value for the design of stirred tank and the improvement of CFD model.In the experimental study with Newtonian fluids,the flow field characteristics near the blade region with transitional flow were investigated by Stereo PIV experiment.On the premise of Stereo PIV results being verified,the influence of Reynolds number,viscosity and phase angle on the flow fields has been investigated,and the results show that: when the flow field change from the turbulent to laminar,the evolution of the trailing vortex structure has unique transitional behavior: as the Reynolds number decreasing,the trailing vortex structure change from the symmetrical structure into a markedly asymmetric structure as the upper vortex disappearing,then as the lower vortex waning the vortex structure changed into the symmetrical structure again,finally,the upper and lower trailing vortex vanished in the flow field at the same time;when Re ?6000,the flow fields near the impeller region were still completely turbulent;when 6000 ?Re ?2600,turbulent similarity is difficult to maintain,which indicates that the flow field changes from turbulent flow to transitional flow;when Re?270,The gradual disappearance of the turbulent kinetic energy indicates that the flow field begins to change from transitional flow to laminar flow.The flow field characteristics under transitional flow,such as the vortex structure,average velocity,velocity fluctuation level,etc.,are independent with viscosity conditions and only dependent on the Reynolds number;when Re=1310,the symmetrical vortex structure formed at ?=10°,and has a slow dissipation speed in its migration process.The region with higher TKE and energy dissipation rate is concentrated near the lower vortex,and its value is much smaller than turbulent condition.In the numerical simulation with Newtonian fluid,this article used a variety of method or model to simulate the flow field characteristics with Newtonian fluid under transitional flow condition,and based on Stereo PIV experimental data,the accuracy of numerical results has been evaluated in detail,and the results show that: the Transition SST model failed to predict the transitional behavior of the flow characteristics as the Reynolds number decreasing and overestimated the turbulent kinetic energy of the flow fields near the impeller region;The direct numerical simulation based on finite volume method(FV-DNS)can roughly predict the evolution of flow field characteristics with decreasing Reynolds number,but it cannot accurately predict the markedly asymmetric vortex structure under Re=1310;The tailing vortex structure predicted by the LB-DNS under the condition of Re=1310showed a more obvious asymmetrical structure,but it is still different from the experimental results;FV-DNS can reasonably predict the variation of power number with Reynolds number under transition flow condition,but the results predicted by the Transition SST model is obviously larger.In the experimental study with Non-Newtonian fluid,the flow characteristics of shear thinning fluid under transitional flow condition were investigated by 2-D PIV experiment in this article.The stability of the working fluid and the accuracy of the experimental results were verified,and the flow field characteristics near the impeller region were studied by both ensemble average and phase resolved methods,and the results show that: The rotational speeds and the rheological behavior have a significant influence on the flow field,and the velocity fluctuation levels decrease with the increase of apparent viscosity;The correlation based on the Metzner constant underestimates the average shear rate near the impeller region,and the difference between the local maximum shear rate and the overall values became larger as rotational speed increasing;As the rotational speed increasing,the difference between the local shear rate calculated from the average velocity and the local shear rate calculated from the instantaneous velocity became larger;The apparent viscosity near the jet structure is low,and the spatial inhomogeneity of apparent viscosity became more significant,as the rotational speed decreasing;With the increase of the phase angle,the region with higher local shear rate gradually migrated from the tip of the blade to the wall of the tank;When the apparent Reynolds number(or Reynolds number)is similar,the differences of the average velocities in the phase resolved flow fields between the shear thinning fluid and Newtonian fluid is small,but the velocity fluctuation level caused by turbulence with the shear thinning fluid is obviously lower than that with Newtonian fluid.In the numerical simulation with non-Newtonian fluids,FV-DNS is used to simulate the flow characteristics of the shear thinning fluid.The simulation results were carefully verified,and the rheological parameters were changed to further predict the influence of the rheological characteristics on the flow field characteristics near the impeller region,and the results show that: The profiles of average velocity predicted by the simulations agree well with the experimental results,but the dimensionless velocity fluctuation levels in the flow field were overestimated.In addition,the simulation results can well predict the local shear rate distribution near the impeller region and the directly proportional relationship between the mean value of the local shear rate and the rotational speeds;The consistency coefficient(K)has little influence on the average velocity,but the velocity fluctuation levels decreases significantly with the increase of K.The radial jet near the impeller region will rise gradually with the increase of the fluid index,and the fluid index has a complex influence on the dimensionless velocity fluctuation level,which is not consistent with the increase of the corresponding apparent Reynolds number.The fluid index and consistency coefficient have great influence on the region with higher local shear rate,but they have relatively little influence on the mean value of the local shear rate.