Study On Mixing And Flow Characteristics Of Shear-thinning Fluids In A Simplified CTM Model

Mixing is a very common unit operation in the chemical production process.The quality of the mixing in the polymer industry such as rubber and textile often affect the competitiveness of the product.These polymer fluids are mostly shear-thinning fluids,and the mixing process is different from that of Newtonian fluids.The complex rheological properties of polymer fluids and special mixing equipment structures make it difficult to quantitatively study the flow and mixing characteristics of shear-thinning fluids in mixers,and to explore the mixing mechanism.The current researches are mostly simulation studies,and lack of mutual verification of experimental and simulated results.In this paper,the flow and mixing characteristics of shear-thinning fluids in a simplified CTM model are studied through a combination of experiments and simulations.In this paper,the planar laser-induced fluorescence(PLIF)and particle image velocimetry(PIV)techniques are used to explore the flow characteristics of a pure shear-thinning fluid,and mixing and flow characteristics of two miscible shear-thinning fluids in an LDC.The quantitative experimental results obtained through visualization technology and the CFD simulation results verify the reliability and accuracy of the models(species transport model)used in the simulation.Through the validated model,the effects of different parameters on mixing two shear-thinning fluids are studied.These parameters include:the power law model parameters k and n,the liquid volume ratios,the lid speeds,the cavity aspect ratios,and the density differences.The effect of buoyancy(density difference)on the mixing process under different conditions were analyzed by the velocity fields,the intensity of segregation(IOS)and dimensionless groups:Re,Ar/Re,Ri.It provides an effective way for designing a better CTM for mixing and further studying the mixing mechanism.Experiments studies have shown that the pure shear-thinning fluids in the LDC have the trend of better flow characteristics as n increases,which is the same with literature results.The mixing efficiency of two miscible shear-thinning fluid in the LDC also increases with increasing n.The effect of k on mixing is weak and changing the value of k have little effect on mixing in the LDC.When there is a density difference between upper-layer and lower-layer fluids,the larger the value of k,the better the mixing efficiency.When the consistency index of the upper-layer and lower-layer fluids is different,placing the higher consistency index fluid in region I-the high shear rate region-makes the mixing efficiency better.The initial interface surface should be as close as possible to the position of the vortex center to strengthen the mixing of shear-thinning fluids while the upper-layer fluid and the lower-layer fluid are mixed with different volume ratios.The higher the lid speed,the worse the mixing efficiency when the lid moves the same displacement,but when the upper-layer and lower-layer of fluids have the density difference of 10 kg/m3,the lid speed has little effect on mixing.The smaller the aspect ratio of the lid-driven cavity,the better the mixing effect.When Re<1,Ar/Re can evaluate the effect of buoyancy on mixing.If Ar/Re is far greater than 1,buoyancy severely hinders mixing;while the opposite effect is observed when Ar/Re is not much different from 1.When Re>1,Ri can evaluate the effect of buoyancy on mixing.If Ri<1,buoyancy promotes mixing,while Ri>1,buoyancy weakens mixing.The larger the value of Ri,the stronger the weaken effect.