Numerical Study On Flow And Heat Transfer Of Non-newtonian Fluid In Stirred Tank

Mechanical stirred tank is a key equipment in many industrial production processes.Usually,the flow dynamics,heat and mass transfer processes often are crucial for the quality and cost of production made inside the tank.The material property plays an important role in designing and operating of the stirred tank.In the real process,the composition of the materials mixed in the tank are complex,mostly non-Newtonian fluids.Non-Newtonian fluids have complex rheological properties,which in turn affects the process of flow dynamics,and heat and mass transfer in the tank.The conventional design methods are not applicable any more.Meanwhile,the traditional experimental methods are imitated as well.In current work,Computational Fluid Dynamics simulations are employed to systematically analyze the flow and heat transfer processes of nonNewtonian fluids in the stirred tank.The effects of key factors are discussed in detail,including the rheological parameters,paddle type,and paddle rotating speed.In addition,the influence of the type of paddle on the mixing process of the non-Newtonian fluid in the tank is studied.The main contents are summarized below:1.The flow and heat transmission parameters in the stirred tank were thoroughly investigated.The results reveal that the viscosity distribution in the tank is inversely proportional to the local shear rate,with considerable variances in viscosity between various locations.The interactions among the flow behavior,heat transfer,and viscosity occurs to non-Newtonian fluids the tank.2.The effects of consistency factor and rheological index on non-Newtonian fluid flow and heat transfer in a stirred tank were investigated.The results show that the increase in the consistency factor causes an overall increase of the viscosity of the medium in the tank,but the distribution of the velocity and shear rate of the medium in the tank are less influenced.In comparison,the rheological index has a more obvious effect on the shear rate,overall viscosity,and velocity distribution of the medium;the larger the rheological index,the more uniform the viscosity distribution tends to be,with the overall viscosity increases.The increase in the consistency factor and rheological index leads to a lower heat transfer coefficient on the coil’s outer surface decreases and a higher stirring power.As the consistency factor and rheological index continue to increase,the changes in heat transfer coefficient will gradually decrease.For the stirring energy consumption,the stirring power will increase significantly when the rheological index reaches a certain value.3.Secondly,the impact of the paddle type on the fluid flow and heat transfer process in the tank was investigated.The results show that the maximum-blade paddle（MB）has the best mixing and heat transfer performance,followed by the disc turbine propeller with six arc blades（HY6）And,the paddle with six inclined blades（PBT6）and the disc turbine propeller with six inclined blades（ZY6）have similar and the worst effect.In addition,the mixing power of MB consumed several times of that of the other three types of paddles,followed by HY6,which also consumed about twice as much mixing power as the remaining two paddles.The rest two types of paddles consume the lowest values of mixing power.4.The effect of the paddle speed on the flow and heat transfer process in the tank was analyzed as well.It’s found that at lower rotating speeds,the flow pattern of the fluid is more uniform,and the viscosity of the fluid is lower only in the stirring and mixing zone.While the paddle speed is increased to 150 RPM,the flow pattern of the fluid in the tank becomes complicated and the viscosity of the fluid becomes lower.With the increasing paddle speed,both the stirring power and the convective heat transfer coefficient on the outer surfaces of the coil rise,but the increment of the former gradually decreases and the increase of the latter gradually increases.5.Additionally,the influence of the type of paddle on the mixing process in the tank was investigated.The results show that the MB has the best mixing performance with the shortest mixing time,followed by the PBT6 and ZY6.PBT6 and ZY6 have similar mixing performance with a difference of about 5 s in mixing time.In terms of mixing effectiveness,HY6 is second only to MB,but HY6 requires the longest mixing time.