Numerical Simulation And Experimental Study On The Flow Field In Tank Stirred By Unsteadily Rotational Impeller

Stirred tanks are widely used in petrochemical, bioengineering, pharmaceutical and several other industries. Numerical simulation and experimental studies on flow fields in stirred tanks show that, when high-viscosity fluids and other delicate materials are involved, mixing can only be carried out in laminar conditions, where the flows are usually characterized by the existence of isolated mixing regions (IMRs) and it is usually difficult to achieve efficient mixing state. With the rapid development of chaotic and nonlinear theories, chaotic mixing can be utilized to destroy the IMRs and improve the mixing effect. Unsteady agitation can induce chaotic mixing, has broad prospects for industrial applications. The experimental studies of flow fields in stirred tanks have been carried out for several decades, but the experimental apparatus used for flow field measuring are always expensive.In the paper, the numerical simulation and experimental studies were done to reveal the flow field and mixing performace in stirred tank under unsteady agitation. The main content of the paper is as follows:The characteristics of the fluid flow in a stirred tank driven by three-bladed helical agitator were studied, and the variation of IMRs with Reynolds number were illustrated. According to the CFD method, considering the complexity of the impeller motion, the simulation model of the unsteady agitation was created, and the dynamic meshing technique was adopted to simulate the flow and mixing characteristics in the stirred tank under unsteady agitation. The influence of unsteady agitation on the IMRs were discussed by analyzing the flow field, concentration profile of the tracer, mixing time as well as the pathlines, and factors that are influential in unsteady agitation were summarized. The acid-base neutralization experiments were also performed to validate the numerical results. The results show that three-bladed helical agitator is one kind of a radial flow pattern impeller with high circulation ability. The velocity fluctuations in the vicinity of the agitator are larger than other regions. Under steady agitation conditions, there are two IMRs located above and below the agitator. The IMRs were hard to destroy and existed for a long time. When the Reynolds number increased, IMRs were far away from impeller along the radial direction and close to the impeller along the axial direction. Whereas, when operated under unsteady agitation conditions, IMRs disappeared quickly and the mixing process were greatly shortened. At the identical agitation period, a larger amplitude of fluctuation in impeller rotational speed always results in a better mixing effect. Under the same amplitude of fluctuation in rotational speed, the mixing effect is better when the impeller rotational speed is higher. This is in good agreements with the experimental results.