Numerical Simulation Of The Flow Field And Mixing Process In The Reciprocating Stirred Vessel

Stirred vessels are widely used in industry mixing process. The mixing efficiency is strongly influenced by the hydrodynamic and mixing characteristics, which depends on the impeller configuration. With the rapid development of computer and CFD technique, substantial research work has been done on the inner flow field in the stirred vessels—which in many instances are under turbulent conditions—during the past decades by means of experiments and numerical simulation, and great achievements have been made. But all these researches are focused on the modification or development of the stirred impellers, and no work is done on the motion form of the impeller. Take this point into account, we intend to substitute reciprocation for rotation, and make a deep research into the reciprocating stir.In this paper, studies have been complemented by the application of advanced numerical modeling techniques based on computational fluid dynamics(CFD) to provide a more detailed insight into the complex hydrodynamic and mixing process in a cost—effective way. A three dimensional model,after being created in the preprocessor GAMBIT with three sets different mesh density,is exported to FLUENT and simulations have been carried out on it.A standard k-ε turbulent model and user—defined function(UDF) are used to predict the flow generated by the impeller.And the "dynamic mesh" technique is used to regulate the motion of the mesh.During the simulation,We solve the momentum equations and the species mass—conservation equations separately.And the overall flow field in the vessel,axial velocity,radial velocity,tangential velocity and tracer concentration distribition with time— which indicates the process of tracer's transportion—in middle vertical plane,as well as the mixing efficiency of the reciprocating stirred vessel are acquired.Based on the numerical simulation,the theoretical analysis of flow field and mixing process is given.We come to the conclusion that reciprocating stirrer is one kind of radial flow stirrer.The flow field generated by impeller is highly complicated turbulent ,and is approximately symmetrical by the stirred axis in the reciprocating direction.On the average,the velocity of flow field in the upper zone of the tank is bigger than that of the below zone,which is dominated by radial flow. The velocity of fluid becomes bigger as it reaches the wall of the tank.And the flow is mainly tangential in the zone from the impeller to the free surface.As a result of the flow field,the mixing efficiency depends much on the tracer feeding point. When the tracer is fed form the free surface,the mixing efficiency of the reciprocating stirrer is higher than that of the standard DT-6 impeller;If we feed the tracer from the below, the mixing efficiency is much lower than that of DT-6.In contrast,no matter where we feed the tracer,the calculation of the mixing efficiency both demonstrate that reciprocating stirrer is no better than the CBY impeller.