Numerical Simulation Of Chaotic Mixing In A Stirred Tank

Stirred tanks are typical process equipment and are widely used in engineering. When the fluid in stirred tanks are turbulent, the fluid can mixing rapidly. But in many cases, the fluid in the stirred tanks are laminar and mixing velocities are slow, so the mixing efficiency are slow.These years chaotic mixing is regarded as an effective method to improve the mixing effect in the laminar stirred tank . Many people have proved that if Re<500 there are two isolated mixing regions in stirred tanks which are the worst obstacle to mixing. Then it's very important to find methods to destroy the isolated mixing regions and will be significant in engineering.Lagrangian tools, developed from dynamical systems theory ,are applied to the stirred tank. Mixing in the stirred tank, for Reynolds numbers below 100,is characterized using Lagrangian particle tracking and stretching calculations. Poincare sections and stretching calculations indicate that the laminar flow in the stirred tank with the use of time-dependent RPM produces a globally chaotic flow.Poincare sections are created by tracking 100 particle in the flow for 1000 periods and plotting their positions at the end of each period on a single gragh. Poincare sections of the flow reveal a complex internal structure consisting of small cluster of points in the form of islets inside the segregated regions surrounded by a cloud of apparently randomly scattered points.Stretching fields are generated by tracking the stretching experienced by 10,000 fluid particles uniformly placed on a vertical plane located atθ=0°, Stretching is calculated by considering a fluid element represented by an infinitesimal vector I. This vector is located at an arbitrary position in the flow domain and it is stretched as it is convected by the flow. Stretching (A) is defined as the ratio of the magnitude of the vector 1 at time t to its initial magnitude. The stretching experienced by fluid elements placed throughout the flow has also been calculated and stretching distributions generated display a "hump" of low stretching values characteristic of the segregate regions are found to increase only linearly.We use the UDF software of CFD software package FLUENT to make model. FLUENT solve the conservation equations to determine particle tracking and stretching calculations. Through the Poincare section and stretching distribution, we can determine the position, magnitude and shape of the isolated mixing regions. These analysis can contribute to the field analysis, structure optimization and the improvement of the mixing efficiency in stirred tanks.