Numerical Simulation Of Fluid Flow In Stirred Tank Reactors Using CFD Method

Stirred tank reactors are extensively used in a variety of industries like the paper and pulp, pharmaceutical, fine chemicals, food industry, petroleum chemical engineering, pharmacy and biochemical engineering etc. In designing such reactors, understanding the fluid dynamics is critical particularly. Untill recently, dimensionless numbers and empirical correlations have been used as tools for design, scale-up and operation of stirred tanks. Since the turbulent three-dimensional flow of single or multiphase fluid are complex in stirred tanks, these dimensionless numbers and empirical relations are clearly inadequate. Some researchers have given some simplified mathimatical models , but these models intensively depend on experimental data. To fully understand the processes occurring in stirred tanks, mathematical modeling of local hydrodynamic behavior are desired in depth. This thesis is devoted to the above tasks on the basis of computational fluid dynamics (CFD ). In accordance with the anisotropic feature of turbulent flow, an anisotropic reynolds stress model (RSM) is adopted to predict the turbulent flow field and turbulent characteristics generated by a Rushton disc turbine with the sliding mesh iterative procedure. The discretization scheme of diffusion term has an important effect on the prediction of turbulent parameters. QUICK discretization scheme shows good ability to accelerate convergence and improve precision. The predicted turbulent flow is compared with experimental data and the simulation results by standard k ? ε, RNG k ? εand SSTk ? ωturbulence models. The RSM model are found to give beter prediction than the other isotropic turbulence models. The distribution of turbulent parameters, vorticity magnitude, helicity and shear rate in the stirred tanks are simulated with the established numerical procedure. Accurate interfacial models are needed in the simulation of the two -phase flow in a gas-liquid stirred tank based on the two-fluid model. The virtual mass force is considered. In order to choose a reasonable drag force equation, investigate the effect on predicting the mean velocity of gas and liquid and the gas fraction by diferent drag coefficien -t equation. Multiple reference frames (MRF) is used to model the action between baffles and impeller. The qualitative and quantitative distribut -ion of gas fraction at different gas flow rate show good approach of the reported experimental data. In order to predict the turbulent transport characteristics of gas bubbles in stirred tanks, a discrete phase model is adopted to calculate the gas bubbles tracks. The discrete random walk model is improved suitably. For gas bubbles, a convenient representation of the gas bubbles size distribution is the Rosin-Rammler equation. The complete range of sizes is divided into an adequate number of discrete intervals, each represented by a mean diameter for which trajectory calculations are performed. Results show that the particle tracking method is very successful in simulating gas flow patterns in dispersed, loaded, and flooded conditions at a given impeller speed for Rushton turbine and down pumped pitched-blade impeller. Results of simulation have shown good agreement with photographs and the simulation results by two-fluid model.