CFD SIMULATION OF MIXING VESSEL

Abstract

Mixing is an ancient art, which, despite being practiced for thousands of years, has often been taken for granted. Mixing as a discipline has evolved from foundations that were laid in the 1950s, culminating in the publication of works by Uhl and Gray (1966) and Nagata (1975). Over the last 30 years, many engineering design principles have been developed, and design of mixing equipment for a desired process objective has become possible. Many of the recent advances are the result of computerized measurement and simulation techniques that are now everywhere in laboratories throughout the world. A fluid flow in Stirred tank fitted with rotating impeller is simulated using Computational Fluid Dynamics in the present investigation. A special emphasis is devoted to the study of the influence of rotation speed, effect of clearance from bottom, with baffles as well as without baffles on flow field of a single phase liquid in a vessel equipped with Rushton impeller. A Rushton turbine, a six-flat-bladed disc impeller is considered in the present study as it is a commonly used impeller type for which extensive experimental data is available, so that comparison between simulation and experimental results could be made to check the accuracy of the CFD simulation. A multiple reference frames (MRF) model is used to model the impeller. This is a more up-to-date technique compared to the rotating frame (RF) method that was used by many previous researchers. When using the rotating reference frame approach, the stirring vessel geometry has to be split into a stationary and rotating part, and position of interface between both regions is nearer to impeller swept region in order to avoid numerical errors, leading to numerical approximations at the interface. The CFD simulation is performed in a 3D domain using commercial code FLUENT. The results obtained from present study include full field values of, mean radial velocity, mean axial velocity and mean tangential velocity, kinetic energy of turbulence, k, and its turbulent energy dissipation rate, e. Simulation results obtained using CFD agrees well with the experimental work of Wu and Patterson et al. (1989) and are within -8.5 % error for Normalized radial pumping capacity. The numerical results are also compared with other published iii experimental and CFD results and a reasonable agreement is also obtained. The tank is divided into two part impeller swept region and fluid region (remaining part) for which volume integral of turbulent energy dissipation, e and kinetic energy of turbulence, k have been evaluated. Baffled tanks provide more mixing (38.8%) as compared to un-baffled tanks. The impact of factors such as RPM, baffles and clearance from the tank bottom is also investigated. It is found that the rotation speed and baffles have significant impact on the level of volume integral of turbulent energy dissipation, e as far as impeller swept- and fluid- regions are considered. The impact of the clearance is found to be insignificant.