CFD Analysis of Viscous Models in the Transition Regime

Mixing is an essential operation in the manufacturing and processing industry. To optimize the mixing efficiency, it is crucial to understand what is going on inside the tank. In industrial mixing, the process is typically modeled in the laminar or turbulent regimes, as the equations governing these regimes are well understood. However, in a real mixing tank, several flow regimes exist simultaneously. At the impeller, turbulent flow is likely to exist. In the bulk and the surface of the tank, the fluid can be in laminar flow. Between the two extremes, there will be a wide range of Reynolds Numbers, all falling into the transitional flow regime. The goal of the work was to use Computational Fluid Dynamics (CFD) simulations to find the best fit model that most accurately depicts mixing in all flow regimes in a tank. CFD simulations are used to analyze the properties of a fluid and how it behaves while being mixed. In this case, we work to model the volume fraction distribution of dye that is released in the tank while being mixed at various speeds. By following the dye distribution, flow patterns become easier to visualize as well as analyze. The goal of this research was to find a viscous model that can accurately predict the mixing patterns of a non-Newtonian shear-thinning fluid, Carbopol, in the transition flow regime. By increasing the rotational velocity of the impeller, the flow regime goes from laminar to turbulent, as shown by the increasing Reynolds Number. After using a series of equations to find shear rate, shear stress, and viscosity, the necessary impeller speeds were determined to achieve laminar, laminar-transition, transition, transition-turbulent, and turbulent regimes. Using these speeds, the corresponding Reynolds Numbers were calculated. Other than the impeller speed, identical conditions for these regimes were used across the Laminar Viscous, Reynolds Stress, and k-kl-omega models to select the most accurate scenario. In the Laminar Viscous model, the transition between each of the five regimes was clear and smooth while the other two models either yielded no mixing patterns or images that were indistinguishable between regimes. After compiling images of the simulated tanks at various stages of mixing, the Laminar Viscous model was determined to show the best mixing patterns throughout the entire tank.