CFD Analysis of Mixing in the Transition Regime: User-Defined Functions and Viscosity Variations | AIChE

CFD Analysis of Mixing in the Transition Regime: User-Defined Functions and Viscosity Variations

Mixing is a common process used in the manufacturing and processing industry. To optimize the mixing efficiency, it is critical to understand what is going on inside the tank. In a physical 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 process for industrial mixing is typically modeled in the laminar or turbulent regimes, as the equations governing these regimes are well understood1-2. Due to the complexity of transition mixing, there is minimal modeling done in the regime despite it being a common working regime for some industries. The lack of accurate transition mixing models hinders these industries in fully understanding their processes1. Computational Fluid Dynamics (CFD) is used in modeling various physical systems to analyze velocity profiles, streamlines, volume fraction distribution, and numerous other fluid and flow properties. In this case, ANSYS Fluent, a commercial CFD software package, was used to model the volume fraction distribution of dye that is released in the tank while being mixed. Inaccuracies of transition mixing models may arise from the inability of CFD to predict large scale instabilities in the flow domain. To circumvent this issue, multiphase transient simulations of the mixing of Carbopol were conducted to study the transition mixing region with higher accuracy than before. Carbopol is a non-Newtonian, shear-thinning fluid whose viscosity profile can be altered with change in pH and/or concentration. Two different viscosities, 7500 cP and 3350 cP, were used to investigate their dependence on flow patterns. Different mixing regimes were achieved by changing the speed of the impeller. Two turbulent models, the Transition Shear-Stress Turbulence Model and the Spalart-Allmaras Model, were used to model the mixing in the tank and compared for analysis. Both models accurately depicted the behavior of the Carbopol at high Reynolds numbers, as expected from the two turbulent RANS models. A user-defined function (UDF) was then created from the Carreau-Yasuda shear-thinning viscosity model to better represent the dispersion of dye inside the mixing tank in the transition regime. The UDF successfully modeled the shear-thinning of the Carbopol throughout the entire mixing tank.