tailieunhanh - Advanced Topics in Mass Transfer Part 8

Tham khảo tài liệu 'advanced topics in mass transfer part 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Simulation of Hydrodynamics and Mass Transfer in a Valve Tray Distillation Column Using Computational Fluid Dynamics Approach 269 where Kpq Kqp is the interphase momentum exchange coefficient and it tends to zero whenever the primary phase is not present within the domain. In simulations of the multiphase flow the lift force can be considered for the secondary phase gas . This force is important if the bubble diameter is very large. It was assumed that the bubble diameters were smaller than the distance between them so the lift force was insignificant compared with the other forces such as drag force. Therefore there was no reason to include this extra term. The exchange coefficient for these types of gas-liquid mixtures can be written in the following general form aap f Kpq T 6 where f and Tp are the drag function and relaxation time respectively. f can be defined differently for the each of the exchange-coefficient models. Nearly all definitions of f include a drag coefficient that is based on the relative Reynolds number. In this study the basic drag correlation implemented in FLUENT Schiller-Naumann was used in order to predict the drag coefficient. In comparison with single-phase flows the number of terms to be modeled in the momentum equations in multiphase flows is large which complicates the modeling of turbulence in multiphase simulations. In the present study standard k - s turbulence model was used. The simplest complete models of turbulence are two-equation models in which the solution of two separate transport equations allows the turbulent velocity and length scales to be independently determined. Eeconomy and reasonable accuracy for a wide range of turbulent flows explain popularity of the standard k - s model in industrial flow and heat transfer simulations. It is a semi-empirical model and the derivation of the model equations relies on phenomenological considerations and empiricism FLUENT Users Guide 2005 . The equations k and s that describe