(457c) Implementation of a Flux-Dependent Anisotropic Diffusivity Model into Resolved-Particle CFD

Implementation
of a flux-dependent anisotropic diffusivity model into resolved-particle CFD

Behnam Partopour, Anthony G. Dixon

Department of Chemical Engineering, Worcester
Polytechnic Institute, Worcester, MA

Species
diffusion inside catalyst particles is often modeled using an effective
diffusivity, which can be derived from the Dusty Gas Model. Conventional
assumptions in these models are 1- the diffusion is isotropic, 2- the ratio of
species fluxes in the computational domain is fixed and equal to their
stoichiometric ratio. In this study, we examine the accuracy of these
assumptions, specifically for 3-dimensional resolved-particle CFD models. A new
approach is developed to calculate a vector of fluxes for each species in each
computational cell. Then the diffusivity tensor components are calculated and
passed to the solver. The iterative approach continues until the diffusivity
coefficients in each computational cell become constant. To make the model
computationally feasible an appropriate transformation is used to change the
anisotropic (full) diffusivity tensor to orthotropic (diagonal) tensor.

The method is
implemented for resolved-particle CFD simulations of ethylene oxidation in
small packed beds of spherical and cylindrical particles. The results show that
the effects of the flux changes in the particles are considerable and reaction,
species and temperature profiles are affected.

Figure.
Temperature and ethylene profiles on an arbitrary radius of a selected
spherical particle. Blue: flux dependent anisotropic model; Red: fixed flux
isotropic model.