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23.8 Modeling Species Transport in Multiphase Flows

Species transport, as described in Chapter  14, can also be applied to multiphase flows. You can choose to solve the conservation equations for chemical species in multiphase flows by having FLUENT, for each phase $k$, predict the local mass fraction of each species, ${Y_i}^k$, through the solution of a convection-diffusion equation for the $i^{\rm th}$ species. The generalized chemical species conservation equation (Equation  14.1-1), when applied to a multiphase mixture can be represented in the following form:


 \frac{\partial}{\partial t} (\rho^q \alpha^q {Y_i}^q) + \nab... ...( {\dot{m}_{\rm p^i q^j} - \dot{m}_{\rm q^j p^i}} ) + {\cal R} (23.8-1)

where ${R_{i}}^q$ is the net rate of production of homogeneous species $i$ by chemical reaction for phase $q$, $\dot{m}_{\rm q^j p^i}$ is the mass transfer source between species $i$ and $j$ from phase $q$ to $p$, and ${\cal R}$ is the heterogeneous reaction rate. In addition, $\alpha^q$ is the volume fraction for phase $q$ and ${S_{i}}^q$ is the rate of creation by addition from the dispersed phase plus any user-defined sources.

FLUENT treats homogeneous gas phase chemical reactions the same as a single-phase chemical reaction. The reactants and the products belong to the same mixture material (set in the Species Model panel), and hence the same phase. The reaction rate is scaled by the volume fraction of the particular phase in the cell.

The set-up of a homogeneous gas phase chemical reaction in FLUENT is the same as it is for a single phase. For more information, see Chapter  14. For most multiphase species transport problems, boundary conditions for a particular species are set in the associated phase boundary condition panel (see Chapter  23.9.8), and postprocessing and reporting of results is performed on a per-phase basis (see Section  23.15).

For multiphase species transport simulations, the Species Model panel allows you to include Volumetric, Wall Surface, and Particle Surface reactions. FLUENT treats multiphase surface reactions as it would a single-phase reaction. The reaction rate is scaled with the volume fraction of the particular phase in the cell. For more information, see Chapter  14.

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To turn off reactions for a particular phase, while keeping the reactions active for other phases. turn on Volumetric under Reactions in the Species Model panel. Then, in the Materials panel, select none from the Reactions drop-down list.

The species of different phases is entirely independent. There is no implicit relationship between them even if they share the same name. Explicit relationships between species of different phases can be specified through mass transfer and heterogeneous reactions. For more information on mass transfer and heterogeneous reactions, see Section  23.9.7 and Section  23.9.6, respectively.

Some phases may have a fluid material associated with them instead of a mixture material. The species equations are solved in those phases that are assigned a mixture material. The species equation above is solved for the mass fraction of the species in a particular phase. The mass transfer and heterogeneous reactions will be associated with the bulk fluid for phases with a single fluid material.




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