The Multiphase Reactions tab is used to define the reaction
properties for three different types of reaction rates. The
Arrhenius option uses a very similar implementation for
multiphase reactions as for single phase reactions. For details, see Arrhenius. Because reactants using Arrhenius reaction type must all
come from the same phase in this release of Ansys CFX, the temperature required to
calculate the Arrhenius rate comes from the single permitted phase. Additional
information on heat release is available in Heat Release/Heat Release Distribution.
Typical coal combustion reactions also require a model for char oxidation. Char oxidation requires higher temperatures than devolatilization and therefore occurs after devolatilization. In Ansys CFX, char oxidation is modeled either as a global reaction of order unity (Field) or using a simple analytic approach to the diffusion of oxygen within the pores of the char particles (Gibb).
The Mass Arrhenius option is the same implementation as
used for single-phase reactions. For details, see Arrhenius.
Theory documentation for this model is available in Field in the CFX-Solver Theory Guide.
In the field model, a char particle is considered to be a spherical particle surrounded by a stagnant boundary layer through which oxygen must diffuse before it reacts with the char. The oxidation rate of the char is calculated on the assumption that the process is limited by the diffusion of oxygen to the external surface of the char particle and by the effective char reactivity. Additional information on Heat Release is available in Heat Release/Heat Release Distribution.
Theory documentation for this model is available in Gibb in the CFX-Solver Theory Guide.
The alternative char
oxidation model, the Gibb model, takes into account the diffusion of oxygen
within the pores of the char particle. The parameters required for this model
include the void fraction 
of the char
particle, the particle volume/internal surface area ratio 
, the effective
internal diffusion coefficient 
of oxygen within the
pores, and the molar ratio 
of carbon atoms/oxygen molecules
involved in the oxidation processes. The molar ratio is determined by the
equilibrium, and the relevant input data is
provided by the values entered in the Char Product Ratio
area.
This option allows you to use a User Fortran subroutine to calculate the reaction rate of a multiphase reaction. General information on creating user routines is available in User Fortran.
In the
<install_dir>\examples\UserFortran
directory, you can find a reaction rate routine,
pt_reaction.F, and corresponding CCL template,
pt_reaction.ccl.
Heat release specifies how much heat is released during a multiphase reaction, and heat release distribution specifies how the heat is distributed over the participating phases. There are two principal methods that define the amount of heat released; it can either be calculated from the material reference enthalpies, or can be user specified.
If the Heat Release option is set to From
Material Properties, the heat release is calculated from material
reference enthalpies. When set to Specific Enthalpy, the
value of heat released per unit mass of reactants must be specified. Because
none of the reactants are known to Ansys CFX as the "fuel" component
at this stage, you must also select the parent phase and reference material, so
that the heat release is correctly related to the mass of the true fuel
material. The Latent Heat option is equivalent to the Specific
Enthalpy setting, except that the value for heat release is
specified with the opposite sign.
The From Material Properties option for Heat
Release is advantageous because the calculation guarantees the
conservation of energy (that is, if a fuel and oxidant react to form some
products by two separate competing reactions, then the amount of heat released
automatically is the same for both reaction paths). The disadvantage is that
correct reference specific enthalpies must be found and entered when the
material properties are defined. Reference specific enthalpies can be difficult
to determine, especially for materials that are not pure substances but mixtures
of inconsistent definition (such as coal). In such cases, it is much more
convenient to specify the heat release for the reaction. Heat releases for
reactions can be measured directly and are therefore usually available for a
given application. When specifying heat release directly, however, you must
ensure that you define consistent values for heat release in the case of
competing reactions or reaction paths.
The distribution of released heat to the participating phases is defined by specifying the fraction of total heat release that the individual phases receive. When setting the Heat Release Distribution parameters, the parent materials list must contain only materials that occur in reactants and/or products. For each phase selected, a value fraction of heat released must be specified (for example, for a gas phase receiving 25% of the heat released, the value would be 0.25). The total of all heat fractions should sum to unity.