To compute the heterogeneous phase interaction rates, a modified Arrhenius type rate expression is provided, which you can define using the graphical user interface (see Specifying Heterogeneous Reactions in the User's Guide), or a user-defined function (see DEFINE_HET_RXN_RATE in the Fluent Customization Manual).

The rate expression is in the general form:

(14–687)

where

(14–688)

Here, is the mass fraction of reactant species in the phase , is the total number of reactants in a given inter-phase reaction, is the bulk density of the phase , is the volume fraction of the phase , the molecular weight (kg/kmol) of the reactant species in the phase , is the rate exponent, and is the rate constant.

The rate constant in the modified Arrhenius form is given by Equation 14–688. is the phase temperature (phase-1, phase-2, and so on) as required by the expression. Make sure you provide the correct phase for the temperature to be extracted. is the normalization temperature. Usually, is set to unity for most reactions, but certain reaction rate constants may have a given value, which is normally 298.15 K.

The reaction rate given in Equation 14–688 will be valid above the kick-off temperature specified by you, where the temperature for the rate calculation is taken from the selected phase.