In hypersonic flows, the recombination of atoms is usually dominant at the wall. The atoms that diffuse to the wall can either reflect from the wall or recombine into molecules. The Ansys Fluent density-based solver offers the partially catalytic boundary condition to model this phenomenon for Earth atmospheric flights [103].
 | (17–7) |
where 
is the Boltzmann constant, 
and 
are the mass of O and N, 
is the diffusion coefficient, 
is the wall temperature, 
and 
are the recombination coefficients for O and N.
is defined as the fraction of atoms impinging on the wall that recombine. As
described in Figure 17.21: Illustration of Partially Catalytic Wall, a 
value of zero results in a non-catalytic wall boundary condition, a value of
one results in a fully-catalytic wall boundary condition, while a value between zero and one
results in applying a partially catalytic wall boundary condition.
Fluent offers two methods to compute the recombination coefficient:
 | (17–8) |
 | (17–9) |
 | (17–10) |
The basic steps for using the partially catalytic boundary condition at the wall are listed below:
As a pre-requisite, you must have the density-based solver enabled and one of the following selected for Mixture Material in the Species Model dialog box (as shown in Figure 17.22: The Species Model Dialog Box with Air Mixture):
air-2species-nitrogen,air-5species-park93,air-11species-park93, orair-11species-gupta.For each wall where you want to apply the partially catalytic boundary condition, select Partially Catalytic Wall under the Species tab as shown in Figure 17.23: The Wall Dialog Box with Partially Catalytic Wall.
Select one of the following options:
Recombination Coefficients For General Materials
Using this option, you must specify the Recombination Coefficient for n and Recombination Coefficient for o directly (
and 
in Equation 17–7, respectively).Recombination Model for Thermal Protection System
With this option, experimental data is used to obtain the temperature-dependent coefficients for the thermal protection system of a space shuttle. Three empirical formulations are available as shown in Figure 17.24: The Wall Dialog Box with Partially Catalytic Wall for Thermal Protection System:
Kolodziej (Air):
and 
in Equation 17–8.Zoby (Air):
and 
in Equation 17–9Scott (Air):
and 
in Equation 17–10.
