Modeling Phase Thermodynamics in Cantera

Here, we describe some of the most commonly-used phase models in Cantera.

Bulk, Three-Dimensional Phases

Ideal Gas Mixtures

Far and away, the most commonly-used phase model in Cantera is the ideal gas model. Many combustion and CVD simulations make use of reacting ideal gas mixtures. The Cantera ideal gas model allows any number of species, and any number of reactions among them. It supports all of the options in the widely-used model described by Kee et al. 1, plus some additional options for species thermodynamic properties and reaction rate expressions.

Ideal gas mixtures can be defined in the legacy CTI format using the ideal_gas() entry, or in the YAML format by specifying ideal-gas in the thermo field.

Stoichiometric Solid

A stoichiometric solid is one that is modeled as having a precise, fixed composition, given by the composition of the one species present. A stoichiometric solid can be used to define a condensed phase that can participate in heterogeneous reactions. (Of course, there cannot be homogeneous reactions, since the composition is fixed.)

A stoichiometric solid can be defined in the legacy CTI format using the stoichiometric_solid() entry, or in the YAML format by specifying fixed-stoichiometry in the thermo field.

Interfaces

Cantera presently implements a simple model for an interface between phases that treats it as a two-dimensional ideal solution of interfacial species. There is a fixed site density \(n^0\), and each site may be occupied by one of several adsorbates, or may be empty. The chemical potential of each species is computed using the expression for an ideal solution:

\begin{equation*} \mu_k = \mu^0_k + RT \log \theta_k, \end{equation*}

where \(\theta_k\) is the coverage of species \(k\) on the surface. The coverage is related to the surface concentration \(C_k\) by

\begin{equation*} \theta_k = \frac{C_k n_k}{n^0} , \end{equation*}

where \(n_k\) is the number of sites covered or blocked by species \(k\).

An interface can be defined in the legacy CTI format using the ideal_interface() entry, or in the YAML format by specifying ideal-surface in the thermo field.

References

1

R. J. Kee, F. M. Rupley, and J. A. Miller. Chemkin-II: A Fortran chemical kinetics package for the analysis of gasphase chemical kinetics. Technical Report SAND89-8009, Sandia National Laboratories, 1989.