Modeling Phases 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
Many combustion and CVD simulations make use of reacting ideal gas mixtures. These can be defined
ideal_gas() entry. 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.
An example of an
ideal_gas() entry is shown below:
ideal_gas(name='air8', elements='N O Ar', species='gri30: N2 O2 N O NO NO2 N2O AR', reactions='all', transport='Mix', initial_state=state(temperature=500.0, pressure=(1.0, 'atm'), mole_fractions='N2:0.78, O2:0.21, AR:0.01'))
This entry defines an ideal gas mixture that contains 8 species, the definitions of which are
imported from dataset
gri30.xml). All reactions defined in the file are to be
included, transport properties are to be computed using the mixture-averaged rule, and the state of
the gas is to be set initially to 500 K, 1 atm, and a composition that corresponds to air.
Two transport models are available for use with ideal gas mixtures. The first is a multicomponent
transport model that is based on the model described by Dixon-Lewis 2 (see also Kee et al.
3). The second is a model that uses the mixture-averaged rule. To select the
multicomponent model, set the transport field to the string
'Multi', and to select the
mixture-averaged model, set it to the string
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.)
stoichiometric_solid(name='graphite', elements='C', species='C(gr)', density=(2.2, 'g/cm3'), initial_state=state(temperature=300.0, pressure=(1.0, 'atm')))
In the example above, the definition of the species
'C(gr)' must appear
elsewhere in the input file.
A stoichiometric liquid differs from a stoichiometric solid in only one respect: the transport manager computes the viscosity as well as the thermal conductivity.
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:
where \(\theta_k\) is the coverage of species \(k\) on the surface. The coverage is related to the surface concentration \(C_k\) by
where \(n_k\) is the number of sites covered or blocked by species \(k\).
The entry type for this interface model is
ideal_interface(). Additional interface
models may be added to allow non-ideal, coverage-dependent properties.
Defining an interface is much like defining a phase. There are two new fields:
phases field specifies the bulk phases that
participate in the heterogeneous reactions. Although in most cases this string
will list one or two phases, no limit is placed on the number. This is
particularly useful in some electrochemical problems, where reactions take place
near the triple-phase boundary where a gas, an electrolyte, and a metal all meet.
site_density field is the number of adsorption sites per unit area.
Another new aspect is in the embedded
state() entry in the
initial_state field. When specifying the initial state of an interface, the
state() entry has a field
coverages, which can be assigned a string
specifying the initial surface species coverages:
The State Entry¶
The initial state of either a phase or an interface may be set using an embedded
state() entry. Note that only one of (
density) may be
specified, and only one of (
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.
G. Dixon-Lewis. Flame structure and flame reaction kinetics, II: Transport phenomena in multicomponent systems. Proc. Roy. Soc. A, 307:111—135, 1968.
R. J. Kee, M. E. Coltrin, P. Glarborg, and H. Zhu. Chemically Reacting Flow: Theory and Practice. 2nd Ed. John Wiley and Sons, 2017.