Before discussion of the theoretical background for reacting-flow simulations, it is important to introduce the concepts of phases, and the species that reside in those phases and take part in the chemical reactions of the system. The Ansys Chemkin phase nomenclature builds from that of Eriksson, [2] but has been extended to account for surface sites. Currently, Chemkin defines three types of species: gas-phase, surface, and bulk. In the gas phase, a species is part of an ideal-gas mixture that may be flowing or stagnant. When reactive surfaces are present, the gas phase is considered to be "above" the surface. A bulk species is one that resides in a solid or condensed material, or "below" the surface. A surface species resides at the bulk-gas interface or is defined to be a chemical species on the top-most layer of a solid or condensed material. Each surface species occupies one or more "sites," where the total number of sites is usually conserved in the system. Only gas-phase species participate in gas-phase reactions, but all types of species may participate in heterogeneous gas-surface reactions. In writing elementary reactions for a surface mechanism in a kinetic model, mass, elemental composition, and charge must all be conserved.
Note: In actuality there is no constraint that the surface must be only one atom thick. However, defining a "surface" that is several monolayers thick may be conceptually much more difficult to deal with.
Beyond the basic gas, surface, and bulk species, we also use the notion of different physical "phases" to group the chemical species in a problem. For example, there can be more than one type of site on the surface. This may be used to specify that a surface consists of "ledge" sites and "plane" sites, where the number of sites of each type might be a characteristic of the solid crystal face. Alternatively, a particular site type may consist of "active" sites vs. "non-active" sites, where different reaction rates are defined on each type. There can be any number of site types. The user may define a species that only resides on a certain type of site. For example, the thermodynamic properties of a hydrogen atom on a ledge site might be different from a hydrogen on a plane site, and they could be specified as different species (even though their elemental composition is the same). The population of different species occupying a given type of site is specified by site fractions. The sum of the site fractions of the species on a given site is 1. (Thus an "open site'' is considered as a distinct species.) This general formalism provides the user extensive flexibility in defining surface chemistry problems, whether they involve material deposition, surface catalysis, or simple radical recombination.
In the bulk there can also be different types of bulk species. The simplest bulk phase consists of a pure (single) species. There can be any number of pure bulk phases. It is also possible to specify a bulk mixture with components A and B, for example. The composition of the bulk phase may then be specified using the " activities" of each of the bulk-phase components.
Note: For gas-phase and surface site species, the mole fractions of the species correspond directly to the species activities. In general this is not true for condensed-phase mixtures unless the bulk phase is either an ideal mixture or a pure bulk species.
When defining problems in Ansys Chemkin, a user may define a chemistry set that is gas-phase only. In all cases one or more gas-phase species must be defined, but problems may be defined that include no gas-phase reactions. When surface chemistry is included, it may include only surface species, only bulk species, or both surface site and bulk species. Reactions may involve any combination of species from different phases, as long as conservation rules are obeyed.