Ansys Chemkin includes several unique capabilities for well mixed reactor modeling. First, the model allows for the description of plasma kinetics, where a system is characterized by more than one temperature (e.g. the electron temperature, the ion temperature, or a neutral gas temperature). In these cases, some reaction rates may depend on one temperature, while other reaction rates may depend on another. This capability is incorporated through the use of the multi-fluid Gas-phase Kinetics package, and should not impact users who are only interested in systems that are in thermal equilibrium. Secondly, the model considers reaction kinetics on multiple surfaces within the reactor. Balances of surface species and bulk material species determine the surface state as well as net etch or deposition rates. This capability requires some hierarchy of information about gas-phase, surface-phase, and bulk-phase information. Only one gas phase is allowed, while more than one surface phase or bulk phase may be defined for each material. The details of this hierarchy are described in the Surface State Variables .

Homogeneous 0-D reactor equations may address problems in both transient and steady-state environments. Even with steady-state equations, the computational algorithm often requires a partial solution of the related transient problem. Therefore, the transient conservation equations are presented here. We begin with global mass conservation in the reactor volume, where the time-rate of change of the mass in the reactor is equal to the difference between the mass flow in and the mass flow out, plus any material that is added to or subtracted from the surfaces within the chamber. This equations is stated as:

(8–1)

Here is the reactor number, is the mass density, is the reactor volume, is the inlet mass flow rate, and is the outlet mass flow rate. is the number of inlets for each reactor , while is the total number of reactor modules in the reactor network. is the fraction of the outflow of reactor that is recycled into reactor . The outlet mass flow differs from the sum of the inlet and recycled mass flow when deposition or etching of materials within the reactor occurs, as represented by the last term on the right-hand side. In this term, is the surface area of the m th material defined within the reactor, and is the molar surface production rate of the k th species on the m th material per unit surface area. There are gas-phase species and materials.

The time-dependent equation for mass conservation of each gas-phase species, including the implicit time dependence of through its dependence on the temperature and molecular weight, is

(8–2)

In Equation 8–2 , is the mass fraction of the k th species, is the molecular weight of the k th species, and is the molar rate of production of the k th species by gas-phase chemical reaction per unit volume. The superscript * indicates inlet stream quantities.

For steady-state conditions, the nominal residence time in the reactor can be related to the reactor volume and the inlet mass flow rate as follows:

(8–3)

where the mass density is related to the pressure, gas temperature and electron temperature through the multi-fluid ideal gas equation of state (see Gas Equation of State and Conversion Formulas ). The residence time is often used as a characteristic parameter of the reactor, rather than the mass flow rate for steady-state flow. In this case, can be calculated from a specified residence time using Equation 8–3 . Alternatively, the effective volume can be calculated given specified values of both the residence time and the mass flow rates, also using Equation 8–3 . If the mass flow rate is zero, then the reactor may not be characterized by residence time.