In addition to chemically reacting flow applications, Ansys Chemkin includes an Equilibrium Reactor model. This model allows users to determine the chemical state of a mixture under equilibrium conditions. Any number of gas-phase or condensed (bulk) species can be included in an equilibrium calculation, while surface site species are ignored. In this way, the Equilibrium Reactor model can be used to determine phase equilibrium, between gas and condensed phases, as well as chemical equilibrium. All that is required is thermodynamic data for all species in each phase.
An established method for evaluating chemical equilibrium is the element-potential method embodied in the Stanford software STANJAN [51]. The Ansys Chemkin Equilibrium Reactor employs the STANJAN library of routines in it’s solution method. The equilibrium determines composition equilibrium and/or phase equilibrium. The results depend only on the thermodynamic properties of the species in the user’s chemistry set, as well as the starting composition and conditions specified. The starting composition determines the relative amount of chemical elements in the system. An initial estimate of the equilibrium temperature can sometimes be used to select a "burned" equilibrium state from an "unburned" equilibrium state in the case where two equilibrium states are possible.
Currently, the equilibrium program assumes that the gas-phase is a mixture of ideal gases and that condensed phases are ideal solutions. The user selects atomic populations through identity of initial species and their fraction in each phase, as well as the state parameters.
The user may specify the state parameters in a number of different ways, including
temperature and pressure;
pressure and entropy;
enthalpy and pressure; and
volume and entropy.
Species composition can be "frozen" in a given calculation, or the equilibrium composition can be determined. Calculations may be linked through continuations, such that the conditions calculated from a previous equilibrium case can be used as the starting point for a subsequent case with different constraints. In this way, the user can employ the Equilibrium Reactor Model to analyze stages in a thermodynamic cycle.
The Equilibrium Reactor Model is also commonly used to determine adiabatic flame temperatures for combustible gas mixtures. Such a simulation is performed by specifying an initial (reagent) gas mixture and constraining equilibrium for constant enthalpy (adiabatic) and constant pressure. The calculation can also be performed using constant internal energy and constant volume. An initial guess for the equilibrium temperature of ~1000 K or above is usually needed to cause the equilibrium solver to find the burned-gas solution. For accurate adiabatic-flame temperature calculations, it is important to include all radical species that might occur in the flame, as well as stable reactants and products.
In the remainder of this chapter, we describe the equations solved and the methodology used for determining chemical and phase equilibria of arbitrary systems.