In Ansys Chemkin, we allow the user to select either pressure or density, temperature(s), and either mass fraction, mole fraction, or molar concentration, to describe the state of a gas mixture. In other words, to define the state of a gas, one variable must be selected from each column of the array below.

(2–1)

In making these options available from among the many possible, we provide combinations of variables that are natural ones for a wide class of chemically reacting-flow problems. For example, pressure is a natural choice in situations where pressure is fixed, and density is a natural variable where volume is fixed. Moreover, density is a natural variable in many problems involving fluid mechanics because it is determined directly from the mass continuity equation. Temperature is always taken as a natural variable because the thermodynamic properties and the chemical rate constants both depend directly on temperature. Mass fraction and mole fraction are convenient variables for describing the composition of a gas. Molar concentration (, in moles/cm³) is sometimes a convenient variable because the rate of progress of chemical reactions depends directly on the molar concentration of the reactants and products. In general, most Ansys Chemkin Reactor Models use species mass fractions to track variations in chemical composition, because of the direct relation to mass conservation principles. The formulas (and their related subroutines) in the following sections show the calculations necessary to convert between state variables, which are used in assembling reaction rates-of-production, thermodynamic properties, and other terms in the reactor governing equations.