The physical properties of general multicomponent mixtures are difficult to specify. The default treatment in Ansys CFX 2024 R2 makes the assumption that the components form an ideal mixture. For details, see Ideal Mixture in the CFX-Solver Modeling Guide.

Now consider a given volume of the fluid mixture. Let be the mass of component present in this volume, then . The partial volume of component is defined to be the volume, , that would be occupied by the given mass of the component at the same (local) temperature and pressure as the mixture. The "thermodynamic density" of the component, which results from evaluating its equation of state at the mixture temperature and pressure, may be expressed as . Because the partial volumes of all components must sum to the total volume, , you have:

(1–156)

or:

(1–157)

Thus, the mixture density may be calculated from the mass fractions and the thermodynamic density of each component, which may require knowledge of the mixture temperature and pressure, as well as an appropriate equation of state for each component.

Note carefully the distinction between and . The component mass density, , is a quantity relating to the composition of the mixture, while the thermodynamic density, , is a material property of the component.

An arbitrary constitutive fluid property may be calculated from:

(1–158)

where is the property value for fluid component . While it may appear anomalous at first sight that density does not conform to this expression, the specific volume (volume per unit mass, that is, ) does indeed conform, as can be seen by considering Equation 1–158. Properties that may be evaluated for a multicomponent mixture using Equation 1–158 include the laminar viscosity , the specific heat at constant volume , the specific heat at constant pressure , and the laminar thermal conductivity .