$IGMixture
It is possible to define up to 6 custom ideal gas mixtures that can be used as a fluid in EES just as any other built in fluid. These are referred to as IGMixture1 through IGMixture6. In order to define a custom mixture you need to use the $IGMixture directive. For example, to defined IGMixture1 as a mixture of 30% CO2, 20% O2 with the rest being H2 you should include a $IGMixture directive that includes the number of the mixture being defined followed by its composition.
$IGMixture1 CO2 0.30 O2 0.20 H2 0.50
These custom ideal gas mixtures can be used just like any of the built-in ideal gas substances, including in the Heat Transfer & Fluid Flow and Component Libraries. All of the functions that are available for ideal gases, e.g., CH4, are available for these custom ideal gas mixtures as well, including Viscosity and Conductivity. The viscosity of the gas mixture determined using the Wilke Approximation as described in Eq. 9-5.2 of Reid, Prausnitz, and Sherwood. The thermal conductivity is determined using the Mason and Saxena modification, as described in Eq. 10-6.2. of Reid, Prausnitz, and Sherwood. The heating values are determined as a mole-fraction average of the gases in the mixture.
The Directive Information command in the Options menu provides a template to enter the gas mixtures into the $IGMixture or $Natural_Gas_2 directives.
Air, Natural_Gas, Natural_Gas2, the CombustionGas substances and the custom ideal gas mixtures IGMixture1 through IGMixture6 are the only substances in the EES database that do not follow the ideal gas name convention. All other substances are modeled as a real substance if the fluid name is spelled out, (e.g., Nitrogen) and as an ideal gas if the fluid name is chemical formula (e.g., N2).
See also: $Natural_Gas_2