Theory

The humid gas is an ideal-gas mixture: every dry-air species keeps its share of the dry part, scaled by $1 - x_{H_2O}$ , and water vapour takes $x_{H_2O}$ . The molar mass, the heat-capacity polynomial and therefore the density, the ratio of specific heats and the speed of sound are the mole-fraction averages. The viscosity stays that of dry air (Sutherland's law).

No saturation check is made for this input: a mole fraction above the saturation value at the gas temperature describes supersaturated vapour, which the gas model accepts.

Equations

x_i = (1 - x_{H_2O})\,x_{i,dry}, \qquad M = \sum_i x_i M_i

(1)

where	meaning	unit
$x_i$	Mole fraction of species $i$ in the humid gas	-
$x_{i,dry}$	Mole fraction of species $i$ in dry air	-
$M$	Molar mass of the mixture	kg/kmol

Default and rule

Default: 0
Rule: 0 (dry air).
Source: The mixing rule of the program's gas model; the species data are those used for dry air and exhaust gas.

Valid range 0 to below 1

Typical values

Saturated air holds about 0.023 at 20 °C and 0.57 at 85 °C at 1 atm, less at a higher pressure.

Theory

Equations

Default and rule

Valid range 0 to below 1

Related 3