Prandtl number (custom gas)

Theory

With a constant $Pr$ , the thermal conductivity follows from the viscosity and the heat capacity, equation (1). The value is used in the same way as the air value Prandtl number:

in the gas-side correlations of heat-loss walls (Gnielinski (Gnielinski, 1975));
in the JCA / JCAL equivalent-fluid models of an absorbing packing (Atalla and Panneton, 2000).

The outer surface of a heat-loss wall is always exposed to dry air. The compressible flow solver does not read this value: it computes the conductivity with the Eucken-type relation of its Sutherland transport model (OpenFOAM user guide, 2026).

Equations

k = \frac{\mu\,c_p}{Pr}

(1)

where	meaning	unit
$k$	Thermal conductivity of the gas	W/(m K)
$\mu$	Dynamic viscosity	Pa s
$c_p$	Specific heat capacity at constant pressure	J/(kg K)
$Pr$	Prandtl number (this setting)	-

Default and rule

Default: 0.71
Rule: 0.71 (air).
Source: Equation (1) is the definition of the Prandtl number; the program applies it with the custom gas's viscosity and heat capacity in the heat-transfer correlations and the packing models.

Valid range 0 < Pr <= 100

Values

Gases lie near 0.7 (monatomic gases about 0.67).