Porous Gas Constants#

Porous Gas Constants = IDEAL_GAS <float_list> [varies]

Description / Usage#

This required card is used for Media Types of POROUS_UNSATURATED and POROUS_TWO_PHASE, and is used to input some standard thermodynamic gas constants needed for vapor-liquid equilibrium calculations (see Media Type card). Eventually more than one model may be allowed for nonideal gas situations.

The IDEAL_GAS model is the only model currently requiring standard constants; they are defined as follows:


the model name requiring constants for the thermodynamic ideal gas law.

  • <float1> - MWair, the molecular weight of the insoluble gas in the gas phase [g/mole].

  • <float2> - R, the universal gas law constant [M-L2/t2/K]

  • <float3> - T, the temperature [deg K]

  • <float4> - pamb, the ambient gas pressure.


The sample input card follows:

Porous Gas Constants = IDEAL_GAS 28.0 8. 315 275 1.06e+5

Technical Discussion#

For POROUS_UNSATURATED media types the ambient pressure dictates the equilibrium pressure for the calculation of the gas-phase density of solvent (viz. the total ambient pressure minus the vapor pressure will be the gas partial pressure, from which the concentration of gas can be computed based on the other gas constants). In POROUS_TWO_PHASE media types, the gas partial pressure is a dependent variable and computed as a part of the Darcy law mass balance. In this case the dynamic pressure is used instead of <float4> here for the calculation of the gas-phase concentrations.

It is important to realize that setting the ambient pressure on this card for Media Types of POROUS_UNSATURATED will potentially affect your saturation curve and the appropriate values of your liquid phase pressure boundary conditions. If possible, you should set this value to zero, and base your Saturation versus vapor pressure curve accordingly. Also, in that case your liquid pressure boundary conditions can all be referenced to zero. However, if you choose a gauge pressure, or thermodynamic pressure, you Saturation/capillary pressure curve must be shifted accordingly, as do your boundary conditions. Also, remember these pressures will affect your solid pressure state in poroelastic problems.


GT-008.2: Porous Media Capabilities/Tutorial for GOMA. User Guidance for Saturated Porous Penetration Problems, August 11, 1999, P. R. Schunk

GT-009.3: GOMA’s Capabilities for Partially Saturated Flow in Porous Media, September 1, 2002, P. R. Schunk

SAND96-2149: Drying in Deformable Partially-Saturated Porous Media: Sol-Gel Coatings, Cairncross, R. A., P. R. Schunk, K. S. Chen, S. S. Prakash, J. Samuel, A. J. Hurd and C. Brinker (September 1996)