| Subscript denoting species A |
A |
| Subscript denoting species B |
B |
| Klinkenberg parameter (F/L2) |
b |
| Moles of gas per unit volume (moles/L3) |
C |
| Moles of species i per unit volume (moles/L3) |
Ci |
| Effective molecular diffusion coefficient (L2/T) |
D |
| Molecular diffusion coefficient (L2/T) |
Dm |
| Knudsen diffusion coefficient for species i (L2/T) |
 |
| Equimolar flux of species i (moles/L2T) |
Ji |
| Intrinsic permeability (L2) |
k |
| Permeability to gas or effective gas permeability (L2) |
kg |
| Apparent gas permeability (L2) |
kga |
| Knudsen number |
Kn |
| Coordinate tangent to path of motion (L) |
l |
| Ratio MA/MB |
MAB |
| Ratio MB/MA |
MBA |
| Molecular weight of specie i |
Mi |
| Molecular mass of specie i |
mi |
| Porosity |
n |
| Number of molecules per unit volume (L-3) |
ñ |
| Mole flux of species i (moles/L2T) |
Ni |
| Mole flux of species i due to diffusion (moles/L2T) |
 |
| Diffusive flux for the gas as a whole (non-equimolar flux) (moles/L2T) |
ND |
| Mole flux of gas due to viscous flow (moles/L2T) |
Nv |
| Mole flux of species i due to advection in viscous flow (moles/L2T) |
 |
| Pressure (F/L2) |
p |
| Mean pressure (F/L2) |
 |
| Partial pressure of species i |
pi |
| Ideal gas law constant (FL)/(moles temperature) |
R |
| Absolute temperature |
T |
| Volume flux (L/T) |
v |
| Mean speed of species i molecules (L/T) |
 |
| Mole fraction of species i |
xi |
| Vertical coordinate (L) |
z |
| Volume of gas per unit bulk volume |
θg |
| Mean free path length (L) |
λ |
| Characteristic pore dimension (L) |
λp |
| Molecular diameter (L) |
σ |
| Dynamic viscosity (FT/L2) |
μ |
| Tortuosity |
τ |
