# DIFFUSION_COEFFICIENT¶

The phase molecular diffusion coefficients may be entered in the GAS_WATER, COMP3 and COMP4 modes. These are entered in FLUID_PROPERTY blocks. See example:

FLUID_PROPERTY
PHASE LIQUID
DIFFUSION_COEFFICIENT 2.0d-9
/
FLUID_PROPERTY
PHASE GAS
GAS_DIFFUSION_COEFFICIENT 2.0d-5
/


The diffusion coefficients are entered in m2/sec.

If not entered, the diffusion coefficients default to the following values:
• liq_diff_coeff = $$1.0 \times 10^{-9} m^2 /\mbox{sec}$$

• gas_diff_coeff = $$2.13 \times 10^{-5} m^2 / \mbox{sec}$$

Internally the simulator uses the liquid molecular diffusion as entered by the user, while it modifies the gas molecular diffusion with corrections that accounts for pressure and temperature:

$D_{\mbox{gas}} = \mbox{gas_diff_coeff} \frac{101235}{P} \left[ \frac{T+273.15}{273.15} \right]^{1.8}$

Note that the pressure and temperature in the equation above are in Pascal and degree Celsius.