Example: Methane Combustion with UDF
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. math::

  \text{CH}_4 (g) + 2\text{O}_2 → \text{CO}_2 (g) + 2\text{H}_2 \text{O}(g)

Notes:

• Global field variables are referenced (RO_g, X_g, T_g, and EP_g )
• The fluid cell index (``IJK``) is passed as a dummy argument.
• Species aliases (O2 and CH4) are used instead of the species indices.
• Reaction identifier (CH4_Combustion) is used in the rates array.
• Reaction rate is stored for post processing (see below).

.. code-block:: none

    ###  mfix.dat:
    NMAX_g = 4
    Species_g(1) = "CH4 ANHARMONIC"
    Species_g(2) = "O2"
    Species_g(3) = "CO2"
    Species_g(4) = "H2O"

    Species_Alias_g(1) = "CH4"
    Species_Alias_g(2) = "O2"
    Species_Alias_g(3) = "CO2"
    Species_Alias_g(4) = "H2O"

    @(RXNS)
    CH4_Combustion { chem_eq = "CH4 + 2O2 --> CO2 + 2H2O" }
    @(END)

.. code-block:: fortran

    ### usr_rates.f:
    SUBROUTINE USR_RATES(IJK, RATES)

    INTEGER, INTENT(IN) :: IJK
    DOUBLE PRECISION, DIMENSION(NO_OF_RXNS), INTENT(OUT) :: RATES
    DOUBLE PRECISION c_O2 ! Oxygen concentration (mol/cm^3)
    DOUBLE PRECISION c_CH4 ! Methane concentration (mol/cm^3)

    INCLUDE 'species.inc'

    ! Calculate species concentrations:
    c_O2 = (RO_g(IJK) * X_g(IJK,O2))/MW_g(O2)
    c_CH4 = (RO_g(IJK) * X_g(IJK,CH4))/MW_g(CH4)

    ! Methane Combustion
    !-----------------------------------------------------------------//
    RATES(CH4_Combustion) = 6.7d12 * exp(-2.4358d4/T_g(IJK)) * &
    EP_g(IJK) * (c_O2**1.3) * (c_CH4**0.2)

    ! Store the reaction rate for output/post processing.
    IF(CH4_Combustion <= NRR) &
    ReactionRates(IJK, CH4_Combustion) = RATES(CH4_Combustion)

    END SUBROUTINE USR_RATES