Example: Char combustion with UDF
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. math::

  \text{C}(s) + 0.5\text{O}_2 (g) → \text{CO}(g)

Notes:

 * The fluid cell index (``IJK``) is passed as a dummy argument.
 * Algebraic expressions for the rate limiting steps are omitted for brevity.

.. code-block:: fortran

    ###mfix.dat: see char combustion example

    ###usr_rates.f:
    SUBROUTINE USR_RATES(IJK, RATES)
    INTEGER, INTENT(IN) :: IJK
    DOUBLE PRECISION, DIMENSION(NO_OF_RXNS), INTENT(OUT) :: RATES

    INCLUDE 'species.inc'
    INCLUDE 'usrnlst.inc'

    ! Fluid Cell Index
    DOUBLE PRECISION, INTENT(OUT) :: RATES(:) ! Reaction Rates

    ! Rate limiting steps:
    DOUBLE PRECISION k_f ! film diffusion (cm/sec)
    DOUBLE PRECISION k_a ! ash layer diffusions (cm/sec)
    DOUBLE PRECISION k_s ! chemical kinetics (cm/sec)
    DOUBLE PRECISION k_eff ! effective rate (cm/sec)

    ! Total surface area of solids phase 1 in IJK
    DOUBLE PRECISION Sa ! (cm^2/cm^3)

    ! C + 0.5O2 --> CO (reacted moles/sec.cm^3)
    !-------------------------------------------------------//
    ! Verify that solids are present
    IF(.NOT.COMPARE(EP_g(IJK),ONE)) THEN
      ! Calculate film diffusion rate
      k_f = < film diffusion rate expression >
      ! (cm/sec)
      ! Calculate ash diffusion rate
      k_a = < ash diffusion rate expression >
      ! (cm/sec)
      ! Calculate kinetic rate
      k_s = < kinetic rate expression >
      ! (cm/sec)
      ! Effective rate (cm/sec)
      k_eff = ONE/(ONE/k_a + ONE/k_f + ONE/k_s)
      ! Calculate total surface area of solids phase 1
      Sa = 6.0 * EP_s(IJK,1) / D_p0(1)
      ! Calculate the reaction rate.
      RATES(Char_Combustion) = 2.0 *(Sa * k_eff * Conc(O2))
    ELSE
      ! No solids --> No reaction
      RATES(Char_Combustion) = ZERO
    ENDIF

    IF(nRR >= Char_Combustion) ReactionRates(IJK,Char_Combustion) = RATES(Char_Combustion)


    END SUBROUTINE USR_RATES

See legacy_tutorials/SpoutedBedCombustor for details on a similar simulation setup.
Legacy tutorials can be found by downloading the source tarball. They are meant to provide representative setups for older versions of MFiX (before the launch of the GUI), and are not guaranteed to run with the latest version of MFiX.