Example: DES droplet evaporation with UDF
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

.. math::

  \text{H}_2 \text{O}(l) → \text{H}_2 \text{O}(g)

Notes:

 * Various algebraic expressions in the sample UDF are omitted for brevity.
 * The global particle index (``NP``), phase index (``pM``), and fluid cell index (``IJK``)
   are passed as dummy arguments.

.. code-block:: none

    ### mfix.dat:
    NMAX_g = 2                          ! No. of gas phase species
    Species_g(1) = "Air" "H2O"          ! Database names
    Species_Alias_g(1) = "Air" "Vapor"  ! Species Aliases

    NMAX_s(1) = 1                    ! No. of solids phase species
    Species_s(1,1) = "H2O(L)"        ! Database names
    Species_Alias_s(1,1) = "Liquid"  ! Species Aliases

    @(DES_RXNS)
    Evap { Liquid --> Vapor }
    @(DES_END)

.. code-block:: fortran

    ###usr_rates_des.f:
    SUBROUTINE USR_RATES_DES(NP, pM, IJK, DES_RATES)
    DOUBLE PRECISION, INTENT(IN) :: NP  ! Global particle index
    DOUBLE PRECISION, INTENT(IN) :: pM  ! Particle solid phase
    DOUBLE PRECISION, INTENT(IN) :: IJK ! Fluid Cell Index
    DOUBLE PRECISION, INTENT(OUT) :: DES_RATES(NO_OF_DES_RXNS) ! Reaction Rates

    INCLUDE 'species.inc'

    !-------------------------------------------------------//
    ! Calculate the concentration gradient (mole/cm^3)
    Cmg_H2O = < expression for calculating gradient >

    IF(Cmg_H2O > ZERO) THEN
      ! Calculate mass transfer coefficient (cm/sec)
      H2O_xfr = < mass transfer coeff calculation >
      ! Calculate droplet surface area (cm^3)
      Sa = Pi * 4.0d0 * (DES_RADIUS(NP)**2)
      ! Calculate the mass transfer rate (moles/sec)
      DES_RATES(Evap) = Sa * H2O_xfr * Cmg_H2O
    ENDIF

    ! Store the reaction rate for post processing.
    IF(Evap <= NRR) ReactionRates(Evap) = &
      ReactionRates(IJK, Evap) + DES_RATES(Evap)

    END SUBROUTINE USR_RATES_DES

See legacy_tests/dem-tests/evaporation for additional details.
Legacy tests 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.