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.