4.13.3.5. Example: Methane Combustion with UDF

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

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).

###  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)

### 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