How does FLUENT calculate the enthalpies in reacting flows reported in the Report/Fluxes panel?
How does FLUENT calculate the enthalpies reported in the Report/Fluxes panel?
1. For non-reacting flows, enthalpy calculation at any boundary is straight forward. Assuming constant cp, enthalpy at each facet of boundary = m * Integral from Tref->T (cp_i * dT) .....1
The total heat transfer rate reported in Report --> Flux panel will be sum of the enthalpy flow rate values over all the facets of boundary.
2. For reacting flows, the enthalpy will be calculated using standard state enthalpy (href_i) and reference temperature value (Tref_i) specified in Define --> Material panel.
For species i, total_enthalpy = m * href_i + m * integral from Tref_i - to T (cp_i * dT) .....2
Hence the enthalpy flow rate from a facet of boundary will sum over all species (htotal_i)
Again, the total heat transfer rate reported in Report --> Flux panel will be sum of the enthalpy flow rate values over all the facets of boundary.
3. To make things more clear, please go through the attached test case. A simple UDF is created which will calculate these values for a reacting flow case using above formulas and report the flux values in detailed manner.
4. Please follow the steps below to reproduce the results at your end.
i. Download the attached zip file (enthalpy_calc.zip).
ii. Read the case and data file.
iii. Compile the enthalpy.c file to create a libudf directory.
iv. Load the newly created libudf directory.
v. Execute the enthalpy_cal using Define --> User Defined --> Execute on Demand.
vi. Print the flux values using Report --> Flux panel to compare the results.