30.3.2 Flux Reporting for Reacting Flows

To report heat transfer for reacting flows, one of models in the Species Model dialog box must be enabled for the Total Sensible Heat Transfer Rate option to appear in the Flux Reports dialog box. For reacting flows, ANSYS FLUENT produces two kinds of reports which use a different treatment at the flow boundaries:

• Total Heat Transfer Rate reports the total enthalpy flux, which consists of the total energy plus the species formation enthalpies. The heat rate based on this definition is a conserved quantity in reacting flows. See this section in the separate Theory Guide for details.

• Total Sensible Heat Transfer Rate reports the total energy flux as defined in this equation in the separate Theory Guide. Note that in reacting flows, this is not a conserved quantity and the addition or removal of heat due to the chemical reactions ( this equation in the separate Theory Guide) is reported separately in the Heat of Reaction Source field, as shown in Figure  30.3.3. If you have more than one reaction defined in your case, the Heat of Reaction Source reported is the sum of the heat for all reactions. For exothermic reactions the Heat of Reaction Source is reported as a positive quantity, while for endothermic reactions it will be a negative quantity.

Figure 30.3.3: The Flux Reports Dialog Box

Note that both the Total Heat Transfer Rate and Total Sensible Heat Transfer Rate options report a Net Result, which may be used as an indication of the energy balance for the case. In general, and if heat sources other than the heat of reaction and DPM are not included in your problem, the Net Result reported in both the Total Heat Transfer Rate and Total Sensible Heat Transfer Rate options should be a small number for a converged calculation. However, if a reacting case is not well converged for both energy and species transport equations, the Net Result reported in the Total Heat Transfer Rate and Total Sensible Heat Transfer Rate options may differ. In that case, you may consider iterating further to achieve a fully converged solution. In addition, please refer to the sections that follow for special considerations when including particles, multiphase models, or other volumetric energy sources.

Please note that for the non-premixed and partially premixed models the Heat of Reaction Source is calculated as the difference of the net Total Heat Transfer Rate and the net Total Sensible Heat Transfer Rate. The Heat of Reaction field function is not available for the non-premixed and partially-premixed models.

Flux Reporting with Particles

If you are using the discrete phase model (DPM), the contributions from the particle injections are reported separately and are included in the net mass and heat balance results. Consequently, the Mass Flow Rate report includes the DPM Mass Source, the Total Heat Transfer Rate report includes the DPM Enthalpy Source, and the Total Sensible Heat Transfer Rate includes the DPM Sensible Enthalpy Source (Figure  30.3.4).

Figure 30.3.4: The Flux Reports Dialog Box with DPM

In the case of reacting flows with the DPM model, the Heat of Reaction Source entry reports the heat of all homogeneous reactions in the continuous phase, while the heat released or consumed due to particle reactions (e.g. char combustion) is reported in the DPM Sensible Enthalpy Source field.

Flux Reporting with Multiphase

If you are using any of the multiphase models, the mass or heat rates can be reported separately for each phase and for the mixture phase. Please note that if your multiphase model includes mass or heat transfer processes between phases, the mass and heat transferred across the phases will be reported as an imbalance in the report of each phase. In order to check the overall balances for the multiphase cases you should select the mixture phase for your report. In that case, the report will include the sum of the fluxes and sources for all phases included in your model.

Finally, if you are solving a multiphase problem that includes chemical reactions, you should be aware of the following conventions when you are requesting a Total Sensible Heat Transfer Rate report:

• If you select one of the phases with gas phase chemical reactions, the Heat of Reaction Source will only include contributions from reactions in the particular phase.

• When you report the Total Sensible Heat Transfer Rate for the mixture phase, the Heat of Reaction Source entry will report the sum of the heat of reaction of all gas phase reactions in all phases plus the heat of any heterogeneous reactions that take place.

Flux Reporting with Other Volumetric Sources

The reported mass and heat balances address the flow that enters or leaves the domain through boundaries and the contributions from DPM sources; they do not include the contributions from user-defined and other volumetric sources, such as the heat exchanged in the Heat Exchanger Model. For this reason, a mass or heat imbalance may be reported. In that case, and in a converged calculation, the reported imbalance will be equal to the volumetric source.