## 4 Predictions with Different Chemical Kinetic Mechanisms  
As previously mentioned, the EDC has been successfully applied to the numerical simulation of combustion for a long period of time, and it is implemented in most available commercial CFD codes. Various models have been validated with the Flame D setup with the detailed chemical kinetic mechanism of GRI-Mech 3.0 [15; 32; 33; 34; 35; 36; 37; 38; 39]. The expense of using a very detailed chemical kinetics mechanism, such as GRI-Mech 3.0, is, however, in general quite high and might be unacceptable for engineering purposes. One way of reducing this cost is by applying mechanism reduction methods that neglect the less important elementary reactions in the detailed mechanism during computation. A second approach is directly applying simplified reaction mechanisms, such as ARM 9 [49] and ARM 19 [49], or even a simple global reaction mechanism [32; 56]. The objective of this section was to investigate the performance of the chemical kinetics mechanisms with different species and elemental reactions from GRI-Mech in comparison to the experimental results when simulating a methane-air non-premixed flame with the EDC at a reasonable computational cost. In order to compare the performance of the mechanisms, a series of calculations was conducted for methane-air combustion with RSM turbulence model, which was validated with the detailed mechanisms in the last section. Five relatively simple mechanisms and the detailed mechanism of GRI-Mech 3.0 were chosen for comparison with the experiments [40; 41; 42]. The details of the mechanisms used in the validation, including the corresponding cases, are given in Table 4. The results for the Sandia Flame D from the six mechanisms were measured with the EDC model. A partially premixed combustion (PPC) model, which is one of the common combustion modes for many practical combustion systems, is based on the non-premixed combustion model and the premixed combustion model [37; 74; 75; 76]. Since the PPC PDF model (referred to as the PDF model below) with chemical equilibrium for state relation and adiabatic energy treatment is widely used in industry-scale turbulent combustion simulations [77; 78], the results with this model are also presented for comparison due to its simplicity and very fast simulation. One thing must be noted: the inlet pilot mixture composition could not be exactly defined in the PDF model, which might have resulted in some uncertainties. The only difference of the work in this section was the reaction mechanism used for each case. The sensitivity to the reaction mechanism was investigated by applying EDC-53, EDC-49, EDC-32, EDC-24, EDC-21, EDC-16, or PDF with the RSM-based turbulence model and compared with the experimental data [40; 41; 42] to determine the candidate reaction mechanism to be used in the simulations of industry-scale natural gas combustion furnaces. The details of the cases for EDCs can be found in Table 4, and the settings were the same as those in the last section. A comparison of the predicted flame temperatures is displayed in Figure 6. The predicted flame constructions by the chemical kinetics mechanisms were found to be quite similar. Compared to the EDC models, the result from the PDF model, shown in Figure 6g, showed little difference regarding flame shape (the high temperature area). A \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Mechanism** & **No. of Species** & **No. of Steps** & **NO Species** & **Case** & **Reference** \\ \hline GRI3.0 & 53 & 325 & With NO & EDC-53 & [44] \\ GRI2.11 & 49 & 279 & With NO & EDC-49 & [46] \\ GRI1.2 & 32 & 177 & Without NO & EDC-32 & [45] \\ sp24 & 24 & 104 & Without NO & EDC-24 & [47] \\ sp21 & 21 & 84 & Without NO & EDC-21 & [47] \\ Skeletal & 16 & 41 & Without NO & EDC-16 & [51] \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Chemical mechanisms used in the calculations. low temperature was found at the pilot exit, and the flame shape was thinner compared to others.