## 4 Predictions with Different Chemical Kinetic Mechanisms
Almost all the radial predictions of H2O were larger than the experiments at \(x/d\) = 30, as shown in Figure 12, meaning that the combustion was intensive at this place with a very similar shape of the temperature, as shown in Table 5.2 Other major composition profiles had similar distributions of the composition of H2O, as shown in Figure 11 for O2 and Figure 13 for CO2. The inert N2 composition profiles, shown in Figure 14, agreed quantitatively well with the experiments. Once again, all the N2 predictions by the EDCs agreed very well with each other, and the  
Table 5.3 Comparison of predicted and measured mean N2 mass fraction profiles for the Sandia Flame D.  
Table 5.4 Comparison of predicted and measured mean H2O mass fraction profiles for the Sandia Flame D.  
Table 5.5 Comparison of predicted and measured mean CO2 mass fraction profiles for the Sandia Flame D.  
One of the possible reasons for the discrepancies of major and minor species, shown in Figures 10-17 and Tables 4 and 5, may have been the inaccurate treatments of heat radiation, which had a significant effect on the predicted mean compositions. Some of the predicted maximum/minimum mass fractions, shown in Figures 10-17 and Tables 4 and 5, were significantly over/under-predicted compared with the respected measured data, which was probably due to the temperature overestimation by the approximate radiation heat transfer treatment when using the DO model along with the WSGGM for absorption coefficients. For the objective of this section, we chose the prediction results by EDC-53 as the comparison base, and the other simulation results were compared with such a base. Generally, the predictions agreed well with those by EDC-53 for the mean temperature, the turbulent kinetic energy, the mean axial velocity, and the compositions for the major species, with exceptions of those by the PDF model. There were some disagreements between the predictions for the axial and the radial distributions for the temperature and the compositions for the Sandia Flame D. Among the EDCs, the results from the smaller number of species by EDC-21 and EDC-24 agreed very well with the results by EDC-53, the detailed chemical kinetic mechanism of GRI-Mech 3.0 for methane-air combustion. The results, shown in Figures 6-14 and Table 5, indicated that the EDCs could give relatively good predictions for the turbulent piloted methane-air diffusion flame of the Sandia Flame D. For the simulations of industry-scale natural gas combustion furnaces, relative simpler chemical kinetic mechanism and lesser computational costs, such as shown by EDC-21 or EDC-24, could be used for combustion simulations at affordable expenses. Since the result of EDC-21 was quantitatively closest to that of EDC-53, as clearly shown in Figures 6-14, EDC-21 could be used for the simulation of the turbulent piloted methane-air diffusion flame, the Sandia Flame D. With more validations, EDC-21 might be used for simulations in engineering applications. On the other hand, most of the results by the PDF model with lesser simulation expenses than EDC-21, as shown in Figures 6-14, agreed better with the experiments. However, NO composition was not included in the EDCs with exceptions of EDC-53 and EDC-49. If the emission of NO has to be predicted during the turbulent methane-air combustion, an NO\({}_{x}\) sub-model (implemented in most commercial CFD codes) could be used after validation. Therefore, the PDF model is suggested for simulations of the engineering applications of methane-air combustion based on the results shown in Figures 6-14 and the deviations shown in Table 5.6  
The disagreements between the measured and the computed scalar values might have mainly been due to the reasons mentioned in this work and partly due to the treatments of reactions, which will the subjects of further investigations. combustion for the Sandia Flame D. The validated turbulence models were the SKE, the SKE-R with the standard constant changed, the RKE model, the SST \(k\)-\(\omega\) model, and the RSM.