# Numerical Simulation of Natural Gas/Hydrogen Combustion in a Novel Laboratory Combestor
## 3 Results and Discussion  
### Temperature and Velocity Fields  
Figure 4 shows the variation of the maximum and average temperatures as a function of the hydrogen content in the fuel for five different combinations of the excess air coefficient (\(\lambda\)) and fraction of primary air (\(\alpha\)). Both temperatures increase with rising hydrogen content, regardless of the specific values of \(\lambda\) and \(\alpha\). This trend is primarily attributed to the higher adiabatic flame temperature of hydrogen compared to natural gas. The increase is more significant for the maximum temperature than for the average temperature, especially at higher hydrogen content. However, when combustion is conducted using only primary air, the maximum temperature increases approximately linearly with the hydrogen molar fraction. The average temperature decreases with increasing excess air coefficients at a fixed \(\alpha=50\%\), as expected, due to the dilution effect of the excess air, as illustrated in Figure 4. However, the maximum temperature exhibits an opposite trend. This reversal is linked to the fact that combustion takes place under fuel-rich conditions in the reaction zone downstream of the burner. As the global equivalence ratio increases, the equivalence ratio based solely on the primary air approaches unity, enhancing combustion intensity in that zone, as discussed later. When the excess air coefficient is held constant at 1.3, both the average and maximum temperatures increase with the fraction of primary air as \(\alpha\) increases from 50% to 66.7%. This is because the combustion in the primary zone occurs closer to stoichiometric conditions at \(\alpha=66.7\%\) than at \(\alpha=50\%\). When only primary air is used, i.e., \(\alpha=100\%\), the average temperature continues to rise, while the maximum temperature retains a near-linear dependence on the hydrogen molar fraction, as previously observed. The mean temperature along the centreline of the combustor is shown in Figure 5 for a hydrogen content in the fuel ranging from 0% to 100% (by volume) and for the studied combinations of \(\lambda\) and \(\alpha\). A comparison of the various profiles reveals that there is a marginal temperature increase when the hydrogen content in the fuel rises from 0% to 80% (vol.). In the case of pure hydrogen, the increase of temperature at the burner exit occurs a little further downstream, and the temperature remains from 50 to 80 K higher than for the other fuel mixtures. Overall, while the temperature profiles for fuels with up to 80% H\({}_{2}\) are relatively close to each other, the pure hydrogen flame temperature profile is clearly distinguishable from those, regardless of the excess air coefficient and fraction  
Figure 4: Maximum (**left** side) and mean (**right**) temperatures as a function of the hydrogen content in the fuel. of primary air. This general observation is consistent with the findings of [PERSON] et al. [25] and [PERSON] et al. [19]. Both studies investigated turbulent non-premixed \(\mathrm{NG/H_{2}}\) flames in scaled industrial low-swirl, considering unconfined flames in the former study and confined flames in the latter one. [PERSON] et al. found minimal visual differences for volumetric hydrogen contents of 10% and 30% (vol. ), with only slight reductions in visible flame length, while [PERSON] et al. observed that the overall flame change remained largely unchanged up to 50% (vol.) hydrogen molar fraction, but a pure hydrogen flame exhibited different characteristics. However, the burner design flame, the combustion system configuration, the operating conditions, and the studied conditions (e.g., constant thermal input, constant volumetric fuel flow rate, constant Reynold number, etc.) vary widely in the literature, and other authors have reported changes in flame shape or length even for relatively small hydrogen molar fractions.