# Numerical Simulation of Natural Gas/Hydrogen Combustion in a Novel Laboratory Combestor
## 3 Results and Discussion
### Temperature and Velocity Fields
Several authors observed a shorter flame and a shift of the temperature peak and reaction zone towards the burner as the hydrogen content in the fuel increased [11; 15; 18; 21]. Figure 5: Temperature profiles along the centreline. Blue and yellow symbols denote experimental data [37] for 20% and 60% H\({}_{2}\) (vol.) in the fuel, respectively. The axial profiles displayed in Figure 5 show a sharp temperature increase near the burner exit. Further downstream, the temperature remains nearly constant up to \(x\approx 20\) cm before rising again to reach a peak in case \(\alpha=50\%\), after which it decreases towards the exit section of the combustor. That peak is attenuated for \(\alpha=66.7\%\) and does not occur for \(\alpha=100\%\), i.e., when there is no secondary air. In the latter case, the peak temperature occurs close to the burner. The higher temperatures observed for hydrogen flames in comparison to blended mixtures of hydrogen and natural gas have also been observed in many previous studies, e.g., [10; 16; 18; 19]. This is due to the increase of the adiabatic flame temperature when the hydrogen content in the fuel rises. However, this increase is relatively small for low hydrogen content and only becomes larger when it approaches 100%, which explains the differences in the temperature profiles between the \(\mathrm{NG/H_{2}}\) blends and the hydrogen flame. In the present work, the thermal load was maintained constant. Therefore, for fixed values of \(\lambda\) and \(\alpha\), an increase in the hydrogen content in the fuel leads to a higher lower heating value (LHV) on a mass basis, resulting in a reduction of the fuel mass flow rate. Notably, the LHV of a mixture containing 20% NG and 80% H\({}_{2}\) (vol.) is approximately 40% higher than that of a pure natural gas flame. In comparison, the LHV on a mass basis of a pure H\({}_{2}\) flame is almost 150% higher than that of a natural gas flame [57], and so the fuel mass flow rate in the latter case is about 150% higher in comparison to the former one. Hydrogen has a molar weight and density that are both approximately one-eighth of those of methane, the primary component of natural gas. The mass flow rate of air decreases, similarly to the fuel mass flow rate, as the molar fraction of hydrogen increases, provided that \(\lambda\) remains constant. Therefore, the amount of nitrogen is reduced due to the lower air mass flow rate, further contributing to the temperature rise as the hydrogen fraction increases, as pointed out by [PERSON] et al. [19]. Experimental data [37] for hydrogen molar fractions of 20% and 60% are also presented in Figure 5. The predictions accurately capture the temperature rise downstream of the burner. However, the maximum temperature is overestimated and exhibits a sharp, nearly right-angled profile, in contrast to the smoother profile experimentally observed. Moreover, the measured temperature reaches a peak upstream of the numerically predicted maximum at \(x\approx 20\) cm, and the measured profile is smoother than the computational one. The velocity field and temperature contours in an axial plane passing through the centre of a secondary air injection hole are presented in Figure 6 and Figure 7, respectively, for \(\lambda=1.3\). Figure 6 indicates that the momentum of the secondary air jets for \(\alpha=50\%\) or 66.7% is not sufficiently high to allow that air to reach the reaction zone near the burner. When \(\alpha=50\%\), the secondary air jets penetrate roughly up to one half of the chamber length, while for \(\alpha=66.7\%\), the penetration is reduced to about 1/3 of that length, regardless of the hydrogen content in the fuel. There is a slight decrease in the size of the central recirculation zone downstream of the burner, induced by the swirler, for pure hydrogen flames compared to \(\mathrm{NG/H_{2}}\) flames. This aligns with the findings reported by several authors regarding the shrinkage of the reaction zone in the former case, as mentioned above.