# Numerical Simulation of Natural Gas/Hydrogen Combustion in a Novel Laboratory Combestor
## 4 Conclusions  
A computational study was conducted on NG/H\({}_{2}\) turbulent swirling flames in a novel combustion chamber, with the molar fraction of hydrogen varying from 0% to 100%. The key findings are:  
1. The size of the central recirculation zone induced by the swirler slightly decreases for pure hydrogen flames compared to NG/H\({}_{2}\) flames. Flow patterns vary depending on the presence of secondary air. When only primary air is used, the central recirculation zone is larger, and a smaller recirculation zone forms beneath the top-mounted plenum.
2. The maximum and average temperatures in the combustor increase with rising hydrogen content in the fuel. At a fixed fuel composition and primary air fraction (\(\alpha=50\%\)), the average temperature decreases while the maximum temperature increases with increasing excess air coefficient. When the excess air coefficient is held constant (\(\lambda=1.3\)), both the average and maximum temperatures increase with the primary air fraction as \(\alpha\) increases from 50% to 66.7%. At \(\alpha=100\%\), the average temperature continues to increase while the maximum temperature maintains a near-linear dependence on the hydrogen molar fraction.
3. The temperature in the region delimited by the inner and outer shear layers of the swirling flow increases with the rise of the hydrogen content in the fuel. The tem  
Figure 13: Molar fractions of the species at the exit for \(\alpha=50\%\) as a function of the excess air coefficient and hydrogen content in the fuel (open symbols—predictions, solid symbols—measurements).  
perature rise at the centreline shifts slightly downstream for pure hydrogen flames, and the temperature remains 50\({}^{\circ}\) to 80 \({}^{\circ}\)C higher than for blended CH\({}_{4}\)/H\({}_{2}\) flames towards the exit section. When secondary air is introduced, the temperature decreases in the reaction zone, but a second temperature peak appears at the centreline, near the exit due to the combustion of unburned fuel transported from the primary reaction zone. 4. Increasing the primary air fraction results in a smoother temperature profile along the centerline. When only primary air is used, the centerline profile exhibits a single temperature peak near the burner, followed by a gradual downstream decline. As the excess air coefficient increases, the initial rise in centerline temperature shifts slightly closer to the burner. Further downstream, a temperature peak appears for \(\lambda\) = 1.3 and \(\lambda\) = 1.5, though it is less pronounced at \(\lambda\) = 1.5 and nearly disappears at \(\lambda\) = 1.7. Beyond this peak, dilution effects dominate, leading to a temperature decrease towards the combustor exit. 5. The O\({}_{2}\) and CO emissions are higher for \(\alpha\) = 50% and 66.6% than for \(\alpha\) = 100%, indicating incomplete combustion in the former case. The NO emissions are lowest for \(\alpha\) = 50% and show little variation between \(\alpha\) = 66.7% and \(\alpha\) = 100%. They do not exceed 50 ppm, except for pure hydrogen combustion and \(\alpha\geq\) 66.7%. For a given fuel composition, CO\({}_{2}\) and CO emissions decrease, while NO emissions increase with the excess air coefficient. 6. The CO emissions decrease and NO emissions increase with rising hydrogen content in the fuel, a trend that becomes more pronounced as the hydrogen molar fraction approaches unity. 7. Secondary air is particularly useful for hydrogen flames, as NO emissions remain relatively low (e.g., -40 ppm at \(\lambda\) = 1.3 and \(\alpha\) = 50%) despite increasing with higher excess air coefficients, and CO emissions are absent. For NG/H\({}_{2}\) flames, secondary air is effective only at sufficiently high excess air levels to keep CO emissions within acceptable limits.  
Not applicable.  
The data that support the findings of this study are available from the corresponding author upon reasonable request. The data are not publicly available due to privacy.  
The authors declare no conflicts of interest.