# Numerical Simulation of Natural Gas/Hydrogen Combustion in a Novel Laboratory Combestor
## 3 Results and Discussion
### Temperature and Velocity Fields
The flow pattern for \(\alpha=100\%\) is different from that for \(\alpha=50\%\) or 66.7%, since there is only primary air. The central recirculation zone is larger, and a small recirculation zone develops underneath the top-mounted plenum. In the region between the inner shear layer (separating the main swirling flow and the central recirculation zone) and the outer shear layer (separating the main recirculation flow and the outer recirculation zone), the temperature is higher for hydrogen flames compared to blended \(\mathrm{NG/H_{2}}\) flames, as illustrated in Figure 7. For \(\alpha=50\%\), the primary air supply is insufficient to achieve complete combustion, resulting in locally fuel-rich conditions near the burner and lower temperatures in the reaction zone. The secondary air jet mixeswith recirculated combustion products and, after losing momentum, entrains the upward flow of the fuel-rich mixture, which contains unburned fuel and combustion products transported from the primary reaction zone. This leads to a temperature increase due to the combustion of the remaining unburned fuel at the mixing layer between the upward flow and the mixture of secondary air and recirculated combustion products. Figure 6: Influence of the primary air fraction and hydrogen content in the fuel on the velocity field. Figure 5, particularly for \(\alpha\) = 50%. These characteristics of the temperature field are also observed for the pure hydrogen flame, even though the temperatures are higher in this case. The inlet fuel and air velocities are also higher for hydrogen flames because the decrease in density, as the hydrogen content in the fuel increases, outweighs the reduction in mass flow rate. The higher velocities contribute to a reduction in the size of the central recirculation zone (see Figure 6) and a downstream shift in the initial temperature rise, despite the greater reactivity of hydrogen compared to natural gas. The temperature field is not uniform at the exit section, suggesting that the combustion chamber may be slightly too short, potentially leading to incomplete combustion. For \(\alpha\) = 66.7%, combustion near the burner still occurs under fuel-rich conditions but with a lower equivalence ratio. This results in higher local temperatures while reducing the amount of unburned fuel available for reactions further downstream, where temperatures are lower compared to \(\alpha\) = 50%. When all the air is introduced through the swirler (\(\alpha\) = 100%), combustion is concentrated near the burner, leading to more complete fuel consumption in this region. As a result, the temperature distribution becomes more uniform further downstream. The influence of the primary air fraction on the temperature profile along the centreline is further illustrated in Figure 8 for \(\lambda\) = 1.3. In the case of \(\alpha\) = 66.7%, the mixture remains fuel-rich in the reaction zone near the burner, but the temperature profiles are smoother compared to \(\alpha\) = 50%. Higher temperatures are reached in the central recirculation zone, while the peak temperature at \(x\approx\) 25 cm is significantly reduced. When \(\alpha\) = 100%, a single peak temperature occurs near the burner, followed by a gradual decrease further downstream towards the exit section. The influence of the excess air coefficient on the velocity field and temperature contours in an axial plane passing through the centre of a secondary air injection hole is illustrated in Figure 9 and Figure 10, respectively, for \(\alpha\) = 50%. The velocity field remains nearly unchanged with varying \(\lambda\) for a given fuel composition. However, a slight increase in the size of the central recirculation zone is observed as \(\lambda\) increases. This occurs due to the enhanced swirl intensity with additional air, which strengthens mixing and recirculation. An increase in the excess air coefficient leads to higher temperatures in the region between the inner and outer shear layers of the swirling flow.