Numerical investigation of the impact of an extended nozzle on combustion stability and flashback characteristics in laminar H₂-Air premixed flames

Hydrogen (H-2) is a promising renewable energy carrier essential in decarbonization and sustainable energy systems, especially when it is derived from low-carbon sources. However, ensuring stable and reliable H-2 combustion requires effectively mitigating flashback risks. Considering the complexity of its combustion process, mitigating flashback risks is crucial for the safe design of combustion chambers, thermal stability, and burner performance. This study aims to reduce flashback velocities in H-2 combustion by extending burner holes. To this end, the effect of normalized nozzle length (e/d) at various equivalence ratios (phi) was investigated. The results were compared with the conventional burner geometry without a nozzle (e/d = 0). The numerical solution procedure was validated by comparing the static temperature and H-2 consumption rate values with the literature. Critical flashback velocities for conventional (e/d = 0) and proposed burner geometries (0 < e/d < 2) were investigated across various phi values by using the validated model. Moreover, temperature, H-2 molecule consumption rate, and enthalpy results were analyzed for different nozzle lengths (e/d = 0 to 2.0), inlet velocities (V-in = 3, 4, and 5 m/s), and equivalence ratios (phi = 0.5 to 1.0). The results showed that a nozzle design with e/d = 0.6 significantly reduced critical flashback velocities. While the critical flashback velocities for e/d = 0 at phi of 0.5, 0.7, and 0.9 were 1.40 m/s, 2.85 m/s, and 3.40 m/s, respectively, for the e/d = 0.6 configuration, these values decreased to 1.10 m/s, 2.05 m/s, and 2.45 m/s, respectively. In conclusion, the proposed geometries effectively mitigate potential flashback risk, enhancing the safety and stability of H-2 combustion systems.