Laminar burning velocity and unburnt temperature: Comparative analysis across a broad temperature range of atmospheric NH3+H2 flames

Abstract

Ammonia (NH₃) emerges as a promising carbon-free fuel, necessitating an understanding of its fundamental combustion properties, particularly the laminar burning velocity ($S_L$), at very high unburnt temperatures ($T_u$). Despite this need, a consensus on the relationship between $S_L$ and $T_u$ across a broad temperature range has not yet been established. This study investigated the $S_L$ vs. $T_u$ relationship from 298 K to above 800 K, analyzing both literature data and simulation results for 40%H₂+60%NH₃+air flames at 1 atm. Seven kinetic models were used in the simulations, among which the models from Shrestha, Stagni, Han, NUIG, and KAUST accurately reproduced the experimental data within uncertainty limits, making them suitable for investigating $S_L$ vs. $T_u$ relationship. The analysis revealed that no tested correlation perfectly captures the $S_L$ vs. $T_u$ relationship across the entire temperature range with their originally defined constants, because the overall activation energy of the global one-step reaction is indeed increasing rapidly with $T_u$. In addition, the much lower reaction sensitivities of the temperature dependence coefficient than $S_L$, along with the same effects of elevated temperature and oxygen enrichment on model validations, were found to be valid for temperatures up to 850 K in these simulations, consistent with those previously identified for $T_u$ < 500 K conditions. Reaction sensitivities were also calculated for the overall activation energy, which exhibits significantly stronger temperature dependence than $S_L$, thus more effective for identifying reactions requiring adjustment for improving predictions across wide unburnt temperature ranges. Based on these findings, a feasible strategy was proposed for future investigation of the laminar burning velocities with broad unburnt temperature range, helping with relevant applications.