Extinction limits and structure of counterflow nonpremixed hydrogen-doped ammonia/air flames at elevated temperatures

The present study demonstrates the potential of hydrogen (H₂)-doped ammonia (NH₃) as a carbon-free fuel. The extinction limits, flame temperature and morphology of the counterflow nonpremixed NH₃-H₂/air flames at elevated temperatures and normal pressure are experimentally determined. Also, the detailed flame structure and the extinction limits are computed using a detailed kinetic mechanism. Results show that the blow-off limits, the concentration of radicals H, OH and O and the maximum flame temperature are enhanced with H₂ substitution in NH₃/air flames. This supports the potential of H₂ as an additive for improving the reactivity and ignition of nonpremixed NH₃/air flames and thus the potential of H₂-doped NH₃ as a carbon-free fuel. Meanwhile, the extinction limits (in terms of the mole fraction of NH₃ in the fuel gas) and the maximum flame temperature are reduced with increasing strain rates, indicating that flames can sustain more NH₃ at low strain rates. Also, it is observed that the blow-off limits and the maximum flame temperature are enhanced with increasing air temperature. Measured and predicted tendencies of the extinction limits and temperature for various conditions show encouraging agreement, but quantitative discrepancies among the measurements and predictions merit additional consideration in boundary condition modeling and the reaction mechanism.