Experiments on the Vented Ammonia–Hydrogen–Air Deflagrations under Various Hydrogen Volume Fractions

Although ammonia as a carbon-free fuel is regarded as an energy source worthy of significant development, how to address the shortcomings of its poor combustion properties has been a hot topic of research. Hydrogen enrichment is a promising solution to improve the combustion properties of ammonia, but at the same time, the potential explosion risk will inevitably be raised. In this paper, the effects of hydrogen volumetric fraction (χ) in NH3/H2 on the NH3/H2/air deflagrations were experimentally investigated in a rectangular duct with an end-opening at an initial pressure and temperature of 101 kPa and 298 K, where χ was varied from 0.1 to 0.9. Results demonstrate that flame propagation and overpressure profiles inside and outside the chamber depend on χ. Buoyancy plays a significant role in flame propagation at χ = 0.1. More prominent deformation of the flame front propagating toward the closed end (CE) was observed in the experiments with higher χs. Helmholtz oscillations of the flame occurred in all tests, which resulted in pressure oscillations with a decreasing frequency as χ was increased. Acoustically enhanced combustion of NH3/H2/air remained at the CE, appeared when χ ≥ 0.7, and acoustic-type oscillations of the internal explosion overpressure and a pressure peak of p2 formed in these tests. The amplitude of p2 dropped as χ varied from 0.7 to 0.9. A proportional relationship between the highest explosion overpressure inside the chamber (pmax) and the square of the laminar burning velocity of NH3/H2/air (sl2). When χ ≥ 0.3, a dominant pressure peak (pext) caused by the external explosion appeared in the external overpressure profiles, and its amplitude increased as χ varied from 0.3 to 0.9.