Explosion characteristics and flame propagation of HHO/NH3/air premixed mixtures in a closed vessel

Brown’s gas (HHO), a highly reactive carbon-free fuel, has attracted increasing attention for hydrogen-based energy applications, while ammonia (NH 3 ) is considered a promising hydrogen carrier. Understanding the explosion behavior of HHO/NH 3 /air mixtures is essential for their safe utilization. In this study, the effects of HHO fraction on explosion characteristics and flame propagation behavior were experimentally investigated in a 20 L closed cylindrical vessel under an equivalence ratio of 1.0, an initial temperature of 300 K, and an initial pressure of 101.3 kPa. HHO volume fractions of 20 %, 40 %, 60 %, 80 %, and 100 % were examined. The results demonstrate that increasing HHO fraction significantly enhances explosion severity. The maximum explosion pressure ( P max ) increased from 0.60 MPa to 0.87 MPa, while the maximum rate of pressure rise ((d P /d t ) max ) increased from 32.06 MPa·s −1 to 702.36 MPa·s −1 . The explosion index ( K St ) increased continuously from 8.70 to 190.65 MPa·m·s −1 , and the mixtures entered the high-hazard category according to NFPA 68 when the HHO fraction reached 40 %. Schlieren visualization revealed that flame propagation underwent a significant transition with increasing HHO concentration. Mixtures containing 60 % or more HHO exhibited significantly accelerated flame propagation, whereas lower HHO fractions exhibited slower flame propagation due to ammonia-induced dilution and reduced overall combustion activity. A noticeable change in flame propagation behavior was observed around 60 % HHO, corresponding to a transition from NH 3 -influenced combustion to enhanced flame acceleration associated with increasing HHO addition.