Flame-pressure wave interaction regulated by vent burst pressure in ammonia/hydrogen/air explosion: Overpressure dynamics and flame morphology evolution

Blending ammonia with hydrogen is a promising way to overcome the poor combustion performance of pure ammonia, but it increases the explosion risk. Venting is an effective way to reduce explosion damage. This paper experimentally and numerically studies the effects of vent burst pressure (P_stat) on the explosion venting characteristics of ammonia/hydrogen fuel blends, focusing on overpressure development and flame morphology evolution during flame-pressure wave interaction. Experiments are conducted in a half-open vertical duct with its open end sealed by polyethylene film. Different P_stat values are achieved by varying the number of film layers. Numerical simulations are performed using a thickened flame model and detailed chemistry. As P_stat increases, the dynamic burst pressure P₁ initially exhibits a linear growth trend, followed by a nonlinear increase. The proportionality constant (ΔP₁/ΔP_stat) during the linear growth stage is influenced by the laminar burning velocity, ignition position, and length-to-diameter ratio. The nonlinear growth is driven by the flame skirt contacting the duct sidewalls before venting. Results reveal that increasing P_stat enhances the flame-pressure wave interaction, shifting the moment of flame skirt touching the duct sidewalls from post-venting to pre-venting. This shift influences the growth of flame surface area and internal pressure, thereby affecting the competition between combustion and venting. This competition controls the overpressure evolution. Only spherical and finger-shaped flames are captured for low P_stat, while a typical tulip flame emerges as P_stat increases to 60.14 kPa. Results indicate that the rarefaction wave generated by the flame-wall contact is too weak to induce a tulip flame. The enhanced collision between the flame and the compression wave reflected from the vent cover due to increased P_stat induces a tulip-shaped axial velocity profile ahead of the flame front. This axial velocity distribution results in the formation of a tulip flame.