Experimental investigation on explosion characteristics of inhomogeneous hydrogen clouds

The concentration of combustible gases is a crucial factor influencing explosion characteristics. This study investigated the effect of average concentration (c‾) on flame propagation, temperature distribution and explosion pressure in inhomogeneous hydrogen cloud explosions. Results showed that, inhomogeneous concentration distributions resulted in non-spherical flames. The concentration gradient induced flame propagation velocity gradient, causing the flame to stretch into a conical shape at high c‾ (c‾ ≥50.7%). Thermal-diffusion and buoyancy instabilities further led to inwardly concave flames which adopting a mushroom shape at low c‾ (c‾ ≤23.08%). At 30.0%≤ c‾ ≤44.9%, the flame tending to be spherical, and both flame propagation velocities and temperatures increased. Furthermore, explosions at low c‾ were more sensitive to concentration distribution variations. Lower c‾ or local concentration at ignition point induced significant ignition delay period and pronounced mushroom flame shape. In the early-stage, flames propagation was highly influenced by concentration distribution, with the propagation time and maximum temperature differing by 74.25 ms and 164.7 °C respectively among different explosions. However, in the later stage, differences in propagation time and maximum temperature dropped to only 4.0 ms and 65.28 °C. In this phase, flame propagation velocities and temperature peaked at the stoichiometric height and were minimally affected by concentration distribution. Additionally, at c‾ ≤30%, explosion pressure curves exhibited a secondary positive peak due to explosion process outside the hydrogen cloud. The maximum explosion pressure (P_max) reached maximum value at a high c‾ while impulse (I) increased continuously with the increasing c‾.