Flame propagation in gasoline vapor–air mixtures with concentration gradient in a closed duct

Once gasoline leaks in confined spaces, vapor–air mixtures with concentration gradients will form above the liquid surface owing to evaporation, and explosion may occur if ignition occurs at a suitable location. This study aims to measure the concentration gradient of gasoline vapor–air mixtures and to investigate the effect of the ignition delay (tig) on the flame behavior and overpressure in a closed duct. The results reveal that the flammable layer of the gasoline vapor–air mixtures exist only at a certain time period. After ignition of the stratified vapor–air mixtures, a blue olive-shaped premixed flame bubble is formed, followed by a bright diffusion flame at its bottom, and subsequently, two symmetrical eddies form. The horizontal speed of the premixed flame tip first increases to approximately 3.5–4.3 m/s and then decreases to 1.5–2.0 m/s. The maximum overpressure of the current stratified gasoline–air mixture initially increases from approximately 320 to 450 kPa as tig increases from 6 to 10 min, and then remains nearly unchanged with further increase in tig. A maximum value of 450 kPa is obtained, which is lower than the results of homogeneous gasoline–air mixtures in previous research.