Flame propagation in methane-air mixtures with transverse concentration gradients in horizontal duct

Stratified methane-air mixtures are formed in the early stages of methane leakage in confined spaces, and an explosion may occur if a source of ignition were to appear at the appropriate time and location. However, this has not been elucidated thus far. The aim of this study is to measure the concentration gradient of methane injected from the top surface of a duct, and to investigate the effects of ignition delay (t(ig)), which is the time interval from methane leakage to ignition, on flame behavior and explosion overpressure. Experimental results show that t(ig) significantly affects flame behavior in stratified methane-air mixtures, including the flame shape and speed. The stratified methane-air mixtures cannot be ignited at t(ig) <= 3 min. The horizontal flame speed and maximum overpressure (P-max) first increase as t(ig) increases from 4 min to 15 min and thereafter remain nearly unchanged with further increase in t(ig). The value of P-max at t(ig) in the range 15-25 min is approximately that observed for homogeneous methane-air mixtures. Tulip flames are observed at t(ig) >= 10 min. Diffusion and convection flames, which follow the leading premixed flame front, are only formed at t(ig) <= 6 min.