Flame propagation in millimeter-scale tubes for lean ethylene–oxygen mixtures

Effects of tube diameter and equivalent ratio on flame propagation characteristics of lean ethylene–oxygen mixtures in millimeter-scale cylindrical tubes were experimentally investigated. Five different flame propagation modes—1) steady propagation mode (SPM), 2) jerky flame mode (JFM), 3) oscillating flame mode (OFM), 4) oscillating flame transition to detonation (OFTD), and 5) jerky flame transition to detonation (JFTD)—are observed experimentally with the increase of equivalent ratio. The results show that the oscillation characteristics strongly depend on the tube diameter and mixture composition. The results show that the smaller the diameter of the tube, the more regular and smaller the flame oscillation cycles and amplitude, and the greater the influence of the heat loss on flame propagation. Under the same tube diameter, the ratio of the heat loss to total chemical heat release rate in the five flame modes is H_{loss SPM} > H_{loss JFM} > H_{loss OFM} > H_{loss OFTD} > H_{loss JFTD} . The coupling between the wall heat loss, wall friction, and the flame boundary layers contributes significantly to the observed different flame modes and the transition from deflagration to detonation. In addition, the mechanism of oscillating flame propagation is revealed for the first time by numerical simulations.