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Properties of flames propagating in propane-air mixtures near flammability and quenching limits

Flammability limits and flame quenching in a narrow channel were studied during flame propagation in propane/air mixtures. Most of the experiments were carried out in a vertical, square (50 mm×50 mm) flammability tube 1.8 m long and in a wedge-shaped quenching channel. Some new details of the extinction mechanism of lean limit upward and downward propagating flames (Le>1) in a vertical tube were deduced using schlieren and direct light photography, temperature measurements, and transient phenomena observations. Behavior and properties of the upward propagating rich flames (Le<1) were also studied. Additionally, flame quenching in a narrow channel for upward and downward propagating flames was examined over the entire range of mixture composition. Reliable data related to flammability limits and quenching distance were obtained. The quenching distance appeared to be twice as wide as the flame thickness. The Peclet number, expressed by the quenching distance and laminar burning velocity, is constant and equal to Pe D ≅42. Flames propagating upward in very rich mixtures are characterized by substantial flame thickness and small laminar burning velocity. The flame front of such flames contains several luminous zones (cool flames). In the hottest part of the flame soot is produced.