Flame Propagation Through Layered Fuel-Air Mixtures

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Abstract
It is observed that a layered, unconfined combustible mixture supports a flame propagation velocity of 4 to 5 times the laminar flame speed of the stoichiometric mixture of fuel and air. This effect which has been observed by some and more recently in this laboratory, has never been adequately explained. It is in contrast to the classical soap bubble experiment, in which the flame propagation velocity is of the order of 7 to 8 times the laminar flame speed, that is, the order of the unburned to burned gas density ratio To explain this new effect two physical models with different degrees of complexity are explored here. The first is a step-wise premixed fuel-air and pure air case in a gallery of infinite length. The second deals with one of finite length. Both have been mathematically defined to give quantitative results so that the effects of environmental factors can be explicitly determined Solutions of the first model show that the maximum flame propagation speed occurs when the gallery is of infinite height. In that case the ratio of the propagation speed to the laminar flame speed is equal to the square root of the ratio of the unburned to the burned gas density. For the second model, in which the galleries have finite length with open ends, the interesting result is that for all but a gallery of infinite height the flame propogation speed is unsteady and accelerating. These results have been experimentally verified.

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