The mechanism of surface flash, fast flame spread over the surface of a napped fabric, was investigated experimentally and theoretically. First, downward surface flash over napped cotton fabrics were experimentally examined; the critical nap density (above which surface flash did not occur) and the dependence of surface flash velocity on the nap density were measured. Then, an analytical model was proposed based on the flame structure revealed by high-speed schlieren images. An activation-energy asymptotics (AEA) technique was applied to obtain an approximate solution of the model, which successfully explained experimental observations. Finally, the surface flash over sheep wool and polyester fabrics was tested. Although surface flash occurred over sheep wool fabrics, the flame-propagation velocity was much slower than that over cotton fabrics; this result was attributed to the more charring nature of sheep wool fibers than cotton fibers. Surface flash over polyester fabrics did not occur under the conditions tested. Because of the thermoplasticity coupled with the tendency to melt, polyester fibers shrank away from a flame and melted drip. Thus, polyester fabrics could not maintain their fibrous structures, reducing the probability of surface flash occurrence.
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