The mechanism of surface flash, fast flame spread over the surface of a napped fabric, was investigatedexperimentally and theoretically. First, downward surface flash over napped cotton fabrics were experimentallyexamined; the critical nap density (above which surface flash did not occur) and the dependenceof surface flash velocity on the nap density were measured. Then, an analytical model was proposed basedon 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 experimentalobservations. Finally, the surface flash over sheep wool and polyester fabrics was tested. Althoughsurface flash occurred over sheep wool fabrics, the flame-propagation velocity was much slower than thatover cotton fabrics; this result was attributed to the more charring nature of sheep wool fibers than cottonfibers. Surface flash over polyester fabrics did not occur under the conditions tested. Because of the thermoplasticitycoupled 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 flashoccurrence.
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