Stability and Flame Spread Over Poly(Methyl Methacrylate) (PMMA) Within the Quenching Distance

摘要

This report focuses on the effects of moving boundary on counter- current flame spread over poly(methyl methacrylate) (PMMA) near the flame extinction limits. Experiments were conducted with a wind tunnel and less than 19.4% oxygen (by volume) air with free stream velocity of 87 cm/s. At these conditions, the flame was observed to retreat from the leading edge after ignition and stabilize downstream, establishing a quenching distance from the leading edge. While the flame was retreating, the entire sample surface was relatively flat. As the flame stabilized, a small valley was formed near the flame leading edge and then the flame started to spread counter-currently, decreasing the quenching distance with time. The results show that the flame could not stabilize within the quenching distance when the surface was flat, molten and pyrolyzing but will spread upstream after the surface has solidified. The size of the valley increased as the flame spread upstream. It appeared that the presence of the valley stabilized the flame, perhaps by creating a stagnation/re-circulating zone that increased the Damkholer number and enabled the flame to spread upstream. Flame spread in these experiments was chemically controlled and the spread rates were observed to be comparable to the regression rates. Both rates were observed to decrease with time, as the valley enlarged. On the other hand, a few tests conducted far downstream with ambient air (thermally controlled regime) showed constant spread rates, which were much larger than the regression rates.