Boundary layer flame spread over pmma within the quench zone: A moving boundary effect

Abstract

This article focuses on the effects of moving boundary on countercurrent flame spread over polymethylmethacrylate (PMMA) near the flame extinction limits. Experiments were conducted with low oxygen concentration ( h 19.4% by volume) and/or high freestream velocities. At these conditions the flame was observed to retreat from the fuel leading edge after ignition and stabilize downstream, establishing a quench zone. While the flame was retreating, the entire sample surface was relatively flat. As the flame stabilizes, a small valley is formed near the flame leading edge and then the flame spreads upstream, decreasing the length of the quench zone with time. The results show that the flame could not be sustained within the quench zone when the surface was flat, molten, and pyrolyzing but will spread upstream after the surface has solidified. It appears that the presence of the valley stabilizes the flame, perhaps by creating a stagnation/recirculating zone that increases the Damkohler number and enables the flame to spread upstream. Flame spread in these experiments is chemically controlled and the spread rates were comparable to the regression rates. Both rates are observed to decrease with time as the valley enlarges.