Upward flame spread along PMMA vertical corner walls Part II: Mechanism of “M” shape pyrolysis front formation

Abstract

Flame spread behavior and the pyrolysis region spread characteristics along polymethylmethacrylate (PMMA) vertical corner walls were studied in detail with an automated infrared (IR) imaging temperature measurement technique. The technique was recently developed for the measurement of transient pyrolysis temperature on both charring and noncharring materials. Temporal isotherms on large PMMA samples were successfully obtained, from which the progress rate of the pyrolysis front was automatically deduced. It was found that the pyrolysis front shape was always M shaped, i.e., no spread along the corner, and the maximum spread is within a few centimeters of the corner. Understanding of the mechanism of the M-shape formation is important in developing a prediction model of the spread rate. Four possible mechanisms, effect of ignition mode, effect of solid phase conduction heat loss, fire-induced flow cooling, and flame displacement effect, were identified. Four different experiments were designed to test each mechanism, among them the flame displacement effect, which causes a large heat loss in a nonflammable gas layer due to a poor mixing of pyrolysis products and air was found to be the principal mechanism. For an upwardly spreading fire, total heat flux distributions above the M-shape pyrolysis peak were measured by a Gardon-gauge heat flux meter, and visible flame height and pyrolysis front height were respectively measured by a video camera and the IR technique.