FORCED SMOLDER PROPAGATION AND THE TRANSITION TO FLAMING IN CELLULOSIC INSULATION

It is well known that a smoldering fuel responds to an increased oxygen supply by becoming faster and hotter until, eventually, flames erupt. This sequence was examined quantitatively for thick horizontal layers of a permeable fuel, i.e., cellulosic insulation. Two configurations are possible, forward and reverse smolder; both were investigated experimentally. Reverse smolder was shown to respond only weakly to an increased air flow and it exhibited no transition to flaming at flow velocities up to 5 m/s. Forward smolder responded strongly to increased air flow and yielded transition to flaming at about 2 m/s for unretarded material. The influence of combustion retardants was also examined; these included boric acid, a smolder retardant, and borax, a flaming retardant. Both prevented the transition to flaming in the absence of adjacent flammable material but were less effective in its presence. The overall response of these various fuel mixtures and configurations suggests that both kinetics (via leading edge heat transfer effects) and oxygen supply rate (not the latter alone) play substantial roles in dictating smolder response to an air flow. © 1990.