Controlling Mechanisms in the Transition from Smoldering to Flaming of Flexible Polyurethane Foam

Experiments have been conducted to study the controlling mechanisms involved in the transition from smoldering to flaming of a porous combustible material, flexible polyurethane foam, with an air flow forced across one of the porous-fuel surfaces. The experiments are performed in a small-scale, vertical, combustion wind tunnel, with the air flow forced upward and parallel to an exposed foam surface that forms one of the walls of the tunnel test section. Smolder is initiated at the bottom of the sample and propagates upward in the same direction as the forced and buoyancy-induced flows. The resulting smolder is therefore twodimensional and forward. Thermocouple measurements of the foam interior, along with visual observations, when compared with schlieren interferometry images of the gas phase at the porous-fuel/air-flow interface, indicate that transition to flaming occurs inside the hot char region below the smolder front and not at the interface. An innovative ultrasonic imaging technique is employed to track the evolution of the char permeability in real time, evincing that the char continues to react and increase substantially in permeability long after the primary smolder front has passed. The ongoing heterogeneous reactions in the char region result in the formation of large voids that provide locations for the onset of homogeneous gas-phase reactions. Furthermore, the higher permeability of the char favors the flow of oxidizer, as well as pyrolysis vapors produced by the primary smolder reaction, into the char interior, which, in conjunction with the reduced heat losses, leads to the onset of a homogeneous gas-phase ignition. This process results in the transition from slow smoldering to fast, exothermic, gas-phase reactions, which rapidly engulfs the entire fuel sample in flames.