| Experiments have been conducted to study the controlling mechanisms involved in the transition from smoldering to flaming of a porous combustible material, flexible (open-cell) 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 exposed porous-fuel surface forming one of the walls of the test-section. Air-flow (ranging from none to 2.5 m/s) is forced upward and parallel to an exposed foam surface to examine how a smoldering fuel responds to increased oxygen supply and heat transfer. Smolder is initiated at the bottom of the sample and propagates upward in the same direction as the externally forced air-flow. The buoyancy-induced flows within the porous-fuel interior are also upward. The resulting smolder is therefore two-dimensional and forward. The smolder results observed by varying the air-flow velocity reveal three regimes. First, for no air-flow (natural convection) to 0.25 m/s, smoldering either dies almost immediately following the ignition period or propagates very slowly. Second, for flow velocities between 0.25 m/s and 2.0 m/s, transition to flaming occurs more rapidly with increasing air-flow velocity, producing consequently a decreasing smolder duration. Thermocouple measurements of the foam interior, visual observations, and schlieren interferometry images of the gas-phase at the porous-fuel/air-flow interface indicate that transition to flaming occurs not at the interface, but inside the hot char region below the smolder front. An innovative application of ultrasonic imaging is employed to monitor the two-dimensional smolder front and the evolution of the char permeability in real time. The results evince 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 which 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. These effects, in conjunction with the reduced heat losses, lead 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 engulf the entire fuel sample in flames. Finally, as the air velocity is increased further ({dollar}>{dollar}2.0 m/s), the smolder reaction is initially strong following the ignition period but always ends in almost abrupt extinction, due ultimately to convective cooling. These results indicate that smolder propagation with transition to flaming is the result of increased oxidizer supply and reduced heat losses. |
