Some Observations of Two-dimensional Smoldering and the Transition to Flaming

Experiments have been conducted to study the upward smolder characteristics and the phenomenon of transition to flaming of a porous combustible material in the presence of an air/porous-solid interface. External air flow velocities ranging from 0 m/s to 2.5 m/s are forced upward and forward across the exposed, flat, vertical surface of a high void fraction, flexible, polyurethane foam to examine how a smoldering fuel responds to increased oxygen supply and heat transfer. The smolder results of varying the air flow velocity reveal three regimes. First, for no 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 with decreasing smolder duration relative to increasing air flow velocity. Visual observations and thermocouple histories, when compared to analyses of differential shear interferometry images of the gas phase at the interface, indicate that transition to flaming occurs inside the hot char region below the smoldering front and not at the air/porous-solid interface. Finally, as the air velocity is increased further (> 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.