Smoldering and spontaneous transition to flaming over horizontal cellulosic insulation

Abstract

The transition from smoldering to flaming is basically a homogeneous gas-phase ignition reaction that has a high potential to engulf the entire fuel bed in flames. However, the transition mechanism, especially under windy environment, is poorly understood due to lack of theoretical study. This work proposes a two-dimensional computational model with multiphase formulations to simulate the forward horizontal smoldering and spontaneous transition to flaming of cellulosic insulation. A chemical scheme of three-step solid decomposition reactions combined with one global gas-phase reaction is used in the model. The multi-physical coupling of heat, mass and momentum fields is implemented to reveal the interactions between the packed insulation bed and the surrounding free flow. This model shows that the hotter surface char layer acts as an ignition source for heating the gas mixture, and both the surface char oxidation and sample pyrolysis reactions behave like a gaseous fuel supplier for gas-phase reaction. With favorable conditions of high-temperature combustible fuel gas and sufficient oxygen supply under windy environment, the transition ultimately occurs at 2-3 cm above the leading edge of smolder front on the top surface, which is totally different from the transition within the porous fuel bed under windless condition. The predicted effects of wind velocity and bulk density of fuel bed on the smoldering velocity agree well with literature data. The modeling results for the first time give an interpretation of the spontaneous smoldering to flaming transition over the fuel bed surface.