Smoldering is a non-flaming form of combustion that takes place in the interior of porous combustible materials. Examples of some common materials that support smoldering are non-flaming embers or charcoal briquettes, and cigarettes. The initiation, propagation, transition to flaming and extinction of the smolder reaction are controlled by complex thermo-chemical mechanisms, which are not well understood.
From a fundamental point of view, smoldering is a basic combustion problem that encompasses a number of fundamental processes, including: heat and mass transfer in a porous media; endothermic pyrolysis of the combustible; ignition, propagation and extinction of heterogeneous exothermic, reactions at the pore's solid /gas interface; onset of gas phase reactions from the on-going surface reactions (transition from smoldering to flaming). From a practical point of view, smoldering presents a serious fire danger because the combustion can propagate slowly in the material interior and go undetected for long periods of time while releasing toxic products, and may undergo a sudden transition to flaming.
Common examples of the potential hazard of smoldering are the initiation of wildland fires by smoldering embers, and of building fires from undetected smoldering in insulation and packing materials and furniture cushioning. Smolder of cable insulation, another common fire hazard, is a particular concern in the space program, to date there have been a few minor incidents of overheated and charred cables and electrical components reported on Space Shuttle flights. Recently, with the planned establishment of the International Space Station and other space facilities, there has been an increased interest in the study of smoldering in microgravity because of the need to preempt the possibility and/or to minimize the effect of a smolder initiated fire during the years of operation of these facilities.
Smoldering is characterized by an exothermic heterogeneous combustion reaction that occurs on the interior surfaces of porous combustible materials. The heat released during the heterogeneous oxidation of the solid is transferred toward the unreacted material by conduction, convection and radiation, supporting the propagation of the smolder reaction. The oxidizer, in turn, is transported to the reaction zone by diffusion and convection. These transport mechanisms not only influence the rate at which the smolder reaction propagates, but also the limits to the smolder process, i.e., ignition and extinction (lower bounds) and transition to flaming (upper bound). The propagation of the smolder reaction is, therefore, a complex coupled phenomena involving processes related to the transport of heat and mass in a porous media, together with surface pyrolysis and combustion reactions.
Smoldering is customarily classified into opposed and forward configurations, according to the direction in which the fuel and oxidizer enter the reaction zone. In opposed smolder, the oxidizer enters the reaction zone from opposite direction of the reaction propagation, and in forward smolder from the same direction. The transport of mass can take place by convection (natural and forced) and by diffusion, which in the absence of gravity leads to a secondary classification of smoldering into forced convection driven and diffusion driven smolder. At normal gravity, buoyancy interferes with both forced convection and diffusion. The two series of smolder experiments currently approved to be conducted in the Space Shuttle are in the opposed smolder configuration, two diffusion driven smolder and two forced convection driven.
In opposed smolder propagation, the heat released by the heterogeneous oxidation (smolder) reaction is transferred ahead of the reaction by conduction, heating the unreacted fuel and the incoming oxidizer. The resulting increase of the virgin fuel temperature leads to the onset of the smolder reaction, and consequently gives way to its propagation through the fuel. The combustion process is generally oxygen deficient, and the propagating reaction leaves behind a char that contains a significant amount of unburned fuel. The rate of smolder propagation is basically given by a balance between the rate of heat released by the reaction and the energy required to heat up the solid fuel and gaseous oxidizer to the smolder reaction temperature.
The complexity of the smolder process requires the use of approximations in the theoretical models and simplifications in the experiments in any study of the fundamentals of smolder combustion. Removal of gravity provides substantial simplifications to smolder investigations, as the influence of buoyancy on the heat and mass transport processes is removed . The instabilities induced by the density stratification, and problems related to the sedimentation and collapse of the porous fuel and char are absent in microgravity. Furthermore, experimental information about smoldering behavior in a microgravity environment, and a complementary theoretical foundation, are necessary to asses the fire risk of a space based installation.
