Smoldering Process Research Articles

Smoldering combustion propagation through a permeable horizontal fuel layer

Although the propagation rate of smoldering through porous horizontal fuel layers has been measured for a variety of materials, there has been little work on the structure of the smolder reaction zone and the factors controlling it. These latter aspects are the focus here for the case of thick (18 cm) layers of wood-based fibers in the form of cellulosic insulation smoldering under natural convection air supply conditions. Two-dimensional profiles of temperature, oxygen mole fraction, and residual organic material have been measured both for unretarded insulation and for insulation having 25 wt% of the smolder retardant, boric acid, added on. It is inferred that the overall wave structure is dominated by oxygen diffusion from above. The heat release chemistry appears to involve both oxidative pyrolysis and char oxidation in a shifting balance depending on depth in the layer. Boric acid is unable to halt the smolder process in these thick fuel layers but it slows its spread by about a factor of 2 by a combination of endothermic and kinetic effects.

Experimental observations of the effect of pressure and buoyancy on cellulose co‐current smoldering

AbstractAn experimental study has been performed to determine the potential effect of buoyancy on the rate of propagation of a co‐current smolder reaction through a porous solid fuel, and the range of flow velocities where buoyancy effects are significant. In the co‐current smolder reaction, the fuel and oxidizer enter the reaction zone from the same direction. In the present experiments this is accomplished by initiating the reaction at the top of the fuel bed, α‐cellulose packed at a void fraction 0.85, so that the smolder wave propagates downward opposing an upward forced flow of air. Since in a stratified density field, buoyancy is proportional to the product of gravity and density difference, buoyancy can be controlled by varying either the gravity vector or the gas density. In this study the latter method is followed, varying gas density through the ambient pressure at which the experiments are performed. The smolder velocity is measured for air flow rates varying from 0.2 to 6 gm−2s−1 at constant ambient pressures of 0.6, 0.8 and 1 atm. The results show that for flow rates larger than 1 gm−2s−1 the smolder velocity increases linearly with the air flow rate but is independent of pressure. The reaction peak temperature is weakly dependent on flow rate and independent of pressure. For the present experimental conditions the effect of buoyancy is only observed at very low air flow rates. The mechanisms by which it affects the smolder process appears to be by altering the transport of air to the reaction zone from upstream and downstream of the reaction.

Smoldering Combustion of Cellulosic Materials

In comparison to flaming combustion, the smoldering process seems more complex and is poorly understood. When heated by an ignition source, cellulosic materials decompose to volatile products and char. The volatile products fuel the flaming combustion which rapidly spreads in the gas phase. The slower oxidation of the char provides the heat flux required for propagation of the smoldering combustion. The initiation, propagation and termination of the latter process are controlled by several interrelated factors, including composition of the substrate, in organic additives, rates of heat release and heat flux and the combustibility of the char and volatile products, which are discussed.

A model of smoldering combustion applied to flexible polyurethane foams

Smoldering combustion, particularly in upholstery and bedding materials, has been proven a serious life hazard. The simplest representation of this hazard situation is one-dimensional downward propagation of a smolder wave against a buoyant upflow (cocurrent smolder); the configuration treated here is identical in all respects to this except for the presence of a forced flow replacing the buoyant one. The complex degradation chemistry of the polyurethanes is here reduced to the two major overall reactions of char formation and char oxidation. The model solutions, which are in reasonable agreement with experimental results, show the smolder process to be oxygen-limited, which leads to some very simple trends. More subtle behavior aspects determine actual propagation velocity, fraction of fuel consumed, and apparent equivalence ratio (all of which are variable). The self-insulating character of the smolder wave makes it viable in a wide-ranging set of conditions if the igniting stimulus is sufficiently long. These results have significant implications regarding the problem of smolder prevention or hindrance.