The spontaneous and piloted transitions from smoldering to flaming in polyurethane foam

Abstract

Results of a study on the effects of fire retardants, external heat flux, oxidizer flow velocity, and oxygen concentration on both the spontaneous and piloted transitions from smoldering to flaming in polyurethane foam are presented.

Tests were conducted on a non-fire retarded polyurethane foam and the fire retarded polyurethane foam Pyrell^®.

For both foams and both modes of transition to flaming, it was found that increasing the oxygen concentration, increasing the radiant heat flux, and/or decreasing the duct flow velocity increases the likelihood of a transition to flaming. Limiting external conditions for both modes of transition to flaming are reported for both foams. The results for both foams show that the piloted transition from smoldering to flaming is possible at lower oxygen concentrations than the spontaneous transition from smoldering to flaming. Thermocouple data, video imaging, infrared imaging, high-speed imaging, and schlieren imaging show that the spontaneous transition to flaming occurs in pores in the char region behind the smolder front in both foams.

For the spontaneous transition to flaming in non-fire retarded foam, an ultrasound probing technique is used to measure the line-of-sight average permeability at various heights in the sample, in order to monitor the reactions occurring in the sample interior. It is found that the spontaneous transition to flaming is preceded by rapid fluctuations in permeability in the char region below the smolder front, indicating the formation of pores by secondary char oxidation. The secondary char oxidation in non-fire retarded foam is shown to provide the gaseous fuel, heat, and sites (large pores) for the gas-phase ignition. It is found that the time derivative of permeability, which provides a measure of the pore formation rate, indicates the likelihood of the spontaneous transition to flaming. The permeability data offers a means to track the propagation of the secondary char oxidation wave, and to calculate the secondary char oxidation velocity, which is found to be strongly correlated to the spontaneous transition to flaming. A simplified energy balance model is able to predict the dependence of the secondary char oxidation velocity on oxygen concentration and radiant heat flux.

The experiments on the fire retarded foam Pyrell show that it undergoes a weak smoldering reaction that requires significant assistance in the form of external heat input. Gas chromatography results show that smoldering in Pyrell produces lower levels of CO and CO₂ than smoldering in non-fire retarded foam, likely due to the fire retardant effects. Although the Pyrell was assisted with five times more igniter power than the non-fire retarded foam, the minimum oxygen concentration for the spontaneous transition to flaming was higher than that for the non-fire retarded foam. The results indicate that the fire retardants act in the solid phase to hinder the smolder reaction, and in the gas phase to impede the gas-phase ignition, however smoldering and the transition to flaming can occur in Pyrell at elevated oxygen concentrations. (Abstract shortened by UMI.)