Transition from forward smoldering to flaming in small polyurethane foam samples

Abstract

Experimental observations are presented of the effect of flow velocity, oxygen concentration, and a thermal radiant flux, on the transition from smoldering to flaming in forward smoldering of small samples of polyurethane foam with a gas/solid interface. The experiments are part of a project studying the transition from smoldering to flaming under conditions encountered in spacecraft facilities, i.e., microgravity, low velocity variable oxygen concentration flows. Because the microgravity experiments are planned for the International Space Station, the foam samples had to be limited in size for safety and launch mass reasons. The feasible sample size is too small for smolder to self propagate because of heat losses to the surroundings. Thus, the smolder propagation and the transition to flaming had to be assisted by reducing heat losses to the surroundings and increasing the oxygen concentration. The experiments are conducted with small parallelepiped samples vertically placed in a wind tunnel. Three of the sample lateral-sides are maintained at elevated temperature and the fourth side is exposed to an upward flow and a radiant flux. It is found that decreasing the flow velocity and increasing its oxygen concentration, and/or increasing the radiant flux enhances the transition to flaming, and reduces the time delay to transition.Limiting external conditions for the transition to flaming are reported for this experimental configuration.The results show that smolder propagation and transition to flaming can occur in relatively small fuel samples if the external conditions are appropriate. The results also indicate that transition to flaming occurs in the char region left behind by the smolder reaction, and it has the characteristics of a gas-phase ignition induced by the smolder reaction, which acts as the source of both gaseous fuel and heat. A simplified energy balance analysis is able to predict the boundaries between the transition/no transition regions.

Numerical examination of two-dimensional smolder structure in polyurethane foam

“…The reaction mechanism includes three pyrolysis reactions and four oxidation reactions. One of the oxidation reactions included is what has been referred to as "secondary char oxidation", which has been identified as the key step leading to transition to flaming [10], [12]. While the current focus is not to examine the transition from smoldering to flaming, the work establishes the framework to potentially do so in the future.…”

Ignition of Combustion Modified Polyurethane Foam

“…It is well known that non-fired retarded (NFR) polyurethane foam is susceptible to smoldering and transition to flaming [1,8,9]. In particular, Bar-Ilan et al conducted a study on forward smoldering and transition to flaming in NFR flexible polyurethane foam of density 26.5 kg/m 3 , using the same experimental apparatus as that in the present study.…”

“…The experimental apparatus is very similar to that used by Bar-Ilan et al for non-fire retarded polyurethane foam experiments [9], and a schematic is shown in (Fig. 1).…”

“…The bottom face of the igniter is insulated with a layer of fiberglass blanket of thickness 15 mm. During the tests, there is a forced flow of oxidizer through the fiberglass blanket and the igniter, and into the fuel sample (hence "internal flow") at a flow rate of 13.4 cc/s, which has been used as the standard internal flow rate for this experimental configuration in previous smoldering experiments with NFR polyurethane foam [9]. The internal oxidizer flow through the igniter is controlled using mass flow controllers.…”

“…In particular, Bar-Ilan et al conducted a study on forward smoldering and transition to flaming in NFR flexible polyurethane foam of density 26.5 kg/m 3 , using the same experimental apparatus as that in the present study. It was found that the smoldering reactions, including strong secondary char oxidation, were responsible for triggering the transition to flaming, by supplying heat and vaporized fuel to the gaseous mixtures within the char region [9].…”