Observations of Forward Smoldering and the Transition to Flaming in Small Polyurethane Foam Samples with Ultrasound Imaging

Abstract

¶The transition from forward smoldering to flaming in small samples of polyurethane foam is observed with the use of thermocouple and ultrasound probing. The experiments are part of a project that includes planned microgravity experiments aboard the International Space Station, and consequently 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 the 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 oxidizer flow and to a variable radiant flux. Thermocouples are placed in the foam at six different heights on the sample centerline. An ultrasound probing technique is used to measure the relative permeability of the sample at the same heights as the top five thermocouples. The thermocouple data shows an increase in temperature as the smolder front approaches the thermocouple location and a sharp increase in temperature at the location of the transition to flaming. The ultrasound data shows that the permeability of the char continues to increase after the primary smolder reaction has passed, indicating the presence of a secondary char oxidation reaction. The ultrasound data also shows that the transition to flaming is preceded by large increases in permeability in the char region below the smolder front. The secondary char oxidation results in the formation of large voids containing gaseous fuel and oxidizer, which can undergo a gas-phase ignition if the mixture is combustible (i.e. there is sufficient oxygen) and the heat losses are sufficiently reduced, producing a transition to flaming.