The effects of buoyancy on smoldering combustion and its transition to flame

Experiments have been conducted to study the effect of buoyancy on one-dimensional opposed flow and forward flow smoldering combustion and on the transition from smoldering to flaming in a porous combustible material, flexible open-celled polyurethane foam. The experiments are part of the Microgravity Smoldering Combustion (MSC) project and the Smolder and Transition to Flaming (STAF) project, which are NASA funded programs to study smoldering combustion in a microgravity environment. Complementary studies are conducted here in normal gravity. In one-dimensional smoldering, a controlled airflow is forced through a cylindrical sample of polyurethane foam, which is ignited at one end of the cylinder. The smolder reaction propagates against the direction of airflow in the opposed configuration, and in the direction of the airflow in the forward configuration. In normal-gravity testing there is an additional upward buoyant flow through the sample. Generally smolder takes place under oxygen-limited conditions, and the rate of propagation is therefore determined by the availability of oxidizer. In the opposed configuration, experiments indicate that the smolder propagation is determined by the total mass flux of oxidizer to the smolder reaction for an oxygen-limited regime of smolder combustion. The smolder velocity is seen to vary linearly from 0.10 mm/s to 0.25 mm/s with oxidizer mass flux in a range of oxidizer mass flux from 0 g-O2/m2s to 1.5 g-O2/m2s. For the present sample size, a critical mass flux of oxidizer is identified below which the smolder propagation is not possible. This critical mass flux is significantly lower in microgravity (0.3 g-O2/m2s) than in normal gravity (0.6 g-O2/m2s), due to the reduced heat losses in microgravity. A calculation of heat losses indicates that heat losses are 6--7 times higher in normal gravity than in microgravity due to buoyantly-induced convective cooling. In the forward flow configuration, two microgravity experiments are conducted at a forced oxidizer mass flux of 0.84 g-O2/m2s and 1.4 g-O2/m 2s, with two complementary normal gravity experiments. A comparison of normal and microgravity experiments reveals that in microgravity the absence of buoyantly-induced convective cooling leads to sustained char temperatures after the passage of the smolder front, while in normal gravity the char cools rapidly after the passage of the smolder front. At a forced oxidizer mass flux of 1.4 g-O2/m2s, microgravity char temperatures are up to 100°C higher than the corresponding normal gravity temperatures. The results show that smolder propagation and the 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 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. (Abstract shortened by UMI.)...