EXPERIMENTAL-OBSERVATIONS OF THE EFFECT OF GRAVITY CHANGES ON SMOLDERING COMBUSTION

An experimental study is conducted to determine the effect of gravity changes on the natural convection smolder characteristics of flexible polyurethane foam. Gravity, and consequently buoyancy, is expected to affect smoldering because it induces convective transport of mass and heat to and from the reaction zone. The overall objective of the work is to provide information about the potential onset of a smolder-initiated fire in a space-based facility. Experiments are conducted in an aircraft following parabolic trajectories that provide up to 25 s of low gravity (KC-135A) and up to 20 s (Learjet Model 25), with a pull-up and pull-out of approximately 2 g per parabola. Measurements are performed, during a series of parabolas, of the temperature histories of the polyurethane foam at several locations along the fuel sample interior, both for upward and downward propagation. The measurements show that gravity plays a significant role in the competition between the supply of oxidizer to, and the transfer of heat to and from, the reaction zone. It is found that within the reaction zone, the supply of oxidizer is dominant in downward smolder, and that the smolder temperature decreases at low gravity for lack of oxidizer. Away from the reaction zone there is a temperature increase at low gravity because of the reduction in buoyantly induced convective cooling. The opposite is observed at high gravity. Similar mechanisms are observed in upward smoldering, although here high gravity results not only in an increase in the smoldering temperature but also in an increase in the temperature of the fuel ahead of the reaction. This increase is either because of the increase in the flow of hot postcombustion gases ahead of the reaction zone or the extra heat generated by oxidative reactions occurring in the char. Although the variable gravity periods are too short to study smolder propagation, they allow the observation of trends in the smolder reaction temperature, aiding understanding of how gravity affects smoldering.