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-initi ated fire in a space-based facility. Experiments are conducted in an a ircraft following parabolic trajectories that provide up to 25 s of lo w 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 perfo rmed, during a series of parabolas, of the temperature histories of th e polyurethane foam at several locations along the fuel sample interio r, both for upward and downward propagation. The measurements show tha t gravity plays a significant role in the competition between the supp ly 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 oxidize r is dominant in downward smolder, and that the smolder temperature de creases at low gravity for lack of oxidizer. Away from the reaction zo ne there is a temperature increase at low gravity because of the reduc tion 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 sm oldering temperature but also in an increase in the temperature of the fuel ahead of the reaction. This increase is either because of the in crease in the flow of hot postcombustion gases ahead of the reaction z one or the extra heat generated by oxidative reactions occurring in th e char. Although the variable gravity periods are too short to study s molder propagation, they allow the observation of trends in the smolde r reaction temperature, aiding understanding of how gravity affects sm oldering.