Natural convection smolder of polyurethane foam, upward propagation

Abstract

An experimental study is conducted of upward smolder of polyurethane foam in natural convection. The objective of the study is to provide further understanding of the mechanisms controlling upward smolder, and verification of theoretical models of the problem. Measurements of the temperature histories at several locations through the foam sample are used to infer the characteristics of the smolder process, and to calculate the smolder propagation velocity as a function of the location along the sample length and the sample size. Particularly interesting is the observation that in upward smolder there is a transition from an initial smolder process that is characterized by the propagation of a single oxidation reaction (smolder) to one that consists of an endothermic pyrolysis reaction that precedes the smolder reaction. Also interesting is the observation that this change in smolder pattern appears to be generated by the onset of oxidation reactions in the char left behind by the smolder reaction, which deplete the oxygen from the oxidizer gas flow. Since in upward smolder the oxidizer is induced upward in the same direction of smolder propagation, the experimental data are compared with the predictions of previously developed models of forward smolder. The good agreement between theory and experiments verifies that the smolder controlling mechanisms and simplifying assumptions implicit in the models are appropriate at least for the present experimental conditions. This type of smolder is only characteristic of forward smolder, since for the above mechanisms to happen the fresh oxidizer must flow through the char prior to reaching the smolder reaction. Also particular to this type of smolder is the transition to flaming since it appears to be generated through the onset of strong oxidation reactions in the char.