Opposed Forced Flow Smoldering of Polyurethane Foam

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Abstract
An experimental study is carried out of the effect on the propagation of a smolder reaction through the interior of a porous fuel of a forced flow of oxidizer opposing the direction of smolder propagation. The potential effect of buoyancy in the process is also analyzed by conducting the experiments in the upward and downward propagation, and comparing the respective results. The experiments are conducted with a high void fraction flexible polyurethane foam as fuel and air as oxidizer, in a geometry that approximately produces a one-dimensional smolder propagation. Measurements are performed of the smolder reaction propagation velocity and temperature as a function of the location in the sample interior, the foam and air initial temperature, the direction of propagation, and the air flow velocity. For both downward and upward smoldering three zones with distinct smolder characteristics are identified along the foam sample. An initial zone near the igniter were the smolder process is influenced by heat from the igniter, an intermediate zone where smolder is free from external effects, and a third zone near the sample end that is affected by the external environment. The smolder velocity data are correlated in terms of a nondimensional smolder velocity derived from a theoretical model of the process previously developed. The analysis of the results confirm that the smolder process is controlled by the competition between the supply of oxidizer and the transfer of heat to and from the reaction zone. At low flow velocities oxygen depletion is the dominant factor controlling the smolder process, and the smolder velocity and temperatures are relatively small. Increasing the flow velocity strengthens the smolder reaction due to the oxygen addition resulting in increased smolder velocities and temperatures. These parameters, however, reach a maximum and as the air velocity is increased further the smolder reaction becomes weaker and eventually dies out due to convective cooling.