The role of secondary char oxidation in the transition from smoldering to flaming
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Date
08/2006Author
Putzeys, Olivier
Bar-Ilan, Amnon
Rein, Guillermo
Fernandez-Pello, Carlos
Urban, David
Metadata
Abstract
The transition from forward smoldering to flaming in polyurethane foam is observed using indepth
thermocouples and ultrasound probing. The experiments are conducted with small parallelepiped
samples vertically placed in an upward wind tunnel. Three of the vertical sample sides are maintained at
elevated temperature and the fourth is exposed to an upward oxidizer flow and a radiant heat flux. An
ultrasound probing technique is used to measure the line-of-sight average permeability of the sample at the
same heights as the thermocouples. The smolder front propagation is tracked by both the thermocouples
and ultrasound data, which show an increase in temperature and permeability upon passage of the smolder
front. The permeability data also show that the transition to flaming is preceded by rapid fluctuations in
permeability in the char region below the smolder front, indicating the formation of pores by secondary
char oxidation. The pores provide locations for the onset of gas-phase ignition (i.e. transition to flaming).
The results from all the tests indicate that the formation of pores is a necessary but not sufficient condition
for the transition to flaming. Two novel measures of the intensity of the secondary char oxidation are
introduced: the time derivative of permeability, and the secondary char oxidation velocity. The time
derivative of permeability, which provides a measure of the pore formation rate, is found to increase as the
oxygen concentration and/or radiant heat flux increase, and to indicate the likelihood of the transition to
flaming. The permeability data offers a means to track the propagation of the secondary char oxidation,
and to calculate the secondary char oxidation velocity, which is found to be strongly correlated to the
transition to flaming. A simplified energy balance model is able to predict the dependence of the
secondary char oxidation velocity on oxygen concentration and radiant heat flux.