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The temperature fluctuations during the smoldering spread in rigid polyurethane foam were measured in order to explore the spread mechanisms and thermal structures of the smoldering combustion supported by external heating. The experiments were conducted under natural and forced draft conditions, and the effects of the heat source intensity H and the air supply content Q on smoldering were examined. It was found that smoldering spreads faster in the upward direction than the downward direction under natural draft conditions but that the spread rates become almost the same as Q increases under forced draft conditions. The smoldering spread rate Vs increases linearly as H or Q increases. The Vs is proportional to the temperature gradient (dT/dx) s at the charring front, i.e., the heat flux transferred to the unburned solid. When the representative smoldering temperature Tg in the smoldering zone is larger than 300C, the smoldering spread is maintained, and Vs increases linearly as Tg increases.