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This paper offers a heuristic, semiempirical approach to prediction of thresholds of spontaneous ignition of materials exposed to ascending heat ramps in terms of the dependence of the time taken to ignite ti on rates of change of heat flux with time. The approach relies on the concept that two conditions are necessary (if not always sufficient) for ignition to occur: (1) the exposing flux must first grow to exceed some critical level Hc, taken as equivalent to the so-called “critical irradiance” determined from constant-flux exposures; (2) the additional time for ignition to occur (ti − tc) is then a prescribable inverse function of the rate of flux increase (that is, the slope of the ramp). Temperature attainment criteria are employed in arriving at a mathematical formalism.
Data to test the approach are derived from a previous experimental study of quarter-scale room fires (propane fueled) in which wood block targets on the floor were observed to ignite spontaneously from exposure to heat radiated by the ceiling. Additional data, acquired specifically to extend the range of rates of flux growth and reported here for the first time, were obtained by exposing similar wood targets to ascending heat ramps produced manually with a tungsten-filament radiant heat source. The resultant data, from the two experimental sources, are reasonably well fit by a thermally thick-solid expression relating ti − tc to the rate of flux growth evaluated with independently derived empirical constants.
ignition, ascending flux-time ramps, spontaneous ignition thresholds, critical irradiance, fire spread, flashover, temperature attainment criteria, thermally thick solids, thin solids, wood blocks, radiant heat source
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