The Biot number for a gauze basket in an air oven—the heating arrangement in tests for spontaneous combustion—is determined by having an inert material in the basket instead of a combustible one, monitoring the central temperature and applying equations for non-steady conduction. The Biot num ber turned out somewhat lower than expected, yet when the same heating ar rangement was employed to heat combustible samples the air circulation was found to be adequate to fulfil the Frank-Kamenetskii boundary condition at the sample surface.
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