Mathematical and experimental studies are being conducted on the thermal processes involved in the ignition of slabs of woodlike substances. The results of extensive finite-difference machine computations suggest that the specified fuel-generation-rate criterion for sustained ignition, proposed by Bamford, Crank, and Malan, is incorrect. These results, however, also suggest that the experimental data of Bamford, Crank, and Malan reflect a thermal criterion of sustained ignition which has not been recognized previously. In order to adequately describe this critical thermal condition for symmetrical, two-sided heating of plane infinite-width slabs, a concept of a “thermal feedback wave” being propagated from the surface of symmetry to the heated surface is introduced, and the time required for this wave to reach the heated surface has been derived. Expressed in terms of dimensionless time (Fourier Number), this critical heating time appears to be constant for inert slabs of constant thermal properties. Also, it is approximately constant for noninert slabs of variable thermal properties, and it may decrease somewhat with increasing slab thickness when significant heat-generation effects are present. Experimental measurements of piloted-ignition thresholds have been conducted in an apparatus designed to simulate convective-source symmetrical heating of plane slabs. These experimental results and published data of prior investigators have been analyzed in terms of various ignition criteria. For such purposes, a generalized correlating concept is described which relegates each of the various ignition criteria to its proper position of relative importance for a particular set of conditions. As an aid for this generalized correlation technique, improved transient-heat-conduction data have been machine computed with the analytical solution for inert slabs, and the results are presented in graphical form.
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