Thermal ignition of self-heating porous slab: Comparison with experiment

Abstract

The experimental results of the thermal ignition of a pulverized charcoal layer by a hot surface are presented against those expected from the theory of porous layer ignition. The theoretical results were found to be very sensitive to a variation in the activation energy. There is a satisfactory agreement between experimental and theoretical results when the layer is sufficiently thick (approximately L > 0.5 cm). Because the Biot number declines with the decrease in layer thickness the deviation of theory from experiment occurs at low L. Oxygen consumption near the hot surface (∼ 15 %) has only a minor effect on the temperature of ignition. The sedimentation of dusts on hot surfaces is a frequently occurring phenomenon in many branches of industry. These situations may cause spontaneous ignition due to self-heating and, subsequently, a fire. Similar problems occur in the case of storage and transportation of powders and reactive materials. An evaluation of the minimum temperature of a hot surface which ignites a thin slab of a pulverized solid is a basic problem well known in the literature on thermal explosion, and has been a subject of many experimental [1–4] and theoretical [5–10] studies. Recently, an important step has been taken toward the unification of the experimental methods used to determine the critical ignition temperature of the porous slab heated from below [4]. The purpose of this paper is to check whether the result of our previous theoretical study [10] can be used in practice.