ROLE OF THE PORE STRUCTURE IN THE FRAGMENTATION OF HIGHLY POROUS CHARPARTICLES

Fragmentation of highly porous particles burning in regime I under two heating modes, uniform and nonuniform, is considered. Oxidation with nonuniform heating is analogous to regime II, because it creates nonun iform reactivity. A uniformly heated particle did not fragment, even f or large extents of burning. The nonuniformly heated ones showed syste matic local fragmentation. Analysis of a particle's behavior under the two heating modes allowed one to substantiate that percolation fragme ntation does not occur by a change in total porosity, but is due to ch anges in the larger pores of the char (meso-and macro-). Calculations were compared with experimental data from the literature and the follo wing conclusions were drawn: (1) Fragmentation is determined by the po rosity of the larger pores with diameters higher than the mean dimensi on of a microcrystal within the microskeleton, (2) Shrinkage is determ ined by the micromedium, or small pores. (3) For uniformly heated part icles the porosity of the large pores barely changes during burning, w hich accounts for nonfragmentation. (4) The reason for local fragmenta tion under nonuniform oxidation is the increase in the local macroporo sity, triggered by nonuniform shrinkage of the micromedium. (C) 1998 b y The Combustion Institute.