Ii. Kantorovich et E. Barziv, MICROSTRUCTURAL EVOLUTION OF CHAR UNDER OXIDATION-INDUCED BY UNEVEN HEATING, Combustion and flame, 105(1-2), 1996, pp. 80-91
Single spherical char particles were intentionally irradiated nonunifo
rmly in an electrodynamic chamber, in the temperature range 600-1000 K
. The char particles were irradiated from one side (bottom) and conseq
uently heated unevenly. Nonuniform shrinkage of an initially spherical
char particle has been observed during oxidation. The features of non
uniform shrinkage are rather peculiar: (1) Up to 40%-60% conversion, t
he particle shrank uniformly. Then spatial preferential consumption in
itiated, indicating the threshold nature of the phenomenon. (2) Prefer
ential consumption, in most experiments, was observed to start from th
e top of the particle. (3) Above the threshold conversion, a disk stru
cture became clear. (4) At very high conversion the particle became li
ke a center-hole doughnut. A model was developed to explain these feat
ures by nonuniform transformation of the micropore structure due to ox
idation of the unevenly heated particle. The threshold nature of the p
henomenon was also explained in terms of the dynamic stability of a pa
rticle. The threshold of nonuniform shrinkage corresponds to the trans
ition of the particle to a stable position. After initiation of nonuni
form shrinkage, the macroporosity distribution inside the particle bec
omes nonuniform. Macroporosity was shown to increase near the particle
's center; this eventually creates a hole at high conversion. The good
agreement of modeling results with experimental observation confirms
the notion that nonuniform shrinkage in regime I (kinetically controll
ed) is an indication of the fundamental microstructural transformation
s in the course of oxidation.