FURTHER DEVELOPMENT OF THE ELECTRODYNAMIC CHAMBER FOR STUDYING SINGLE-PARTICLE OXIDATION AND NONUNIFORM SHRINKAGE OF CHAR PARTICLES

The aims of this study are (1) to present the further development of some of the characterization methods used with the electrodynamic chamber (EDC) for high-temperature studies of single particles, and (2) to present in a qualitative manner new results on char oxidation under kinetically controlled conditions. The following methods were either developed or improved for higher accuracy: (1) Shadowgraphy for size and shape measurements. (2) Two-dimensional Mie scattering for sizing and determining optical properties. (3) Drag force measurements (by forced convection) for the determination of the density. (4) Optical pyrometry using wide-band detectors in the visible and infrared regions for temperature determination. In this study, synthetic char particles, spherically shaped (Spherocarb), as well as spherical polystyrene particles were oxidized in atmospheric air. The particles were suspended in the center of the EDC and heated by a CO2 laser beam. The particles were characterized prior to reaction by the above methods. Mass and size changes as well as the particle temperature were measured as a function of time during the oxidation process. The particles were consumed in the following manner: First they were consumed uniformly, following previous shrinkage observations. At about 60% conversion nonuniform shrinkage was observed, mostly preferential shrinkage from the top of the particle. At 70% conversion a clear cut on the top of the particle was monitored. At 80% conversion preferential consumption from the other side of the particle was established, showing a clear disk configuration. At 95% conversion a hole in the center of the disk was developed. The explanation of this preferential consumption is not yet clear; however, it is possible that the nonsymmetrical boundary layer around the particle, caused by free convection, generated a temperature difference where high temperature is always at the top of the particle. It is due to the capability of the EDC that we were able to observe this unique phenomenon using its ability to suspend a particle at the center of the chamber motionless.