OCR Text |
Show leads were threaded back through the 1/8 in. tube and attached to a strip chart recorder. Procedure for the Mass-Transfer Experiments Because of the entrance effects and flow disruptions caused by the sample basket and quartz wool, we were uncertain as to the relationship between the flow condition and the effective particle Sherwood number. A correlations for calculating the effects of external mass transfer was developed by running experiments with subliming naphthalene Acrylic spheres 0.635 cm (0.25 in.) in diameter were coated with naphthalene and placed in the sample basket. Weight loss measurements were made over a range of Reynolds numbers in room temperature air. Although the reactor was not hot, the gas flow was adjusted to cover the range of particle Reynolds numbers expected for the combustion experiments. Weight loss measurements were converted into the mass-transfer correlation described later. Vapor pressure data for naphthalene were obtained from Sogin (12). Procedure for the Combustion Experiments Two types of runs were made for each set of combustion conditions. In the first set of runs, particle temperature was measured as a function of time with the thermocouple technique described previously. Each experiment was repeated five times, and the temperature vs time curves were averaged to develop composite curves. The second type of run was made to measure weight loss as a function of time. In these, particles were carefully weighed before and after being inserted into the reactor for a fixed time interval. Runs for each time interval were repeated five times and the results averaged. Particle dimensions were measured before and after combustion with calipers or with a calibrated magnifying glass. Particles greater than 8 mesh (0.218-cm opening) were characterized by three dimensions: length, width, and thickness. Equivalent spherical diameters based on particle volume and surface area were calculated from these using an ellipsoidal approximation, with the length of the axes equal to the three measured dimensions. For particles less 14 |