Radiative and total heat transfer in circulating fluidized beds

Suspension-to-surface total and radiative heat transfer in circulating fluidized beds at elevated temperatures were studied using a dual tube assembly, a membrane membrane wall and a multifunctional probe. All experiments were carried out in a pilot scale circulating fluidized bed combustor having a 0.152 m by 0.152 m square cross-section and a 7.3 m height. The effects of density, suspension temperature and particle size on the total and radiative heat suspension transfer coefficients were investigated. Results were obtained with silica sand particles of mean diameter 137, 286, 334 and 498 {dollar}rmmu m{dollar} at suspension temperatures ranging from 794 to {dollar}rm 913spcirc C{dollar} for suspension densities up to 110 {dollar}rm kg/msp3.{dollar}; In one set of experiments, heat transfer rates were measured simultaneously for two tubes having different surface emissivities. The radiative component was estimated by comparing the total heat fluxes measured for each tube. It was found that the total suspension-to-tube heat transfer coefficients, which ranged from 150 to 250 {dollar}rm W/msp2{dollar} K, increased with increasing suspension density and decreased with increasing mean particle size under the conditions of this study. The measured radiative suspension-to-surface heat transfer coefficients, which lay between 40 and 100 {dollar}rm W/msp2{dollar} K, also increased with increasing suspension density and decreased with increasing particle size. For the bulk suspension temperature range of 800 to {dollar}rm 900spcirc C,{dollar} it was found that radiative heat transfer comprises 25 to 45% of the total heat transfer from the suspension to a tube located near the wall.; Experiments were also carried out using a membrane wall containing embedded thermocouples as the heat transfer surface. Total suspension-to-pipe and suspension-to-fin heat transfer coefficients were estimated from the temperatures measured inside a pipe and a fin. The suspension-to-pipe heat transfer coefficient is higher than the suspension-to-fin coefficient when radiation is significant for the conditions investigated, indicating that the fin is not as efficient as exposed pipe surface in extracting heat.; A multifunctional probe was designed and fabricated. This probe combines the advantages of the differential emissivity method and the window method for measuring radiative heat transfer fluxes. Cylinders of well-oxidized stainless steel 347 and polished stainless steel 316 with distinctly different surface emissivities were incorporated in the probe, while zinc selenide was chosen as the window material.; A comprehensive radiative heat transfer model was developed based on the known core-annulus structure of the suspension in the circulating fluidized bed. The predictions from the model are about 20% higher than the experimental data obtained by the probe using the differential emissivity method, while the predictions agree well with the results using the window method under the conditions in this study. The model predicts that radiation contributes about 35% of the total heat transfer coefficient for typical CFB combustion conditions. (Abstract shortened by UMI.)...