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Radiative and total heat transfer in circulating fluidized beds

University of British Columbia

Suspension-to-surface total and radiative heat transfer in circulating fluidized beds at elevated temperatures were studied using a dual tube assembly, a 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 suspension density, suspension temperature and particle size on the total and radiative heat transfer coefficients were investigated. Results were obtained with silica sand particles of mean diameter 137, 286, 334 and 498 μm at suspension temperatures ranging from 794 to 913 °C for suspension densities up to 110 kg/m³ . 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 W/m² 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 W/m² K, also increased with increasing suspension density and decreased with increasing particle size. For the bulk suspension temperature range of 800 to 900 °C, 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. An empirical correlation for the total suspension-to-surface heat transfer coefficient is proposed for broad ranges of suspension density, particle size and suspension temperature. This correlation represents within ±50% the results of this work, as well as the high-temperature results of previous workers. 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 the 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. The probe was calibrated using a cylindrical cavity heated by an electric furnace. Net radiation and ray tracing methods were employed to develop a model to calculate the radiative component through the window. Comparison of the results from these two methods showed that the radiative heat transfer coefficient determined by the window method was higher than that obtained by the differential emissivity method. The discrepancy between the two methods is attributed to the unreliable values of the surface emissivities used in the differential emissivity method. The heat transfer coefficients measured by the probe were higher than those obtained using either the dual tube or the membrane wall due primarily to the attenuating effect of the length of the heat transfer surface. A comprehensive radiative heat transfer model was developed based on the known coreannulus structure of the suspension in the circulating fluidized bed. In the core region, the temperature and solids concentration are assumed to be uniform, while the annulus region is composed of a gas layer and an emulsion layer in the vicinity of the wall, with heat being transferred through the parallel layers by conduction and convection, respectively. For radiative heat transfer, the gas layer is assumed to be transparent. The emulsion medium is considered to be non-gray, absorbing, emitting and scattering, while the scattering within the emulsion layer is taken as multiple, independent and anisotropic. The temperature profile as well as the solids concentration profile in the emulsion layer were considered. 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.

Thesis/Dissertation

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2009-04-03

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http://hdl.handle.net/2429/6756

Chemical and Biological Engineering

University of British Columbia

UBCV

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