Measurement of radiative heat transfer coefficient in a high temperature circulating fluidized beds

Experimental measurements of the radiative heat flux were made, and radiative heat transfer coefficients were determined for a circulating fluidized bed of sand particles of mean diameters of 137 and 264 microns. The bed used in this study measured 0.05 m in diameter. The heat transfer test section was 0.9 m long and located in the middle of CFB riser. Operating temperature was varied from 200–600 °C, and the gas velocity in the CFB riser varied from 6 m/s to 11 m/s. The suspension densities covered a range from 3 to 35 kg/m3. Time-averaged radiative heat flux was directly measured with a radiometer. Radiative heat flux and suspension emissivity showed strong dependence on the suspension density. Particle size effect on suspension emissivity was observed. Experimentally determined suspension emissivities, which ranged from 0.3 to 0.85, were in good agreement with the predicted suspension emissivity based on independent scattering theory. The radiative heat transfer coefficients were determined from the measured radiative heat fluxes and were found to be well predicted by the Stefan-Boltzmann law. It was also found that for a dilute system, the prediction of suspension emissivity by Hottel and Sarofim, in conjunction with independent scattering theory of Brewster and Tien, showed good agreement with experimentally determined suspension emissivity.