Experimental and theoretical study of total and radiative heat transfer in circulating fluidized beds

A new heat transfer model is proposed for suspension-to-wall heat transfer in circulating fluidized beds. The model assumes an emulsion layer with varying temperature and solids concentration close to the heat transfer surface and a thin gas layer between this layer and the wall. The packet model of Mickley and Fairbanks (1955) is used to predict convective heat transfer through the emulsion layer, while conductive heat transfer through the gas layer is estimated based on its thickness. Independent scattering theory is employed for radiative heat transfer in the non-gray, absorbing, emitting and anisotropically scattering emulsion layer. The discrete-ordinate method is used to solve the radiative heat transfer equation. The predicted total and radiative heat transfer coefficients are compared with new experimental results obtained in a 152 mm x 152 mm s x 7.3 m tall CFB combustion pilot unit using a multifunctional probe and silica particles of mean diameter 286 and 334 mu m at temperatures from about 800 to 900 degrees C. The model generally gives good predictions of these experimental results, as well as others from the literature. (C) 1999 Elsevier Science Ltd. All rights reserved.