RADIATION HEAT-TRANSFER IN FLUIDIZED-BEDS - A COMPARISON OF EXACT ANDSIMPLIFIED APPROACHES

Radiation heat transfer at heat exchanger walls in fluidized beds has never been examined through a complete formulation of the problem. In this paper a wall-to-bed heat transfer model is proposed to account fo r particle convection, gas convection, and radiation exchange in a var iable porosity medium. Momentum, energy, and intensity equations are s olved in order to determine the velocity, temperature, radiative heat flux profiles and heat transfer coefficients. The discrete-ordinates m ethod is used to compute the radiative intensity equation and the radi ative flux divergence in the energy equation. Both the gray and the no n-gray assumptions are considered, as well as dependent and independen t scattering. The exact solution obtained is compared with several sim plified approaches. Large differences are shown for small particles at high temperature but the simplified solutions are valid for large par ticle beds. The dependency of radiative contribution on controlling pa rameters is discussed.