Investigation of heat transfer in packed beds at high temperatures and low Reynold's numbers

At high temperatures in packed bed heat exchangers or reactors, one must consider the contribution of radiative transfer as well as conduction and convection to the total heat transfer process. In this study, the frequency response method was used to investigate heat transfer in a packed bed subject to gas flow at low Reynold's numbers (less than 16). The frequency response method requires the use of a mathematical model for data analysis. A new model was developed, the extended single particle model, which includes axial conduction in the solid, solid-to-solid particle radiative transfer, gas-particle interphase transfer, and is expressed in terms of the surface temperature of the solid. Experiments were performed using four different sizes of alumina spheres, ranging from 0.003175 to 0.01905 m in diameter, with temperatures from 600 to 1300 K and particle Reynold's numbers varying from 0.6 to 15.4. The model was used to obtain estimates of the heat transfer parameters h (gas-to-particle heat transfer coefficient) and k/sub eax/ (effective axial thermal conductivity), using a nonlinear least squares parameter estimation method. The Nusselt number was found to follow Nu = 0.154 Re/sup 1.48/ for 0.6 < Re < 16, and were only weakly dependent on temperature