A frequency response study of packed bed heat transfer at elevated temperatures

Abstract

Interphase heat transfer coefficients and effective axial conductivities were obtained for packed beds of uniform alumina spheres with gaseous throughflow in the temperature range 375–1300 K. The method used was parameter estimation from frequency response measurements at two axial locations in a bed when a small sinusoidal temperature disturbance was imparted to the inlet gas temperature. A new model was proposed and frequency response expressions derived in order to take into account the large effective axial conductivity resulting from radiative transfer, unsteady temperature distribution in the solid, and gas-solid interphase heat transfer. A key feature of the model is the use of the local average particle surface temperature as the dependent variable. Results showed that the interphase Nusselt number was independent of temperature but exhibited characteristically low values at low particle Reynolds numbers with a dependence given by Nu = 0.054 Re 1.48. Effective axial conductivities showed strong temperature dependence typical of radiative transfer. Calculations showed that the observed Reynolds number dependence of the Nusselt number could be partially explained by a microscopic distribution of porosity. These also showed why results with carbon dioxide—a radiatively participating gas—were little different from radiatively non-participating gases.