Measurements of heat transfer between particles and gas in packed beds at low to medium Reynolds numbers

The heating of packed beds by a hot gas flow depends on the heat transfer between the gas and the particles beside the heat conduction and the radiation between the particles. In order to consider chemical reactions within the particles, it is necessary to take the local convective heat transfer into account. Therefore it is useful to have informations about the variations of the heat transfer coefficient as a result of non-uniform pore velocities in randomly packed beds. Transient heat transfer data is obtained for a nitrogen gas flow through packed beds of porous slate particles and of wooden cubes by local measurements of the particle and the gas temperature. The special arrangement of the test particles insures that heat conduction is excluded. The convective heat transfer coefficient between the flowing fluid and the solid particles is calculated from the temperature measurements. Variations of the gas temperature and the flow rate are taken into account. Increasing gas flow leads to larger values of the heat transport coefficient. The experimental results are compared with the functional relationships of the Nusselt number Nu(dp) = f (Re-o.dp) given by multiple authors. Due to inhomogeneities of the pore velocities the experimental data shows a large variance in particular at low Reynolds numbers, i.e., Re-o.dp < 200. This is treated by a statistical method resulting in logarithmic probability distribution functions for the different materials which are useful for the mathematical treatment of the heating of randomly packed beds. The influence of radiation on the heat transport is considered numerically by solving the equations of a one-dimensional particle model. It is shown, that below 500 degrees C, the influence of radiation is negligible. (C) 2005 Elsevier Inc. All rights reserved.