Numerical and experimental investigation of the heat transfer of spherical particles in a packed bed with an implicit 3D finite difference approach

Heat transfer in packed or fluidized beds in the presence of a surrounding fluid is an important phenomenon which is relevant to numerous industrial applications. Here we extend an earlier derived 3D heat transfer model (Oschmann et al. in Powder Technol 291: 392-407, 2016) to take into account particle-fluid heat convection in the case of Biot numbers Bi >> 1. The Discrete Element Method (DEM) which is coupled with the commercial Computational Fluid Dynamics (CFD) package ANSYS Fluent is used as the modelling framework. As a first approximation of the flow induced inhomogeneity of the local heat transfer on the particle surface a distribution function is employed. To validate the resolved heat transfer model, we compare DEM/CFD simulations of three different materials (wood, Polyoxymethylene (POM) and aluminum) with performed experiments. This firstly includes cases where particle surface temperatures are compared with measurements of an infrared camera. Secondly, a numerical study of the average bed temperatures of particle core and surface is conducted to show the differences of the used materials. Thirdly, the core temperatures of three selected particles are compared against experiments. The DEM/CFD framework provides an accurate description of the temperature evolution where the wall effects are negligible. Close to the walls a qualitative agreement can only be achieved for materials with low thermal conductivities. As a consequence of this, in the second part of our investigation we provide various CFD simulations for the heating of an aluminum oxide wall which is required for the evaluation of the particle surface temperatures measured by an infrared camera. The simulation results show the same tendencies as the experiments, underline the complexity of the heat transfer at the walls and are a first step for the formulation of a complex particle-wall heat transfer model in the context of a DEM/CFD framework.