Analytic Modeling of Heat Transfer to Vertical Dense Granular Flows

The high packing fractions of dense granular flows make them an attractive option as a heat transfer fluid or thermal energy storage medium for high temperature applications. Previous works studying the heat transfer to dense flows have identified an increased thermal resistance adjacent to the heated surface as a limiting factor in the heat transfer to a discrete particle flow. While models exist to estimate the heat transfer to dense flows, no physics-based model describing the heat transfer in the near-wall layer is found; this is the focus of the present study. Discrete element method (DEM) simulations were used to examine the near-wall flow characteristics, identifying how parameters such as the near-wall packing fraction and number of particle-wall contacts may affect the heat transfer from the wall. A correlation to describe the effective thermal conductivity (ETC) of the wall-adjacent layer (with thickness of a particle radius) was derived based on parallel thermal resistances representing the heat transfer to particles in contact with the wall, particles not in contact with the wall, and void spaces. Empirical correlations based on DEM results were developed to estimate the near-wall packing fraction and number of particle-wall contacts. The contribution from radiation was also incorporated using a simple enclosure analysis. The ETC correlation was validated by incorporating it into dense flow models for chute flows and cylindrical flows and comparing with the experimental data for each.