(150d) Experimental Validation of Indirect Conduction Model and Biot Number Analysis for Wall-to-Particle Heat Transfer

Heat transfer between particles and a hot surface is the governing step in many unit operations, such as rotary dryers, heat exchangers, packed beds, etc. In such dense-phase processes, conductive heat transfer from the hot surface (wall) to particle is often the dominating mechanism. Particulate conduction can further be divided into two categories: (i) conduction at the direct contact between the particles and wall (direct conduction), and (ii) conduction through the interstitial fluid surrounding the particles (indirect conduction). The relative importance of the two mechanisms is gauged by the ratio k_gR_p /k_effR_c, where k_g is the thermal conductivity of the interstitial fluid, R_p is the radius of the particle, k_eff is the effective thermal conductivity for the particle and wall, and R_c is the radius of the contact area between the particle and wall; indirect conduction dominates if the ratio is greater than unity. Hence, for particles having exceedingly small contact area and relatively low thermal conductivity – sand, glass, steel, etc. – indirect conduction is important. The indirect conduction theory developed by Rong and Horio [1] is being widely used for MFIX-DEM simulations, though experimental validation for the above model is lacking in the literature. The present work is focused on experiments to assess the validity of the indirect conduction model for wall-to-particle heat transfer. To achieve this goal, experiments were carried out using glass and steel particles, where indirect conduction is expected to dominate. These experimental results were compared to DEM simulations performed with the aforementioned indirect conduction model, with good agreement for steel but not for glass. The mismatch for glass particles was ultimately traced to a non-uniform temperature gradient within the particle due to indirect conduction. Accordingly, a modified Biot number which includes the effect of indirect conduction is introduced.

Reference

[1] D. Rong, M. Horio, DEM simulation of char combustion in a fluidized bed, Second International Conference on CFD in the …. (1999).