Prediction of conductive heating time scales of particles in a rotary drum

Modeling conductive heat transfer from rotary drum walls to a particle bed via discrete element method simulations, three time scales were determined: 1) the characteristic heating time of the bed, τ 2) the particle thermal time constant, τ_p; and 3) the contact time between a particle and the wall, τ_c. Results fall onto a monotonic curve of τ/τ_c vs. φ (τ_p/τ_c), with three heating regimes. At low φ, conduction dominates, and the system heats quickly as a solid body. At high φ, granular convection dominates, and the bed heats slowly at a nearly uniform temperature. At intermediate φ, the system heats as a cool core with warmer outer layers. The results of this work have important implications for improving the design and operation of rotary drums (e.g., energy-intensive calcination processes). By calculating τ_p and τ_c from material and operating parameters, the characteristic heating time, τ, can be predicted a priori.