Particle Scale Heat Transfer Analysis in Rotary Kiln

Abstract

Rotary furnaces have multiple applications including calcination, pyrolysis, carburization, drying, etc. Heat transfer through granular media in rotary kilns is a complex phenomenon and plays an important role in the thermal efficiency of rotary furnaces. Thorough mixing of particles in a rotary kiln determines the bed temperature uniformity. Hence it is essential to understand the particle scale heat transfer modes through which the granular media temperature changes. In this study, numerical simulations are performed using coupled Discrete Element Method (DEM) and Computational Fluid Dynamics (CFD) to analyze heat transfer in a non-reacting rotary kiln. The microscopic models of particle-particle, particle-fluid, particle-surface and fluid-surface heat transfer are used in the analysis. The heat transfer simulations are validated against experimental data. The effect of particle cascading on the bed temperature is measured and contributions from various modes of particle scale heat transfer mechanisms are reported. Particles are heated near the rotary kiln walls by convection heat transfer as they pass through the thermal boundary layer of the heated fluid. These particles are transported to the center of the kiln where they transfer heat to the cooler particles in the core of the kiln and back to the cooler fluid at the center of the kiln. It is found that 90% of the heat transferred to particles from the kiln walls is a result of convection heat transfer, whereas only 10% of the total heat transfer is due to conduction from the kiln walls.