THE CONVECTIVE HEAT TRANSFER COEFFICIENT IN FLIGHTED ROTARY DRUMS
Rotary drums are used in industrial applications for the thermal treatment of granular materials. Additional flights lift particles out of the bulk bed to shower them as curtains in the airborne phase of the drum, resulting in an improved heat transfer. The amount of particles and their distribution in the airborne phase is influenced by operational, design and bulk material parameters, which was researched in previous studies about transverse particle motion. In order to analyze the convective heat transfer, experiments were carried out in an indirectly heated (Pel = 21 kW) flighted rotary drum (D = 0.5 m, L = 1.76 m). The temperature distribution inside the drum is measured using k-type thermocouples at different axial and radial positions. After the drum and the test material (4 mm glass beads) are heated to the set temperature in batch mode, ambient air is sucked through the rotary drum to cool the system. During both phases the temperature profiles for the gas, the granular material, the flights and the drum wall are measured. The contact heat transfer coefficient is calculated for the heating phase. The convective heat transfer coefficient between gas and particles in the airborne phase (curtains) is analyzed in the cooling phase based on the energy balance of the granular material. The operational parameters rotational speed (1 – 8 rpm), air volume flow (100 – 800 Nm3/h) as well as filling degree (10, 20, 30 %) are varied and their effect on the convective heat transfer is discussed.