(161b) Damping of Ultrafine Powders during Vibrations and Shear Flow

The production of ultrafine powders requires reliable powder storage and handling equipment. The enormous interparticle adhesion forces of ultrafine powders disturb the powder flow. This powder flow in a silo or bunker? normally driven by the force of gravity ? can be stopped by channelling or bridging. To avoid these flow problems, to promote and control the discharge of cohesive powders, mechanical vibrations are widely used in practice of powder handling. The flow and the damping behaviour of ultrafine cohesive powders (limestone, d = 1.3 µm; titania, d = 0.6 µm; alumina, d = 1.0 µm) are analysed by a vibrating shear tester according to ROBERTS. The vibrations are directly applied to the shear base and transmitted through the shear zone to the shear ring. Frequencies between 50 and 200 Hz and oscillation displacement amplitudes from 16 to 64 µm are used. The measurements are accomplished in a stress range from 0.5 to 8 kPa and with the constant shear rate 2 mm/min. At each shear ring and shear base an acceleration sensor is mounted. From the attenuation or amplification between exciting and resulting signals the parameters eigenfrequency, viscous damping ratio and COULOMB-damping ratio are determined. These characteristics are evaluated by the augmentation function proposed by DEN HARTOG. The applied vibrations lead to a reduced shear strength of the powders. Depending on the particle size and the particle hardness the powders show different damping behaviours. The evaluation of the results shows an increasing eigenfrequency of the powders with increasing normal stress. The viscous damping ratio decreases with increasing normal stress. The influence of the COULOMB-damping on the total damping behaviour was comparatively small and was found to be nearly constant. Additionally first experiments with limestone powder at higher moisture levels were performed. An influence on the damping parameters can be seen. Compared to the dry powder there are lower eigenfrequencies and viscous damping ratios but no clear tendency in the COULOMB-damping.