Since fine powders tend strongly to adhesion and agglomeration, their processing with conventional methods is difficult or impossible. Typically, in order to enable the handling of fine powders, chemicals are added to increase the flowability and reduce adhesion. This contribution shows that instead of additives also vibrations can be used to increase the flowability, to reduce adhesion and cohesion, and thus to enable or improve processes such as precision dosing, mixing, and transport of very fine powders. The methods for manipulating powder properties are described in detail and prototypes for experimental studies are presented. It is shown that the handling of fine powders can be improved by using low-frequency, high-frequency or a combination of low-and high-frequency vibration.

No abstract

Ultrasonic acoustic standing wave systems find use in many industrial applications, such as sonochemical reactions, atomization of liquids, ultrasonic cleaning, and spray dry. In most applications, highest possible sound pressure levels are needed to achieve optimum results. Until now, the atomization of liquids is limited to fluids with low viscosity, as systems generating sufficient sound pressure for atomizing fluids with higher viscosities are often not marketable due to their low throughput or high costs. For the production of polymer or metal powders or the dispensing of adhesives, highest sound pressures should be achieved with systems in suitable size, with good efficiency and at low cost but without contamination of sonotrodes and reflectors by the dispersed media. An alternative to the use of more powerful transducers is increasing the intensity of the acoustic standing wave field by optimizing the boundary conditions of the acoustic field. In most existing standing wave systems a part of the radiating sound waves does not contribute to the process, as the waves spread into the wrong direction or wipe themselves out due to interference. In order to obtain maximum sound pressure amplitudes in the standing wave field, all waves should be trapped between the sonotrode and the reflector. In addition, the resonance condition should be met for all radiated waves. These conditions can be fulfilled by optimizing the shapes of sonotrode and resonator as well as the distance between them. This contribution reports on a model, which is able to simulate the sound field between a transducer surface and a reflector. Using a linear finite-element model, the boundary conditions of the standing wave system are optimized. Sound pressure levels of the standing wave field are calculated for different shapes of reflectors and boundary conditions like the distance between the transducer and the reflector. The simulation results are validated by sound-field measurements via refracto-vibrometry and a microphone. Finally, optimization guidelines for the generation of high-intensity acoustic standing wave fields are shown and verified by measurements.

The transportation of dry fine powders is an emerging technologic task, as in biotechnology, pharmaceutical or coatings industry particle sizes of processed powders are getting smaller and smaller. Fine powders are primarily defined by the fact that adhesive and cohesive forces outweigh the weight forces. This leads to mostly unwanted agglomeration (clumping) and adhesion to surfaces, what makes it more difficult to use conventional conveyor systems (e. g. pneumatic or vibratory conveyors) for transport. A rather new method for transporting these fine powders is based on ultrasonic vibrations, which are used to reduce friction and adhesion between powder and the substrate. One very effective set‐up consists of a pipe, which vibrates harmoniously in axial direction at low frequency combined with a pulsed radial high frequency vibration. The high frequency vibration accelerates the particles perpendicular to the surface of the pipe, which in average leads to lower normal and thereby smaller friction force. With coordinated friction manipulation the powder acceleration can be varied so that the powder may be greatly accelerated and only slightly decelerated in each excitation period of the low frequency axial vibration of the pipe. The amount of powder flow is adjustable by vibration amplitudes, frequencies, and pulse rate, which makes the device versatile for comparable high volume and fine dosing using one setup. Within this contribution an experimental set‐up consisting of a pipe, a solenoid actuator for axial vibration and a piezoelectric actuator for the radial high frequency vibration is described. An analytical model is shown, that simulates the powder velocity. Finally, simulation results are validated by experimental data for different driving parameters such as amplitude of low frequency vibration, pipe material and inclination angle. (© 2016 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Since fine powders tend strongly to adhesion and agglomeration, their processing with conventional methods is difficult or impossible. Typically, in order to enable the handling of fine powders, chemicals are added to increase the flowability and reduce adhesion. This contribution shows that instead of additives also vibrations can be used to increase the flowability, to reduce adhesion and cohesion, and thus to enable or improve processes such as precision dosing, mixing, and transport of very fine powders. The methods for manipulating powder properties are described in detail and prototypes for experimental studies are presented. It is shown that the handling of fine powders can be improved by using low-frequency, high-frequency or a combination of low-and high-frequency vibration.