Optimization of Ultrasonic Acoustic Standing Wave Systems

Paul Dunst,Walter Sextro,Tobias Hemsel,Walter Littmann,Peter Bornmann

doi:10.3390/act9010009

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.

Highlights

Every vibrating system emits sound waves to its environment, which can be disturbing but can be used to improve processes like cleaning, chemical reactions, or dispersing of fluids and powders.To generate ultrasonic waves, typically piezoelectric bolt-clamped Langevin transducers are used [1,2,3,4,5], which can generate high power ultrasonic radiation at electrical power of up to a few kilowatts
To achieve maximum sound pressures by only improving geometrical dimensions of the sound field, it must be ensured that the standing wave condition is fulfilled for all sound waves
With the concave reflectors on both sides, the maximum sound pressure level is in the middle of the sound field and far away from the reflectors and the transducer

Summary

Introduction

Every vibrating system emits sound waves to its environment, which can be disturbing but can be used to improve processes like cleaning, chemical reactions, or dispersing of fluids and powders. Due to dispersion of the sound waves, the pressure amplitude of standing wave systems in the far field drops rapidly with increasing distance from the transducer. The maximum pressure at the resonant distances (see Equation (1)) decreases with increasing number of n. When a plain reflector is used, as depicted, only sound waves, that are roughly parallel to the rotation axis, are reflected back to the transducer and contribute to the standing wave field. The standing wave condition (see Equation (1)) is fulfilled only for waves near the rotation axis. To overcome these issues, the geometries of transducer and reflector have to be optimized so that the highest possible sound pressure is achieved in the sound field. Modeling results for optimized geometries are shown, discussed, and validated by measurements on an experimental setup of a standing wave system

Model Setup

Model Based Optimization

Experimental Validation

Findings

Conclusions and Outlook