10.03.2011
Ultrasonic baths are widely used in industry, in manufacturing and in the domestic field. The construction of an instrument for a specific application has to this date, however, been based on long test series and on individual empirical values of the manufacturers. The reason for this is that the underlying physical effect – cavitation – is, by nature, highly stochastic and depends on a whole series of ambient parameters. PTB has now succeeded in bringing some order into the chaos.
Ultrasound is used for the most diverse applications in medicine and technology. In liquid media, these are often based on cavitation processes. Due to the stochastic nature and the many influence parameters, it is extremely difficult to check, to control and to optimise all application processes. This hinders especially the manufacturers – who are usually small- and medium-sized companies – from developing and designing ultrasonic baths for the broad range of application possibilities. To overcome this problem, a project has been carried out at PTB which was funded via the Arbeitsgemeinschaft industrieller Forschungsvereinigungen (AiF). The aim of this project was to improve the quantitative description of the phenomena occurring in ultrasonic baths of the small and the medium performance classes with the aid of simple measurements of physical quantities.
The most essential approach hereby was the measurement of the sound field quantities. The objective of this project was to develop this approach further and to complement it with new measurement techniques by which the cavitation effects can be determined. For this purpose, different model processes were investigated and suitable indicators for cavitation were developed – among these, indicators for the erosion (see Fig. 1), for the chemical effect and for sonoluminescence (see Fig. 2). The indicators serve to describe possible applications quantitatively, that is to determine the actual effect. For this purpose, the relationships of the indicators among each other were investigated and their dependence on the operating parameters was determined. An essential element of this was multivariate data analysis. By means of a factor analysis, statistical analyses were carried out and a general procedure was developed by which indicators – or other output quantities of a cavitation process – can be described as a function of easily measurable input quantities (see Fig. 3).
Figure 1: Cavitation corrosion on an aluminium foil which has been exposed to ultrasound for only a few seconds.
Figure 2: The bright areas show the chemoluminescence of a sensor placed in the ultrasonic bath and the sonoluminescence of the surrounding water.
Figure 3: Several easily measurable input quantities at the ultrasonic cleaning vessel – of which each one for itself does not seem to have any relation to the cavitation effect – can be grouped together to one fictive quantity which describes the process well. In this example, the water temperature, the sound pressure of the fundamental and the sound pressure of the cavitation noise constitute a generalised quantity zc which exhibits a good relationship with the erosion effect.
Another task was to investigate a filled cleaning bath. For that purpose, representative "workpieces" were selected and a new control system of the sensor was developed which takes the position of the workpiece during the measurement into account. It was investigated to what extent the cleaning product changes the sound fields in the bath, and where cavitation takes effect and where not.
The methods developed and the results obtained are very helpful for a process description. Cleaning and reaction effects can be estimated quantitatively, and the operating parameters can be optimised. This not only makes it possible for the manufacturers to develop ultrasonic baths for a special purpose, but the users also obtain a tool for quality management.
The project "Investigation, measurement and optimisation of the sound field and its effects in cleaning baths and sonochemical reactors" of the AiF Research Association DECHEMA Gesellschaft für Chemische Technik und Biotechnologie e.V. (Society for Chemical Engineering and Biotechnology) was funded via the AiF within the scope of the "Programme for the promotion of industrial joint research and development (IGF)" of the German Federal Ministry of Economics and Technology.
Contact person:
Matthias Jüschke, Dept. 1.6, WG 1.62, E-mail: matthias.jueschke@ptb.de