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Cavitation phenomena in waste water treatment by ultrasound

Objective

Original research objectives

The use of ultrasound in water treatment has gained increasing interest recently, as a method of high efficiency that avoids the use of chemical additives. Ultrasound can be used in conjunction with, or even as an alternative to, standard chemical processes in wastewater or drinking water treatment. Acoustic irradiation of water can influence cleaning processes in different ways, depending on the type of the pollutant. Indeed, it has been shown that in the presence of strong enough acoustic fields cavitation takes place, i.e. bubbles emerge, grow, and oscillate intensely. Under such conditions they are responsible for the following mechanisms of pollutant removal:
-Degassing of water from pollutants through rectified diffusion, bubble coagulation and bubble drift, as a means of odor and corrosion control.
-Separation of solid pollutants dispersed in a liquid by accumulation at bubble surfaces and agglomeration due to bubble drift.
-Neutralization of chemical pollutants as they react with free radicals, at high temperatures, which are produced during bubble collapses (sonochemistry).
-Deactivation of living micro-organisms as a result of the dynamics of bubble collapse and the subsequent shock wave formation, or through the additional corrosion due to generated free radicals.

Ultrasound can also be used in cases where bubbles are not involved:
-Separation of hydrocarbon droplets from water in preliminary oil separation processes
-Removal of fine dust particles from a gas stream through ultrasonic agglomeration. So far the design of units based on ultrasound treatment is characterized by a high degree of empiricism. Not much has been done to understand the influence of important parameters from a fundamental viewpoint. In particular, the effect of the characteristics of the acoustic pressure field on the cavitation bubble distribution and dynamics is largely unclear. The acoustic field is determined by the resonator shape, the acoustic pressure amplitude and frequency, the fluid properties, and finally by the retroaction of the bubble population. An understanding of collective cavitation bubble behaviour, including nucleation, coalescence, drift and structure formation, is essential for the modelling of the processes ongoing in an ultrasonic cleaning device. Such a modelling, in turn, is an important step towards the optimisation of existing and future ultrasound treatment facilities of wastewater. The recent advances in the field of cavitation, whether dealing with bubble-bubble interaction, or pattern formation in bubble fields, constitute a solid basis for an investigative effort.

The objectives of the proposed research effort are:
- Investigation of interaction forces for arbitrary bubble radii, bubble separation, and sound wavelength.
- Introduction of dissipation mechanisms, such as, viscous dissipation, heat conduction, and compressibility in the gas and/or liquid phases.
- Effect of non-linear dynamics on bubble coalescence, resonance and break-up.
- Modelling of the collective behaviour of bubble populations in acoustic fields.
- Numerical simulation of pattern formation in bubble fields.
- Experimental observations and high-speed video recording of bubble dynamics.
- Streamlining of numerical simulations with industrial applications.
- Improvement and optimisation of existing and/or planned cleaning technologies. In summary, the research effort will be twofold. Partners 1 and 2 will focus on the micro-scale aspects of the problem (bubble-bubble interaction level), whereas Partners 3 and 4 will concentrate on the macroscale aspects (pattern formation in large bubble populations). Finally, all the Partners in close collaboration with the industrial subcontractor (a German SME specializing in ultrasound applications) will incorporate the findings of the proposed investigation in industrial applications.

Expected deliverables
Through the proposed research activity, a more thorough understanding of the inter-bubble acoustic radiation forces will be obtained, in the presence of different dissipative effects and for a wide range of bubble radii and acoustic pressure characteristics. In addition, a deeper insight will be provided into the physical mechanisms responsible for pattern formation in large bubble populations and hence, an interface will become possible with existing technologies implementing acoustic purification processes. Specifically, the major expected results of the proposed investigation are:
- Derivation of acoustic radiation forces for arbitrary bubble radii and sound wavelength
- Incorporation of dissipation mechanisms such as, liquid viscosity, gas and/or liquid heat conductivity and, gas and/or liquid compressibility
- Non-linear bubble-bubble interaction
- Investigation of bubble coalescence
- Experimental data regarding bubble behaviour in cavitating liquids
- Mathematical model for simulation, control, and optimisation of collective bubble behaviour in strong acoustic fields
- Numerical code for the simulation of cavitation bubble structures, including state of the art graphics presentation
- Improvement and optimisation of ultrasound cleaning technology.

The main results of the project will be published in reputed international scientific journals, such as, the Journal of Fluid Mechanics, the Physics of Fluids, and the Journal of the Acoustical Society of America, the Physical Review etc. They will also be presented at relevant international conferences that will take place in the duration and after the expiration of the project, for example, at the joint meeting of EAA Forum Acusticum and the Acoustical Society of America to be held in Berlin in March 1999, at the 4th European Fluid Mechanics Conference to be held in the year 2000, IUTAM conferences.
The main objective of the project was to advance the understanding, modelling and control of cavitation phenomena emerging in sonicated liquids (cavitation is the phenomenon of liquid rupture and bubble creation and oscillation under dynamic tension, initiated, for example, by strong sound fields in liquids). The key application in mind was wastewater treatment by ultrasound, where cavitation is typically the most important cause for the observed effects (mechanical, e.g. rupture of bacteria, or chemical, e.g. oxidation of aromatics via creation of free radicals). The investigations followed a small-to-large approach, based on the small spatial scale of single or few interacting bubbles (Partners 1 and 2), and advancing to larger scales of bubble clusters and structures (primarily Partners 3 and 4). The research group from Belarus (Partner 2) extended the theory of acoustic radiation forces, known as Bjerknes forces, which are experienced by gas bubbles in acoustic fields and cause them to migrate, cluster in certain space areas, interact with each other, etc. The pre-existing theory was based on a large number of simplifying assumptions (such as relative bubble size and distance, importance of various dissipative forces, etc.), which restricted its accuracy and applicability and did not account for many experimental observations. So, owing to these new extensive studies the old theory was considerably extended. The approach used by the Belarus group represents a combination of analytical techniques, based on the methods of mathematical physics, and numerical simulations.

The research group from Patras (Partner 1) targeted its research in the study of the effect of viscous dissipation and nonlinear dynamics on bubble-bubble interaction and in the modeling of collective behavior of bubble populations as well as their numerical simulation. It showed that the drag force on a bubble translating due to its interaction with another bubble is given by the Levich formula. Use of this formula in numerical simulations on bubble-bubble long distance interaction has revealed when stable bubble pairs can be formed. The same formula were combined with the findings of the Partner from Belarus on the interaction forces in a bubble cloud subject to small sound amplitudes to simulate many-bubble interactions. The Russian group (Partner 3) first developed mathematical models and fast numerical algorithms for calculating single bubble oscillations and their translational motions under large pressure variations and then implemented them in new numerical codes for the prediction of the dynamics of cavitation bubble clouds. Collective bubble behaviour was simulated on the basis of the particle model taking into account nonlinear multi-bubble interactions, the two-dimensional continuum model of wave propagation in slightly compressible liquids using the method of multi-scale expansion and the model of bubble cluster including the effects of shape oscillations and rectified diffusion. The German group (Partner 4) investigated experimentally single bubbles by laser-induced optical breakdown and bubble traps.

High-speed optical and acoustical recordings were employed to resolve bubble dynamics and their shock wave emissions. For few-bubble systems, bubble traps and larger acoustic resonators were used for a detailed observation of translating and interacting bubbles and for recording of the dynamics of bubble structures. These investigations included a large and high power device, which is representative for industrial applications. A comparison between experimentally observed and simulated bubble behaviour was made both in a quantitative and qualitative way. For dendritic bubble structures, three-dimensionally reconstructed bubble trajectories were re-calculated in simulations, and in another case a double-layer bubble structure was re-modelled. Overall, the results of this joint effort provide solutions to a number of theoretical and practical problems in the field of bubble dynamics. Altogether, they make major contributions to the modelling and dynamics of individual bubbles and multi bubble clusters in acoustic fields, and thus serve for the development of ultrasound applications in wastewater treatment.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

University of Patras
Address

26500 Patras
Greece

Participants (3)

Belarusian State University
Belarus
Address
Bobruiskaia
220050 Minsk
Georg-August-Universität Göttingen
Germany
Address
Bürgerstraße
37073 Göttingen
Institute of Mechanics - Russian Academy of Sciences (Ufa Branch)
Russia
Address
K. Marx Street
450000 Ufa