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An investigation of the mechanisms at the interaction between cavitation bubbles and contaminants

Periodic Reporting for period 2 - CABUM (An investigation of the mechanisms at the interaction between cavitation bubbles and contaminants)

Reporting period: 2020-01-01 to 2021-06-30

The CABUM project explores the phenomenon of cavitation, where bubbles appear in a liquid in case of a sudden decrease in pressure, which later collapse in a higher pressure area. It is a process that until recently was considered undesirable, because it causes noise, vibration and erosion in water machines. In recent years, however, under specific conditions, it has started to be used for the cleaning of surfaces, improvement of chemical processes, waste water treatment, and elsewhere.

The idea behind the CABUM project is based on the lessons we learned from the multi-year cooperation with the business sector. Clearly, gaps in the knowledge of the physical background of cavitation have been shown, which limit the introduction of this technology into advanced industrial processes.

The 5-year long project pursues a better understanding of the physical background of the phenomenon of cavitation, which could be used in the future for the environmentally friendly cleaning of drinking and waste water.
During the first 2.5 years of the project, we have conducted a review of the current understanding of the mechanisms taking place behind the phenomenon and proposed guidelines which should be followed to ensure reproducibility and clearness of the studies and to avoid oversimplification of the problem and uncritical dissemination of the conclusions over various application areas.

We have conducted experimental campaign to study, for the first time, the effect of hydrodynamic cavitation on liposome stability and compare it to the effect of well described chemical, physical and mechanical treatments. Here we showed, for the first time, that hydrodynamic cavitation was among the most effective physico-chemical treatments in destroying lipid vesicles.

We investigated the chemical reactions which accompany cavitation. During the investigation we found anomalies in the existing methodology, which was used routinely used by researchers for the past 20 years and also suggested amendments.

We indicated the possibility of time resolved visualization and measurements of shock waves which are emitted upon cavitation cloud collapse. Shock wave front velocities and local pressure waves caused by cloud collapse were estimated from visualization, reaching values to more than 700 m/s and over 5MPa respectively. The methodology was already employed for optimization of pilot cavitation reactor for wastewater treatment.

Finally, simulations were used to investigate the bubble-particle interaction on a micro scale. This way the likelihood of previously suspected mechanical effects to cause bacterial cell damage, such as impact of a high-speed water jet, propagation of bubble emitted shock waves, shear loads, and thermal loads could be determined. In the near future the simulations will be evaluated by experiments.
Progress beyond the state of the art in several areas was achieved. Among other:
1. We developed a methodology which enables time resolved visualization of the shock waves in cavitating flows. These were predicted by simulations but never before visualized experimentally. Moreover, the methodology also enables to determine the magnitude of the pressure peaks.
2. We have amended the methodology for determining the radical production in cavitation. We found that the existing methodology, which was routinely used by researchers, significantly influences the cavitation dynamics, hence the results are not reliable.
3. Using numerical simulations we have pinpointed several candidates for the mechanical mechanisms, which could contribute most to the aggressive nature of cavitation against bacteria.
4. We have designed several, robust, test facilities, which enable studies at demanding conditions – high pressures, high temperatures, low temperature, aggressive fluids etc.. This confirms our group as the leader in the filed of cavitation exploitation studies.

By the end of the project we expect to gain full understanding and control of cavitation and its interaction with contaminants. A clear goal is the transfer of the scientific outputs into application – pilot devices are already being developed.
The idea behing the ERC CoG CABUM project