Figure 1: Sonolator sketch

Figure 2: Sonolator orifice

David Ryan1 2, Mark Simmons1, Mike Baker2 – June 2012Cavitation in Sonolators

Acknowledgements:• Duncan Court, Unilever R&D

Port Sunlight, UK• Bob Sharpe, Bill; Chem

Eng, Univ. of Birmingham

For additional information contact:David Ryan, Chemical Engineering,

University of Birmingham, B15 2TT (davidryan1998@hotmail.com)

1University of Birmingham, UK; 2Unilever Research and Development, Port Sunlight, UK

What is a Sonolator?

Conclusions

Aims and Objectives

Overall EngD project aim: to determine how the Sonolator makes emulsions and disperses fluids, and apply the findings to industry.

Objective of this poster: to present recent results showing evidence of cavitation in the Sonolator.

• A Perspex Sonolator section has been made, allowing the flow in the Sonolator to be seen clearly

• Cavitation was observed visually and aurally• Measurement of cavitation onset is possible using audio spectra• Smaller orifices cavitate at higher pressures, disagreeing with theory

Future Developments• Explaining why smaller orifices cavitate at higher pressures• Checking whether the blade affects cavitation• Comparing experimental data to computational models (CFD)• Understand how cavitation affects mixing and emulsification

References: •Håkansson, A., et al (2010) “Visual observations and acoustic measurement of cavitation in an experimental model of a high-pressure homogenizer” Journal of Food Engineering 100 (3), 504–513• Quan, K. M., Avvaru, B., Pandit, A. B. (2011) "Measurement and Interpretation of

Cavitation Noise in a Hybrid Hydrodynamic Cavitating Device" AIChE Journal 57 (4), 861-871

Figure 1 shows a schematic diagram of a Sonolator. A mixture of water and oil droplets passes through a narrow orifice (Figure 2). This subjects the droplets to intense forces, and breaks them. An emulsion of very small oil droplets in water forms. This technique can be used to make many industrially useful fluids, such as, foods and personal care products.

Experimental equipment

Cavitation observations

Acoustic measurement

Acoustic results and frequency spectra

Plot of cavitation measurement vs flow rate

Summary

Cavitation Measurement

Figure 3: Perspex rig

Figure 3 shows the Sonolator rig used for experiments. It has a large clear section made from Perspex. This allows the flow inside to be seen. It was designed to allow small particles in the flow to be photographed, helping determine local flow speeds using a technique called Particle Imaging Velocimetry (PIV).

Figure 4 shows a white jet after the orifice, observed for higher flow rates. Hissing noises were heard at the same time. The close up, Figure 5, shows that the jet is split into upper and lower sections. This is cavitation coming off the sharp upper and lower edges of the orifice. (When fluids travel fast, their pressure reduces. When fast enough, pressure goes below vapour pressure. Gas bubbles form and collapse, which is cavitation, and can be heard.)

Figure 4: Cavitation jet

Figure 5: Close-up of jet

Figure 6: Microphone

Figure 6 shows a microphone placed on the rig. A soft putty seal reduces external noises. Sound damping is especially good at higher frequencies. So high frequency sound is only picked up if it comes from within the Perspex, e.g. cavitation noises. A computer records the audio output for analysis.

Figure 7: Audio file for cavitating flow Figure 8: Frequency spectrum

The microphone output (Figure 7) is not particularly useful by itself, however the frequency spectrum (Figure 8) reveals dB peaks at frequencies between 3kHz and 11kHz when the flow cavitates.

Figure 9: Spectra

Figure 9 shows audio spectra recorded for (top to bottom): no flow, non-cavitating low speed flow, cavitating high speed flow. Extra high frequency sounds only appear between 3-11kHz when the flow cavitates. Cavitation measurement is defined as “average dB measurement in 3-11kHz band”.

Figure 10: Cavitation vs flow rate Figure 11: Cavitation for 3 orifices sizes

Figure 12: Results Table

Cavitation shows a sharp onset at a specific flow rate (Figure 10). The onset varies according to the orifice (Figure 11).

Theory predicts a fixed pressure drop for onset of cavitation (Figure 12). This disagrees with experiment. Literature values fall in the middle of experimental values.