Bubble acoustics

There is a huge range of industries and natural systems in which a gas is dissolved into a liquid. And in all of these, bubbles crucially affect the performance of the system.

Now, the sound that bubbles emit has been exploited by researchers at CSIRO who have built a measurement tool that can be used in the chemicals, minerals-processing and wastewater industries.

Bubbles form an interface across which gas dissolves into liquid. Their behaviour and size affects the rate of gas dissolution. As a bubble is formed or broken up, it suffers a severe distortion to its surface. As the surface recoils, the gas within the bubble gets compressed. Like any natural system with inertia and stiffness, the gas and surrounding liquid vibrate in response to this shock. In effect, the bubbles `ring' like bells. Bubbles moving freely through liquid may also emit sound, probably as they are impacted by turbulent eddies.

Bubbles have a natural frequency that depends on their size. Big bubbles emit low-frequency sounds while small bubbles emit high-frequency sounds - like big bells and small bells. This simple fact, first quantified in 1933, is the key to bubble acoustics. As the bubbles get larger, the pitch of the note gets lower.

For any instrument system, a frequency is easier to measure than amplitude. This has allowed the CSIRO researchers to analyse the performance of a bubbly system with a probe that is little affected by fouling, noise, chemicals, high temperatures or pressures.

The system itself comprises a sensor linked to a PC running software, dubbed StreamTone, that analyses the bubbly fluid flow using the acoustic signals emitted by the bubbles.

The sensor is based on a piezoelectric crystal, solid-state ceramic sensor, which means there is potential for operating close to refractory temperatures.

The StreamTone software 'understands' the physics of bubble acoustics, providing data that can be used for process analysis. StreamTone is particularly suited to the biotechnology and minerals-processing industries, but is finding much wider applications.

CSIRO has discovered that In trials in a 20,000 litre bio-reactor under full commercial operation, StreamTone successfully demonstrated that poor oxygen update of the culture was due to poor sparging performance at a critical stage in the process. There is potential for StreamTone in this industry as both a diagnostic tool and as part of a process feedback system.

For a wastewater-industry aeration diffuser, the bubble size distribution measured by the technology was directly related to the fouling of the diffuser. In work for the chemicals industry, the acoustic signature of a bubbly flow clearly distinguished 'satisfactory' from 'unsatisfactory' operating regimes. And in research for the mineral-sands industry, the variation in bubble size in different parts of a model aeration tank was successfully determined. For more information contact:

Dr Richard Manasseh or Dilip ManuelCSIRO Thermal & Fluids Engineering PO Box 56, Graham Road, Highett, Melbourne, VIC 3190, Australia

Tel: 61 3 9252 6000Fax: 61 3 9252 6252

Email: Richard.Manasseh@dbce.csiro.au Or: Dilip.Manuel@dbce.csiro.au

More information on the web at

http://fluids.mel.dbce.csiro.au/~richman/StreamTone/streamtone002.html