‘Singing’ cracks clean combustion

A phenomenon known as ‘singing’ can develop in clean combustors when they are switched on – the cleaner the combustor, the more likely the problem. If not checked, singing can go on to damage the combustor, causing it to be shut down. At a Royal Institution talk, a Cambridge professor outlined her strategy to combat rogue sound waves.

“Any small unsteadiness in combustion can generate sound at a discrete tone – like singing one note,” Professor Ann Dowling told edie. “The sound waves increase the unsteadiness and develop into an oscillation. You get a frequency particular to a combustor, with amplitudes slowly building.”

Singing, also known as ‘rumbling’ or ‘noise’, occurs when manufacturers test or start-up their combustors. “They may have to spend a long time fine-tuning the furnace to eliminate the problem,” says Dowling. “Some research is looking at [furnace] designs that are less susceptible to this problem. Another way to eliminate it is to use the same kind of techniques used in anti-sound, like the cancelling headsets worn by helicopter pilots.”

Singing is often overcome by injecting the combustion fuel unsteadily to generate acoustic waves that cancel the damaging ones. Dowling’s approach is to use adaptive controllers – systems that start off ignorant and learn how to alter the fuel injection rate to reduce the oscillations. Her research team is collaborating with the US Massachusetts Institute of Technology to help manufacturers develop anti-sound systems that employ adaptive control.

Even the smallest disturbance can cause big problems, says Dowling, like the famous theory of the beating of a butterfly’s wings altering the Earth’s climate. Low emissions or clean combustors, like those used in gas fire power stations, are especially prone to singing because of the way they are designed.

“Companies have had to take power stations off the grid because of such instabilities,” continues Dowling. “Because of the way electricity is priced, companies have to bid in advance to deliver electricity. Singing can make power stations unreliable and therefore unable to meet their commitments to deliver electricity, so it can cost companies quite a lot.”

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