THERMOACOUSTIC ENGINES

Over 100 years ago, Rayleigh understood that heating and cooling could
create acoustic power “if heat be given to the air at the moment of greatest condensation, or be taken from it at the moment of greatest rarefaction.” Rayleigh’s criterion is met in two classes of thermo acoustic engines. In standing-wave engines, a gas oscillates with standing-wave time phasing in a channel with a steep axial temperature gradient, the lateral thermal contact between the gas and the channel wall being deliberately imperfect. In travelling-wave engines, the gas oscillates with travelling-wave time phasing in a channel with a steep axial temperature gradient, the lateral thermal contact between the
gas and the channel wall being as perfect as possible . Both classes of engines have been under vigorous development since Ceperley’s 1979 realization that Stirling engines are of the travelling-wave class, and, hence, that acousticians could play a key role in the development of powerful, efficient heat engines. Today, throughout the world, the necessary heat exchangers are being imbedded in an interesting variety of acoustic cavities and networks, creating the time phasings and other acoustic conditions needed for the creation of heat engines with the simplicity and elegance of sound waves.

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