25-04-2012, 12:43 PM
NEW VARIETIES OF THERMOACOUSTIC ENGINES
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INTRODUCTION
Rayleigh understood that oscillatory thermal expansion and contraction of a gas 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,” and that the oscillatory thermal
expansion and contraction could themselves be caused by the acoustic wave under
consideration, in a channel with a temperature gradient.
STANDING-WAVE ENGINES
Rayleigh’s criterion for spontaneous thermoacoustic oscillation—that heat should flow
into the gas while its density is high and out of the gas while its density is low—is
accomplished in the Sondhauss tube and in other standing-wave engines according to the
process illustrated
TRAVELING-WAVE ENGINES
In Stirling engines and traveling-wave engines, the conversion of heat to acoustic power
occurs in the regenerator, which smoothly spans the temperature difference between the
hot heat exchanger and the ambient heat exchanger and contains small channels through
which the gas oscillates.
CASCADED STANDING-WAVE AND TRAVELING-WAVE ENGINES
None of the systems described thus far provides high efficiency and great reliability and
low fabrication costs. For example, the traditional Stirling engine (Fig. 1) has high
efficiency, but its moving parts (requiring tight seals between the pistons and their
surrounding cylinders) compromise reliability and are responsible for high fabrication
costs. The thermoacoustic-Stirling hybrid engine (Fig. 5) has reasonably high efficiency
and very high reliability, but the toroidal topology needed is responsible for high fabrication
costs, for two reasons: It is difficult to provide flexibility in the toroidal pressure
vessel to accommodate the thermal expansion of the hot heat exchanger and surrounding
hot parts, and some structure or control must be provided to suppress Gedeon streaming
around the torus.
SUMMARY
Ceperley’s realization that Stirling engines require what acousticians call traveling waves
triggered much research activity in the 1980s and 1990s, with both standing-wave and
traveling-wave engines under vigorous development. The newly discovered “cascade”
combination of standing-wave and traveling-wave engines shows that even more
innovation may be possible.