ABSTRACT

Environmentally Sound: High Performance, Compact Thermoacoustic Refrigeration

The focus of this proposal is research that will advance the development of an efficient thermoacoustic Stirling engine for a range of small-scale applications. The thermoacoustic effect was first discovered when glass blowers noticed that glass pipes emitted a noise during production when one end was hot and the other had cooled. It was not until Ceperley connected the thermoacoustic effect with the Stirling cycle by building a traveling wave heat engine in 1978 that modern thermoacoustics was born.

The young field of thermoacoustics is divided into two parts: standing wave (where the crests of the sound waves appear to be fixed) and traveling wave (where the crests of the sound waves appear to move) engines. The latter is more complex but also more efficient. This complexity is one of the reasons why traveling wave technology has been largely overlooked, which allows for significant advances to be made both to this technology and its impact on a widespread basis.

A thermoacoustic engine is commonly comprised of a looped oscillator and a resonator tube. Two heat exchangers and a regenerator are located inside the looped oscillator. The two heat exchangers serve as a heat source and sink, and the regenerator allows the working gas to gradually change temperature from the cold side temperature to the hot side temperature. Thermoacoustic engines utilize the self oscillation imposed on the system by the addition of heat to the hot side. Over the course of several work cycles, the amplitude of the oscillations is continuously increased, eventually reaching a level where the energy can be used in a subsequent device (such as a thermoacoustic refrigerator). One advantageous facet of the thermoacoustic Stirling process is its ability to be powered through external heating of the working gas, rendering it a prime application to be powered by solar heat and waste heat.

We propose to achieve a smaller scale for a thermoacoustic Stirling heat engine by coiling the resonator tube and to drastically improve the engine's efficiency. A computational analysis of the engine's components will be conducted, focusing primarily the resonator tube and regenerator. A detailed understanding of the underlying physical phenomena occurring in these components is a significant contribution to the field of thermoacoustics. This analysis will result in an optimized tube geometry and will support the efficient downsizing of each component. The construction of an experimental prototype will incorporate the use of microfabrication for the heat exchangers and regenerators.

One of the major advantages of thermoacoustic refrigeration over conventional refrigeration and a primary motivation for this work is that it does not utilize harmful refrigerants such as CFCs or HCFCs. It will be shown that thermoacoustic refrigeration can significantly decrease the environmental impact of refrigeration. Broad applications of the improved and downsized units could include refrigerators and air-conditioners for both buildings and vehicles. At the current size, efficient traveling wave thermoacoustic refrigerators are not feasible for implementation in these fields. This work will be used to promote thermoacoustics as an environmentally sound and well-developed technology. Also, the proposed work will support education in the fields of thermodynamics, fluid mechanics and acoustics through minority student outreach, classroom demonstrations, and undergraduate research.

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