Optimizing miniature thermoacoustic coolers

Abstract

Because of limited sound intensity output from commercial drivers at midaudio and ultrasonic frequency ranges used in thermoacoustic coolers, it is important to optimize their performance. To achieve this, studies were conducted on heat transfer at the cold heat exchanger and at the hot heat exchanger. Measurements were taken on stray heat influx to the cooler by mechanisms of convection, conduction, radiation, and streaming, and from the driver. PIV studies show the contributions of streaming, both from the driver and other parts of the devices. A special 4-layer copper mesh heat exchanger was designed and tested to cope with heat pumped acoustically from the cold heat reservoir as well as the streaming from the piezoelectric driver. For typical sound levels of 155–160 dB achieved inside the resonator by commercial bimorph piezoelectric drivers, a cooling power density of 0.5 watts/cm2 was achieved with air at 1 atm. Pressurizing the working gas to 17 atm raises the cooling density. The choice of working gas provides a increase in power density. With the above optimizing approaches, a cooling power density of over 50 watts/cm2 appears feasible for the frequency range of 4 to 24 kHz. [Work supported by a grant from the ONR and the State of Utah.]