Experimental Investigations of an Electro-Dynamically Driven Thermoacoustic Cooler

Abstract

Experimental investigations of an electro-dynamically thermoacoustic cooler prototype were performed. The prototype was designed to provide 140 W of cooling across a 22 °C temperature lift. Operation using a 55% helium-argon mixture at a mean pressure of 20 bar and a frequency near 180 Hz was targeted. The prototype used a tuned “moving magnet” electro-mechanical actuator. Initial investigations aimed at characterizing the electro-mechanical behavior and performance of the driver. The acoustic response of the system with no cooling elements was then investigated to validate the experimental procedures. The thermal performance of the complete system was then measured over a range of operating conditions, for varying gas mixtures. Detailed sound pressure and temperature measurements provided information from which the overall efficiency, capacity, and temperature lift of the cooling system were estimated, in addition to the heat exchange coefficients and performance of the heat exchangers. Net acoustic power inputs of up to 120 W were achieved with an electro-acoustic transduction efficiency varying between 20% and 50%, reaching values as high as 60% in a few cases. In comparison, the theoretical maximum driver efficiency was 65%. The measured cooling capacity varied greatly and peaked near 130 W for a temperature lift of 12°C. The acoustic pressure amplitudes were near 3% of the mean pressure in the stack region, and the heat rejected to a secondary fluid reached 250 W. The best relative coefficient of performance achieved was less than 3% of Carnot, based on the net input acoustic power. The best overall efficiency achieved was thus 1.2% of Carnot. While the acoustic power level exceeded the target value for the desired cooling load, the cooling power was well below the expected value, and the target temperature lifts and efficiencies were not achieved. This was generally attributed to “nuisance” heat loads, acoustic streaming effects, and migration of species within the inhomogeneous mixture. The non-dimensional heat exchanger performance in the thermoacoustic system was found to be slightly less than that in a steady uniform flow when the root-mean-square particle velocity is used for a velocity scale, and the stack end temperature is used in the calculation of the temperature lift. It was also found that this performance value is significantly better than that predicted by linearized boundary layer models often used in linear acoustic models.