Heat and sound wave interactions could run refrigerators

Leaky systems have limited how engineers design thermoacoustic devices.
These rely on the interplay between temperature oscillations and sound waves.
Researchers at Purdue and the University of Notre Dame have demonstrated for the first time that thermoacoustics could theoretically occur in solids as well as fluids, recently presenting their findings at the 175th Meeting of the Acoustical Society of America.
A solid can serve as a medium for heat and sound wave interactions just like a fluid does for thermoacoustic engines and refrigerators — resulting in leak-free machines that can stay operating longer.
Thermoacoustics has been an established and well-studied phenomenon in fluids — whether as a gas or liquid — for centuries. “Applying heat to a fluid enclosed in a duct or cavity will cause the spontaneous generation of sound waves propagating in the fluid itself,” said Carlo Scalo, an assistant professor of mechanical engineering at Purdue. “This results in so-called singing pipes, or thermoacoustics machines.”
“Although still in its infancy, this technology could be particularly effective in harsh environments, such as outer space, where strong temperature variations are freely available and when system failures would endanger the overall mission,” said Fabio Semperlotti, Purdue assistant professor of mechanical engineering.
While fluids have been historically used for these systems, the extra step of building something to contain the fluids and prevent leaks is cumbersome. This led the researchers to consider solids as a replacement.

Thermoacoustics enables either waste heat or mechanical vibrations to be converted into other useful forms of energy. For refrigerators, sound waves generate a temperature gradient of hot and cold. The vibrating motion makes cold areas colder and hot areas hotter.

Engines use an opposite process: a temperature gradient provided by waste heat leads to mechanical vibrations.
Researchers still need to complete an experimental setup to validate this design idea and better understand the thermoacoustics of solids as discovered through mathematical calculations and modeling.

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