Thermoacoustics

Thermoacoustics is the study of the interaction between thermal and acoustic processes in fluids, focusing on how temperature variations, heat transfer, and sound waves influence one another and how acoustic energy can be generated, amplified, or converted into thermal energy and vice versa.

A central principle is the Rayleigh criterion: when heat is added to a gas in phase with the pressure fluctuations of an acoustic wave, work is done on the gas and the wave grows. Conversely, heat removal in phase with the pressure fluctuations damps the wave. In thermoacoustic devices a temperature gradient along a porous medium, such as a stack or regenerator, interacts with the oscillating flow to produce net energy transfer between heat and sound.

Devices in thermoacoustics include engines that convert heat into acoustic power and refrigerators that use acoustic power to move heat. Configurations are typically standing-wave or traveling-wave resonators, with a hot end and a cold end and a porous stack to enable thermoacoustic heat pumping. The same physical principles underpin both energy generation and heat transport in these systems.

History and status: theoretical foundations were laid by Lord Rayleigh in the 19th century, with modern experimental and practical development advancing in the late 20th century and onward. Current research spans theory, materials, and applications such as waste-heat recovery, low-emission power generation, and solid-state cooling. Challenges include efficiency limits, material losses, and control of instabilities, but thermoacoustic devices offer robustness and simplicity due to their lack of moving mechanical parts.