On the relationship between acoustic absorption and temperature gradient in a thermoacoustic liner

Abstract

The thermoacoustic effect provides a means to convert acoustic energy to heat and vice versa without the need for moving parts. This makes thermoacoustic devices of interest for multifunctional applications in harsh or remote service environments. Embedding thermoacoustic stacks within the framework of acoustic liners could provide opportunities to simultaneously optimize both acoustic absorption and thermal output as well as to integrate energy harvesting and sensing and monitoring functionalities. In this study, the influence of sound pressure level and frequency as well as stack length and position on the performance of a thermoacoustic liner is investigated using normal incidence impedance tube tests and DeltaEC simulations. Using Rott’s approximation, the stack’s pore width and wall thickness are optimized for additively manufactured thermoacoustic liner test articles. A peak steady-state temperature gradient of 9.5°C is obtained at 790 Hz for 140 dB tonal dwells. It is found that the temperature gradient correlates directly with the acoustic power available at the leading stack-face, which is highest when the stack is near the center of the resonator. In contrast, acoustic absorption diminishes when the stack is nearer to the closed-end of the resonator. At higher excitation pressure levels, the contribution of viscous heating to the deviation in the thermoacoustic gradient becomes significant. Further, the relationship between the acoustic absorption and the thermoacoustic temperature gradient was examined. In application scenarios requiring multifunctional optimization, a tradeoff may be required in terms of the absorption in order to maximize the temperature gradient. With current additive and hybrid fabrication processes and materials reaching commercial maturity, opportunities exist to realize thermoacoustic liners with enriched functionalities.