Advancement of small-scale thermoacoustic engine

Sungmin Jung

Thermoacoustic engines are energy-conversion devices that produce acoustic power using heat flowing from a high-temperature source to a low-temperature sink. Thermoacoustic engines can be made without moving parts and using various gases as working fluids. The simplicity of manufacturing such engines results in low cost and low maintenance and, therefore, is desirable in industry. A recently proposed candidate for small-scale electricity generation involves a thermoacoustic engine coupled with a piezoelectric transformer. A simple thermoacoustic engine is composed of a resonator with one end closed and the other end open with a piece of porous material, referred to as a stack, placed inside the resonator at a specific location. In this research, reticulated vitreous carbon was used as the stack material and atmospheric air as the working fluid. The engine was tested with resonators of variable lengths in the range 57-124 mm. The temperature difference across the stack and the acoustic pressure amplitudes inside and outside of the engine were measured and compared with theoretical values. The engine starts generating sound at the temperature differences of 200-300°C between the hot and cold parts of the system. The acoustic pressure amplitudes up to 2 kPa are measured inside the resonator in the excited regimes. The acoustic pressure amplitude in the quasi-steady tests shows a monotonic change and no significant hysteresis. In the test with fast change of the heat supply rate, the pressure amplitude shows strong hysteresis. A simplified energy-balance theory adequately predicts a trend in the temperature onset, while underestimating actual values. Model estimations show that the stack-generated acoustic power reaches 100 mW with the stack-based efficiencies of several percent. Optimized geometries and conditions of the engine yield the lowest critical temperature difference of 153 ℃ and optimized efficiency is about twice the result from experimental data in 67-mm engine.

Advancement of small-scale thermoacoustic engine

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