52 Characterization and Analysis of a High-Frequency Traveling Wave Thermoacoustic Engine

Faculty Mentor: Orest Symko (Physics and Astronomy, University of Utah)

Traveling wave thermoacoustic engines are a potentially-high-efficiency method of converting heat into mechanical (or ultimately electrical) energy. A small, high-frequency engine with low onset temperatures was characterized experimentally, including its frequency spectrum and sound pressure level. The addition of a quarter-wave standing wave resonator was also investigated on a similar high-frequency engine, with inconclusive results.

The traveling wave engine was placed in an ice bath to maintain a constant temperature for the cold heat exchanger, and an electrical current was passed through a wire wrapped around the hot heat exchanger to create a temperature difference of 200 ∘𝐶 across a ~1 mm gap. Sound pressure measurements were taken with a pressure probe carefully inserted into the side of the engine.

The fundamental frequency for the characterized engine was 2.45 kHz. The measured frequency spectrum showed a 130 Hz full width at half maximum for the fundamental peak; in addition, the second harmonic was found to be present at 4.91 kHz, with -20 dB amplitude from the fundamental peak and a width of 80 Hz. No higher order harmonics or other frequencies were observed at more than twice the noise decibel level.

With a ~3-Watt thermal input, the maximum observed sound pressure amplitude of the characterized engine was 403 Pa, corresponding to 146 dB, a significant oscillation. Multiple experiments confirmed consistent amplitudes in the range of 300-400 Pa, depending on the quality of the engine resonance.

Finally, Peter Ceperley proposed the addition of a quarter-wave resonator to looped traveling- wave thermoacoustic engines such as this [1], a practice common for larger, low-frequency thermoacoustic engines [2]. A 3 kilohertz engine was assembled with a ~3 cm quarter-wave resonator added. The engine with the added resonator was functional, but results on the resonator performance were inconclusive due to high variability caused by difficulties in engine-resonator alignment. A maximum pressure of 585 Pa was recorded, but results fluctuated between that result and a low of 170 Pa. Further investigation is highly warranted but remained outside the scope of this project.

An additional area of future work is that of coupling multiple thermoacoustic engines into arrays for a larger potential low-onset temperature power source.

References:

1. Ceperley, Peter H. “Resonant travelling wave heat engine.” (1980). 4355517, U.S. Patent and Trademark Office. https://patents.justia.com/patent/4355517
2. Yu, Z.B., Li, Q. et al. (2005). “Experimental investigation on a thermoacoustic engine having a looped tube and resonator.” Cryogenics, Vol. 45 (Issue 8), 566-571. https://doi.org/10.1016/j.cryogenics.2005.06.007