Miniature Thermoacoustic Engines

The thermal-to-acoustic energy conversion occurs when heat is added to the acoustically oscillating fluid in phase with the acoustic pressure oscillations (Rayleigh criterion). The unsteady heat release inside acoustic resonators can lead to highly intensive sound, which is one of the reasons for rocket motor malfunctioning. However, thermoacoustic instabilities can be controlled, and acoustic energy can be produced and harnessed in Thermoacoustic Engines. A schematic of a standing-wave engine is shown below (a). The heart of thermoacoustic engines is the stack (made of porous material), where acoustic power is generated in the presence of externally maintained temperature gradient. At the proper location of the stack inside the resonator, the heat is transported to the gas parcels oscillating in the fundamental acoustic mode (b) at the time of their compression and extracted at the time of rarefaction (c). Besides simple standing-wave engines, more complicated and more efficient travelling-wave and cascade engines were developed at Los Alamos that demonstrated the second-law efficiencies up to 41%.

One of our objectives in thermoacoustics research is to develop efficient miniature power systems based on thermoacoustic engines. A schematic, photograph, and video clip of our small-scale engine demonstrator are shown below. The resonator is made of copper tubes and a ceramic stack holder. Reticulated vitreous carbon (RVC) is applied as a stack. Copper mesh screens placed on both sides of the stack serve as heat exchangers. The heat is suppleid either by flame or an electric heater. A water-cooling jacket is arranged at the cold part of the engine. The system is equipped with a pressure transducer measuring acoustic pressure inside the resonator and two thermocouples measuring temperatures at the stack ends. This engine-demonstrator generates sound at temperature difference about 200 degrees Celsius. The sound amplitude reach values of 2 kPa. In the future, we plan to optimize this concept for thermal-to-electric energy conversion. An electroacoustic transformer will be added. We hope to reduce temperature differences to about 50 degrees and reach overall efficiencies about 5-10%. (Typical efficiencies of other types of centimer-scale energy conversion systems are around 1%.)

Short video

Featured Articles:

Matveev, K.I. and Jung, S., 2011, Modeling of Thermoacoustic Resonators with Non-Uniform Medium and Boundary Conditions, ASME Journal of Vibration and Acoustics, 133(3), 031012, pp. 1-7.

Jung, S. and Matveev, K.I., 2010, Study of Small-Scale Standing-Wave Thermoacoustic Engine, Journal of Mechanical Engineering Science, 224(1), pp. 133-141.

Matveev, K.I., Wekin, A., Richards, C.D., and Shafiei-Tehrany, N., 2007, On the Coupling between Standing-Wave Thermoacoustic Engine and Piezoelectric Transducer, International Mechanical Engineering Congress, Seattle, WA, ASME paper IMECE2007-41119.