3/ 27/13 Thermoacoustic Engine w w w.mme.wsu.edu/~matv eev /tae. htm 1/ 3 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 acoust ic resona tors c an lead to highly i ntensiv e sound, which is one of the reasons for rocket motor malfun ctioning. Howev er, thermoacoustic instabilities can be c ontrolled , and acoust ic 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 ofexternally 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 dev eloped at Los Alamos that demonstrated the s econd-la w eff iciencies up to 41%. One of ou r objective s in thermoacoustics research is to dev elop eff icient miniature pow er sys tems based on thermoacoustic engine s. A s chematic, photogr aph, and v ideo clip of our small-scale engine demo nstrator are show n below. T he resonator is made of coppertubes 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 arr anged at the cold part of the engine. The syst em is equipp ed with a pressure transducer measuring acoustic pressure inside the resonator an d two t hermo couples measuring temperature s at t he st ack ends. This engine-d emonstrator gene rates s ound 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-e lectric ener gy c onv ersion. An electroacoustic transfor mer will be adde d. W e hope to reduce t emper ature 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%.)
