Nearly five years ago, there was a great deal of excitement in the thermoacoustics community after the announcement by Backhaus and Swift [1] of the creation of a regenerator-based thermoacoustic-Stirling heat engine that showed a measured 50% efficiency improvement over the efficiency of earlier standing wave stack-based thermoacoustic engines. (For a brief overview of the differences between stack-based and regenerator-based thermoacoustic engines and refrigerators, see the introduction section of Garrett's resource letter [2] or Garrett and Backhaus's introductory article [3].) Application of their ideas to electrically-driven thermoacoustic refrigerators in order to realize efficiency gains over previous electrically-driven stack-based thermoacoustic refrigerator technology [4, 5, 6] was intriguing and has proved to be fertile ground for innovation. To generate power using a regenerator, Backhaus and Swift [7] created an acoustic phasing network that could present the high acoustic impedance found in standing wave resonators but with traveling-wave phasing between pressure and volume velocity within the regenerator. In that engine, their acoustical network produced a toroidal gas path that allowed acoustically-induced streaming [8] that had to be suppressed by the introduction of a "jet pump" to produce a static pressure gradient sufficient to oppose the streaming. That engine also used a standing-wave resonator that was about 1/3 of a wavelength long to provide the required high acoustic impedance at the acoustical network location.
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