Optimizing impedances in the traveling wave heat engine

Abstract

Thermal coupling between solid surfaces and an acoustic working fluid in a regenerator is essential to the operation of a thermoacoustic engine, while the associated viscous coupling, also between the surfaces and the working fluid, is detrimental to the engine's performance and efficiency. The Prandlt number (roughly 0.7 for most gases over a considerable range of temperatures and pressures) fixes the ratio of these two types of coupling for a thermoacoustic engine with a continuous regenerator. This ratio of couplings can be improved in several ways in traveling wave heat engines with short regenerators [P. H. Ceperley, J. Acoust. Soc. Am. 66, 1508–1513 (1979) and P. H. Ceperley, J. Acoust. Soc. Am. 77, 1239–1244 (1985)]. This paper will discuss the optimization of the flow resistance and length of a short regenerator and also resonant impedance enhancement using split mode excitation [P. H. Ceperley, U.S. Patent 4,686,407 (1987)] to significantly reduce the viscous losses while maintaining good thermal coupling.