Thermoacoustics: A Unifying Perspective for Some Engines and Refrigerators

I am thrilled by the power density and efficiency recently achieved by thermoacoustic engines and refrigerators, and I am fascinated by some of the latest developments in thermoacoustics.''With that sentence, Dr. G. W. Swift begins a textbook that employs 21st century pedagogical tools to introduce students and researchers to a 21st century technology.In addition to a logical and complete development of this multidisciplinary subject, the 300-page textbook includes a CD-ROM with computerized animations and DELTAE software, with its own 200-page manual in searchable PDF format.In many ways this textbook represents a milestone in the literature of acoustics.Unlike Swift's excellent 1988 review paper, ''Thermoacoustic engines,'' published in this Journal ͓84͑4͒, 1145-1180͔, this textbook is destined to become the gateway by which many trained both inside and outside the acoustics community can enter this fascinating and challenging field.Swift's use of computer animations as an integral part of the text is sure to become more common in this century.These are not just pretty ''moving pictures.''I consider myself to be quite well-versed in the physics of the oscillatory thermoviscous boundary layer, but I was pleased with the new insights I gained just by watching his ''Lagrangian marker'' whip through the viscous medium like a limp linguine in a sea of thick minestrone.The more complex animations can be an even richer source of intuitive insights.Certainly, forsaking the wave equation in favor of an approach more closely tied to the fundamental conservation equations, and consistent with the DELTAE software solution of complex quasi-onedimensional acoustical networks, brings a fresh perspective that should be of interest to all of us who teach classes in acoustics.At the very least, a textbook that is independent of the software, yet consistent with it, provides a new ͑and welcome!͒ bridge between concept and computation.Swift makes his ambitious goals clear from the start: He attempts to interweave the physical, the mathematical, and the intuitive into a coherent picture that supports his claim that thermoacoustics describes ''a mental framework, not a class of devices.''The thermoacoustic paradigm abandons the 19th century crankshaft-based technology used to enforce control of gas motion in today's engines and refrigerators.Instead, it relies on oscillating pressure gradients to produce oscillatory gas motion, and on the oscillating gas motion to produce oscillatory pressure gradients.This ''circularity'' is inherently challenging, but it is also both fascinating in concept and elegant in its execution.Swift's first chapter provides a short historical overview, presents some very basic concepts whose understanding is significantly enhanced by computer animations, and introduces the four examples that are used throughout the text to apply the thermoacoustic principles.He chooses two engines and two refrigerators, produced over the past ten years in his laboratory, to provide physical dimensions and to compare the results of the calculations to actual measured performance as the theory is developed.