Certain thermoacoustic effects are described which form the basis for a heat engine that is intrinsically irreversible in the sense that it requires thermal lags for its operation. After discussing several acoustical heating and cooling effects, including the behavior of a new structure called a ‘‘thermoacoustic couple,’’ we discuss structures that can be placed in acoustically resonant tubes to produce both substantial heat pumping effects and, for restricted heat inputs, large temperature differences. The results are analyzed quantitatively using a second-order thermoacoustic theory based on the work of Rott. The qualities of the acoustic engine are generalized to describe a class of intrinsically irreversible heat engines of which the present acoustic engine is a special case. Finally the results of analysis of several idealized intrinsically irreversible engines are presented. These suggest that the efficiency of such engines may be determined primarily by geometry or configuration rather than by temperature.

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Thermoacoustic heat transport and its applications, thermoacoustic engines, have been discussed in a number of articles over the past several years. Lacking from these articles is a thorough, quantitative experimental investigation of the basic theory underlying thermoacoustic heat transport. A logical starting point for such a study is to investigate the simplest class of thermoacoustic engine—a stack of short plates referred to as a ThermoAcoustic Couple (TAC). The utility of this choice is that the theory can be reduced to its simplest form for analysis of the results. The results of measurements of thermoacoustically generated temperature gradients in TACs subjected to acoustic standing waves are reported. The value of the temperature gradient, which results from an acoustically generated entropy flow in the gas in thermal contact with the TAC, is a function of the peak acoustic pressure amplitude, the mean gas pressure, the Prandtl number of the gas, the configuration of the TAC, and its position in the standing wave. Measurements were made with a computer-controlled apparatus for drive ratios (the ratio of the acoustic pressure amplitude to the mean pressure of the gas) from approximately 0.1%–2.0%, in argon and helium having mean pressures from approximately 100–368 kPa, for three different TACs as a function of their positions in the standing wave. The results are compared with predictions based on a theory by Wheatley et al. [J. Acoust. Soc. Am. 74, 153–170 (1983)]. The measurements agree well with theory for drive ratios less than approximately 0.4%. However, the agreement diminishes at higher drive ratios, where two regions of behavior are observed. Agreement is, in general, best in the vicinity of acoustic particle velocity nodes at all drive ratios investigated.

Abstract-A fiber-optic interferometric sensor has been developed which consists of a seismic mass of 520 gm supported by two rubber mandrels, each wound with a single layer of single-mode optical fiber 6.5 m long. One end of each fiber is cleaved to enhance reflection. The other ends are interconnected via a fiber-to-fiber 3-dB coupler, forming a Michelson interferometer. When the case of the sensor is displaced, the fiber around one mandrel extends in length while the other contracts. The resulting "push-pull" mechanical operation of the sensor allows both legs of the interferometer to be active, providing good common mode rejection of spurious effects, as a reference leg is not required. This, together with the fact that the light traverses each leg of a Michelson interferometer twice due to reflection, provides the sensor with four times the sensitivity of a conventionally constructed interferometric sensor. sensitivities of 8500 rad of optical phase shift per micrometer of case displacement have been measured above the massspring resonance, where the sensor operates as a seismometer. Below resonance the sensor operates as an accelerometer with a measured sensitivity of 10 500 rad/g, the highest reported to date. Including both thermodynamic and demodulator noise sources ( = 10 p r a d / J H z ) , below resonance the sensor has a detection threshold of l ng/JHz, a 20-dB improvement over the best existing conventional low noise vibration sensors.M

Thermoacoustical phenomena have a long history and are frequently characterized by great complexity. In the present paper, we describe how, by the use of suitable acoustical structures, the phenomena can both be simplified and readily demonstrated. A heuristic discussion is emphasized, which we hope will be useful in teaching the principles. The qualities of certain model apparatus that demonstrate acoustically stimulated entropy flow, a thermally driven acoustic oscillator, and an acoustically driven refrigerator are also presented in semiquantitative detail.