Thomas J. Hofler

An intrinsically irreversible thermoacoustic heat engine

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 temp...

Understanding some simple phenomena in thermoacoustics with applications to acoustical heat engines

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.

Acoustic cooling engine

An acoustic cooling engine with improved thermal performance and reduced internal losses comprises a compressible fluid contained in a resonant pressure vessel. The fluid has a substantial thermal expansion coefficient and is capable of supporting an acoustic standing wave. A thermodynamic element has first and second ends and is located in the resonant pressure vessel in thermal communication with the fluid. The thermal response of the thermodynamic element to the acoustic standing wave pumps heat from the second end to the first end. The thermodynamic element permits substantial flow of the fluid through the thermodynamic element. An acoustic driver cyclically drives the fluid with an acoustic standing wave. The driver is at a location of maximum acoustic impedance in the resonant pressure vessel and proximate the first end of the thermodynamic element. A hot heat exchanger is adjacent to and in thermal communication with the first end of the thermodynamic element. The hot heat exchanger conducts heat from the first end to portions of the resonant pressure vessel proximate the hot heat exchanger. The hot heat exchanger permits substantial flow of the fluid through the hot heat exchanger. The resonant pressure vessel can include a housing less than one quarter wavelength in length coupled to a reservoir. The housing can include a reduced diameter portion communicating with the reservoir. The frequency of the acoustic driver can be continuously controlled so as to maintain resonance.

Study of a thermoacoustic prime mover below onset of self‐oscillation

The frequency response of a thermoacoustic prime mover has been measured as a function of the mean gas pressure and temperature gradient across the prime mover stack. The quality factor Q and resonance frequency can be determined from the response. As the temperature gradient is increased, the Q increases, indicating a decrease in attenuation across the stack. At sufficiently large temperature differences (∼300 K), the resonator goes into self‐oscillation, indicating negative attenuation. Measurements are reported for helium and argon at pressures ranging from 170–500 kPa and temperature gradients ranging from zero to that required for onset of self‐oscillation. The results are explained in terms of a counterpropagating, plane‐wave analysis, based on techniques commonly used in porous media investigations. In general, the predictions of the analysis are in good agreement with experiment. The predictions of Q and the change in resonance frequency with mean gas pressure are within approximately 5% and 0.4% ...

Fiber optic flexural disk accelerometer

An accelerometer or seismometer has a pair of flat spirals of optical fiber and has one or more elastic disks bearing a mass and supported for flexure. Each spiral is fixedly attached to a corresponding disk side so that disk flexure lengthens a spiral on one disk side and shortens a spiral on an oppositely facing disk side, the spirals being connected as legs of a fiber optic interferometer so that the interferometer provides an output corresponding to the flexure. A pair of the disks may be mounted oppositely of a sealed body with a pair of the spirals arranged to minimize the effect of pressure changes on the sensor, and a pair of the spirals may be mounted oppositely of a thermally conducting disk to minimize temperature differences between the spirals. The mass may be centrally mounted on a disk with the disk peripherally supported, or the mass may be distributed around the disk periphery with the disk centrally supported for isolation from mounting strain. Several of the disks may be coaxially mounted to provide increased sensitivity.

Design and construction of a solar-powdered, thermoacoustically driven, thermoacoustic refrigerator

A thermoacoustically driven thermoacoustic refrigerator powered by solar thermal energy has been successfully built and tested. A 0.457 m diameter Fresnel lens focuses sunlight onto the hot end of a 0.0254 m diameter reticulated vitreous carbon prime mover stack, heating it to 475°C, thereby eliminating the need for the most troublesome component in a heat driven prime mover, the hot heat exchanger. The high intensity sound waves produced by the prime mover drive a thermoacoustic refrigerator to produce 2.5 watts of cooling power at a cold temperature of 5°C and a temperature span of 18°C.

Theory and calculations for an intrinsically irreversible acoustic prime mover using liquid sodium as primary working fluid

A theory describing a thermoacoustic heat engine using a liquid as primary working substance is developed and applied to the design of a high‐power prime mover using liquid sodium. After an introduction that explains the physical principles of this type of engine in a heuristic way, we derive expressions for the acoustic variables and consequent energy flows in the engine. These expressions are easily understandable only in special limiting cases; to discuss practical applications we resort to numerical computations. We find that a reasonably designed thermoacoustic prime mover using liquid sodium and operating between thermal reservoirs at 1000 and 400 K can generate about 60 W/cm2 of acoustic power at about (1)/(3) of Carnot’s efficiency. We also discuss the relative importance of various sources of inefficiency, such as viscosity, that are not essential to the production of power in this type of engine but are unavoidable in practice because of the thermophysical properties of real substances.

Thermal noise in a fiber optic sensor

The mean intrinsic thermal noise in a fiber optic interferometric sensor is calculated from the equipartition theorem. The spectral density of the fluctuating force is the mechanical equivalent of the well‐known thermally induced voltage noise (Johnson noise) in electrical resistors. The consequent phase noise generated by this fluctuating force in a high‐sensitivity fiber optic interferometric seismic sensor is found to dominate the phase noise of present opto‐electronic interferometric demodulators, thus setting the limit for minimum detectable vibration amplitudes. This effect is identical to that found to limit the sensitivity of ‘‘mirrored galvanometers’’ as the optical leverage of the transducer was increased. It is shown that this contribution to the sensor noise is reduced when the sensor is used above resonance as a seismometer rather than below resonance as an accelerometer. The further increase in noise‐limited detection thresholds introduced by aliasing of the noise concentrated about the reso...

Fiber optic flexural disk microphone

An interferometric fiber optic microphone consisting of a 10‐m‐long, 4‐cm‐diam, flat‐wound (spiral) single mode optical fiber bonded to a simply supported, 8‐cm‐diam, 3‐mm‐thick aluminum disk will be described. In the presence of a pressure difference across the plate or an acceleration of the plate, the surface strain induced in the plate is transmitted to the optical fiber. The optical phase shift induced by the strain is detected in an all‐fiber Michelson interferometer. The calculated strain [A. E. H. Love, A Treatise on the Mathematical Theory of Elasticity (Dover, New York, 1944), 4th ed., Sec. ♯309] and optical strain measured using static pressure, acoustic pressure, and a calibrated aecelerometer, are in good agreement (± 10%) and yield a sensitivity of 30 milliradians per Pascal per meter of optical fiber below the plate resonance frequency of 3 kHz. [Work supported by the Office of Naval Research and Office of Naval Technology.]

Effective heat transfer between a thermoacoustic heat exchanger and stack

Typically, hot and cold copper fin heat exchangers couple heat into and out of the stack. Effective heat transfer means that large amounts of heat are carried across the stack interface with a small temperature difference. Effective heat transfer is also efficient if thermoviscous losses in the heat exchanger are minimized. This is particularly difficult to achieve in engines having high‐power density and large gas displacement amplitudes. A simple model for heat exchanger effectiveness and its relationship to fin length and spacing is discussed. Heat exchanger fins that are shorter than the peak gas particle displacement can be effective if the fin spacing is sufficiently small.