About 30 years ago, Ceperley proposed “a pistonless Stirling engine”, which became sufficient motivation for thermoacousticians to regard thermoacoustic phenomena as a kind of heat engine. Since then work flux, heat flux, and their mutual conversion have been suggested to be fundamental ideas for understanding thermoacoustic engines. Such new concepts can be equally applicable to some reciprocating heat engines. In this paper, based on experimental results, I discuss the working mechanism of Stirling engines from the standpoint of a thermoacoustics framework.
The fundamentals of thermoacoustics constitute understanding thermoacoustic phenomena, the proposal of new thermoacoustic devices, and the development of new experimental techniques. This paper describes a practical guide to measurements of important physical quantities in thermoacoustic systems to help beginners understand the basic techniques. The direct method, involving measurements of pressure and velocity oscillations, and the two-sensor method are introduced for the measurement of work flow. A method for the dynamic calibration of a thermocouple is also presented to make measurements of temperature fluctuation possible. Particular emphasis is given accurately determining the phasing between oscillatory quantities.
This research describes the oscillatory flows inside and outside a thermoacoustic sound-wave generator. Two types of thermoacoustic sound-wave generators are employed. One is a conventional thermoacoustic sound-wave generator, 32 mm in inner diameter and either 860 mm or 1,133 mm long including the high- and low-temperature heat exchangers inside the resonance tube. The other is a simplified thermoacoustic sound-wave generator, 72 mm in inner diameter and 860 mm long, and has no heat exchangers inside the resonance tube. Simultaneous measurements of velocity and pressure were performed inside the resonance tube. Typical oscillatory flows, which include the Richardson effect near the tube wall, were observed inside the resonance tube of both types of sound-wave generator except immediately before the tube outlet. The differences between the inflow and outflow were ascertained near the tube outlet as well. Flow rates based on velocity amplitude are newly proposed for evaluating the flow singularities outside the resonance tube outlet. The forward flow rates, which increase up to 30 mm downstream from the tube outlet, decreased more than 30 mm downstream. On the other hand, the backward flow rates vanished more than 15 mm downstream. We also confirmed that the flow outside the resonance tube outlet changes from oscillatory flow at the tube outlet to pulsating flow far downstream.
We built and tested a double-loop thermoacoustic Stirling cooler. The cooler consists of a prime-mover loop, a branch resonator and a cooler loop. Work flow measurements were conducted to evaluate the efficiency of the cooler. It was found that efficiency corresponding to 6.6% of the Carnot efficiency was expected for the prime-mover loop, whereas a Carnot efficiency of 11.5% was expected for the cooler loop. Measurement of heat rejected by cooling water showed that unwanted heat loss significantly lowered the efficiency in the prime-mover loop. When the effective input heat to the regenerator was used to estimate the efficiency, the efficiency was found to be 20.4% of the Carnot efficiency, comparable to that of mechanical Stirling engines.
In thermoacoustic devices with a looped tube, it has been pointed out that acoustic streaming circulating the loop leads to unwanted heat loss. In this research,, we measured the acoustic streaming velocity in a double-loop thermoacoustic Stirling cooler, and show that the streaming circulating the loop accounts for a heat loss of 150 W when input heat to the prime-mover loop is 408 W. It is also shown that the installation of an elastic membrane effectively suppresses the heat loss and thereby increases the performance of the prime-mover loop and cooler loop.
A numerical analysis has been performed to investigate the behavior of gas when thermoacoustic self-exited oscillation begins to be generated and to clarify the influence of heat input at a heater at the time of generating self-exited oscillation. The simplest device in which a regenerator is inserted into a tube is applied to the physical model. Transient one-dimensional equations of continuity, momentum and energy are solved utilizing a TVD scheme. It was found that the amount of heat input is related to the initial infinitesimal oscillation, which is considerably important as a disturbance factor for generation of real self-exited oscillation. It was also found that the infinitesimal oscillation is generated by temporally discontinuing heat input.
A gas in a tube spontaneously oscillates when the temperature ratio between the tube ends exceeds a critical value. This spontaneous gas oscillation is applied to a liquid helium level finder. In the present study, to design a liquid nitrogen level finder using spontaneous gas oscillation, the critical value of the temperature ratio causing spontaneous gas oscillation for the case of nitrogen gas was numerically investigated. It was found that a tube in which an array of narrow circular tubes is located can work as a liquid nitrogen level finder. Hence, a tube with an array of narrow tubes was constructed and the liquid level of nitrogen put into a vessel was measured with the constructed tube. The measurements also demonstrated that the tube can work as a liquid nitrogen level finder.
The onset temperature ratio for spontaneous gas oscillation occurring in a thermoacoustic engine with a looped tube and resonator was calculated. The calculated result was compared with actual measurement and a quantitative agreement between them was obtained. Further, the effect of changing the shape of the thermoacoustic engine on the onset temperature ratio was numerically investigated. As a result, the optimum shape to realize a low onset-temperature ratio was determined.
Taconis oscillation in a closed long tube is studied by the numerical simulation of two-dimensional compressible Navier-Stokes equations. Conditions are used where both end walls of the tube are hot (T = TH), and the central regions of the side walls are cold (T = TC). The oscillation can be spontaneously generated when the temperature ratio (TH/TC) is larger than 7.1, whereas it can be damped when the temperature ratio is smaller than 5.7. We obtained two different critical temperature ratios and observed the hysteresis phenomenon.