Thermoacoustic spontaneous oscillations are now in topics of conversation. Studies on thermoacoustic phenomena, starting with the investigation of the Taconis oscillation, have led to the thermoacoustic theory, which contributed to understanding of the Stirling engine, regenerative refrigerators and the dream pipe. A variety of the thermoacoustic oscillations are classified into open tube type, closed tube type and looped tube type. Geometries of the tube restrict the oscillation frequency. A regenerator (or a stack) in these tubes is a key component. Although the regenerator in the open tube and closed tube is a source of work flow, the regenerator in the looped tube sometimes becomes an amplifier of work flow. Thermodynamic discussion on heat engine indicates that the increment of entropy flow, ΔS, has a decreasing tendency, and thermodynamic discussion on thermoacoustic spontaneous oscillation indicates that nature selects a branch of smallest ΔS. A law of minimizing increment of entropy flow is proposed.
A primitive understanding of fluid oscillation in resonator tube is based on the standing wave approximation. Recent progress in studies on thermoacoustic spontaneous oscillation requires us to make a more realistic image of fluid oscillation in resonator tube. Finite Q-value means some dissipation of work flow in the resonator. Analogous discussion to a transmission line with finite damping successfully leads to empirical equations on the resonance curve and distribution of pressure and velocity amplitudes. The result, however, does not exclude the possibility of negative damping, since this treatment abandons completely information on phase difference between pressure and velocity. The phase difference and the work flow are included in the thermoacoustic theory. Recent progress on experimental techniques makes it possible to observe the phase difference and the work flow, which is rarely discussed in traditional acoustics.
The oscillation of gas in a looped tube is different from that in a non looped tube. Although the oscillation mode in the non looped tube is dominantly decided by boundary conditions at both ends of the tube, the oscillation mode in the looped tube is put under the control of the law of minimum entropy production. The oscillation frequency is near the resonance frequency of the looped tube, if the effect of the branch tube is negligibly small. But if the effect is large enough, a frequency significantly lower than the resonance frequency of the loop is available. The other roles of the branch tube are discussed for both the liquid Stirling engine and the thermoacoustic Stirling engine developed at Los Alamos. Traditional thermoacoustic devices with solid displacers are regarded as variations of a looped tube system with a branch. The final discussion is on the possibility of a looped pulse-tube refrigerator, which is equipped with neither the buffer tank nor the 4-valve controller.
A gas column oscillation in a resonance tube is studied experimentally by measuring the pressure and velocity of a gas. The oscillation is driven by a loud speaker at one end of the closed tube. The quality factor of the resonator is determined precisely from the experimental data of the work flow.
We have built a thermoacoustic engine consisting of a looped tube and a resonator. We performed work flow measurements on the present engine and found that the 1/2-wavelength standing acoustic waves are excited. This result indicates that the sponaneous gas oscillations generated by the stack in the looped tube is not always in the traveling wave mode.
We have made a simple loop-type thermoacoustic engine that will be used for a refrigerator and examined a temperature gradient in the stack and pressure amplitude of oscillation along the loop tube by changing supplying heater power, the mesh size of stack, and the charge pressure of helium gas in the loop tube. The temperature gradient in the stack decreases remarkably in the small values of ωτ and increases in the large values of ωτ as ωτ increases, where ω is the oscillation frequency and τ is the thermal relaxation time between stack materials and helium gas. This result shows an optimum value of ωτ in the minimum onset temperature gradient of oscillation. We have tried to find the efficiency of energy conversion in the stack by assuming that the acoustic wave could be expressed as the superimposition of the standing wave component and the progressing wave component that attenuates linearly along the loop. It is found that the maximum efficiency in this work was about 30% and the energy losses in the stack may depend on its mesh size.
A thermoacoustic refrigerator and a thermoacoustic prime mover, because of its simple structure, would serve as very desirable systems, because thermoacoustic prime movers can be driven with waste heat, such as exhaust gas from engines, or with the heat from natures such as sunlight and geothermal heat. The combined thermoacoustic refrigerator and prime mover system would become a perfect cooling system without moving parts, CFCs, and HFCs. Thus this “Thermoacoustic-driven Thermoacoustic Refrigerator” will be a candidate for the next-generation refrigeration systems. In this paper, we set up our thermoacoustic prime mover to investigate the fundamental characteristics. On the prime mover tests, the fundamental characteristics of stack dimensions are mainly checked and the condition of the oscillation generation is observed with the instantaneous measurement of temperature and pressure oscillation. The way to improve the thermal efficiency of the prime mover is mentioned in terms of the operating condition.
The characteristics of a new loop-type thermoacoustic sound wave generator, in which two closed-end connected-type generators are connected to each other at the open ends by four bent tubes and sealed hermetically, are described. It has been reported that two types of sound wave generators, in which two identically shaped systems are connected to each other at either the open ends or the closed ends, can operate stably. In the new loop-type sound wave generator, which has the benefits of the above-mentioned types of sound wave generators, the pressure, velocity amplitude, acoustic intensity, and heat transferred by sound waves are greater than those of the other connected-type ones. And harmonics in pressure signals and no harmonics in velocity signals are observed at the connected part. An optimum theoretical sound wave frequency of 100Hz is also confirmed for this type of sound wave generator.
The operation frequency dependence of the performance of a Stirling engine was examined in a wide frequency range employing a simulation code, THERMOACOUSTICA, based on the thermoacoustic theory. Results of the simulation were roughly in accordance with expectations. For example, the results satisfies both the first and second laws of thermodynamics. The efficiency and the output work depend on the operation frequency. The frequency corresponding to the maximum output work is considerably higher than that corresponding to the maximum efficiency. The temperature distribution in the regenerator is also dependent on the operation frequency.