Shin-ichi Sakamoto

Generation Mechanism of Heat Flows near the Stack as a Prime Mover in a Thermoacoustic Cooling System

Temperature variations in a thermoacoustic cooling system consisting of a loop-tube were investigated. The heat flows were discussed using observation results of the temperature variations in the tube. The regenerator, stack 1, was employed as a prime mover and stack 2 as a heat pump. At the cooling point, on the stack 2, the temperature decrease of 22°C was confirmed. Heat flows did not appear in the location far from the stack 1 except near the stack 2. On the other hand, heat flows induced by the sound energy flow and acoustic streaming, appeared near the stack 1. The sound energy flow was caused by the thermoacoustic effect. The directions of the sound energy flow and heat flow were opposite, according to the energy conservation law. In the loop-tube, the strong nonlinearity was observed in the generated sound, and the acoustic streaming was induced.

Applying Diverging Tube for the Low-Temperature Drive in a Loop-Tube-Type Thermoacoustic System

In this report, we propose a low-temperature driving method for loop-tube-type thermoacoustic cooling systems with a diverging tube. We specifically examine the sound field within a loop tube and the temperature at the top of a prime mover stack when the diverging tube position is changed. Experimental conditions included constant input heat energy and the temperature at the bottom of the stack. The diverging tube position was adjusted to increase the efficiency of energy conversion from heat energy to sound energy. Furthermore, the temperature at the top of the prime mover stack decreased by about 500 °C, from 845 to 345 °C, compared with that in systems without a diverging tube. The results suggest that the phase difference between the pressure and particle velocity in the prime mover stack improved. The diverging tube system was driven by the low-temperature difference because the energy conversion efficiency was increased.

Relation between Acoustic Impedance and Sound Intensity Amplification in a Stack of Standing-Wave Thermoacoustic Prime Mover

A standing-wave thermoacoustic prime mover and a traveling-wave one realize cooling systems of not having moving parts. The prime movers have a challenge of being miniaturized for a practical use. For the miniaturization, in contrast to lower energy conversion efficiency than the traveling-wave thermoacoustic prime mover, the standing-wave one has some advantages of its simple structure and no acoustic streaming. The efficiency depends on the amplification quantity of sound intensity in the stack. However, the intensity measurement in the stack is difficult. In this paper, for improving the efficiency of the standing-wave thermoacoustic prime mover, the amplification quantity of sound intensity in the stack is calculated and discussed especially with the acoustic impedance in the stack. Consequently, results show that the efficiency improves when the absolute value of the acoustic impedance is low in the stack.

Miniaturization of the Loop-Tube-Type Thermoacoustic Cooling System: Effect of the Installation Position of Heat Pump and Working Gas in the Tube

A loop-tube-type thermoacoustic cooling system uses heat energy as a power source. Therefore, it is possible to put a loop tube into practical use in different areas. Several reports have described total loop tube lengths greater than 3 m. Additional miniaturization is necessary to apply this system to electronic devices such as personal computers. As described herein, to realize a miniature loop tube, an A4-size loop tube (850 mm total length; 24 mm inner diameter) was designed. Furthermore, to assess this systems cooling properties, two experimental investigations were carried out. One is to elucidate the heat pump installation position. The other is to assess the working gas in the tube. An A4-size loop tube driven on the basis of those experimental results achieved cooling from room temperature to 0 ?C.

Numerical analysis of the effect of local diameter reduction on the critical temperature of thermoacoustic oscillations in a looped tube

The installation of a phase adjuster (PA) has been proposed to improve the cooling effect of loop-tube-type thermoacoustic cooling systems. Installing a PA in a circular tube is equivalent to reducing the tube diameter locally. In this paper, we report our numerical investigation of the effects of the configuration parameters of a PA, i.e., diameter reduction and installation position, on the critical temperature of thermoacoustic gas oscillations by utilizing the transfer matrix method. The PA was modeled considering reflections due to the impedance mismatch at the boundaries between different diameters. The calculation results indicate that the critical temperature significantly decreases when a certain level of diameter reduction at appropriate positions is set, and these results correspond to empirical configurations that were experimentally chosen and verified in past studies.

Fundamental study for a working mechanism of Phase Adjuster set on thermoacoustic cooling system

Thermoacoustic cooling systems are drawing attention as next-generation eco-systems that can reuse and transform unused energy such as exhaust energy and solar energy. For the practical use of such systems, improvement of heat-sound energy conversion efficiency remains as an important challenge. We proposed a Phase Adjuster as a device for improving energy conversion efficiency However, the mechanism for that improvement remains unclear. Herein, to ascertain the reason, a parameter prediction method for determining the acoustic field was investigated by dividing the system into two parts: a Prime Mover (PM) part and a Phase Adjuster (PA) part. Consequently, the acoustic impedance cross-point of the PM part and the PA part on a complex plane is expected to represent a resonance frequency and a steady-state temperature ratio of the self-excited oscillation system.

Finite Element Analysis of the Standing Wave Field in a Tube with Lossy Wall for Thermoacoustic System

The finite element method (FEM) is applied to the analysis of the steady-state sound field in a thermoacoustic tube in the frequency domain. Although the thermoacoustic field is nonlinear and isothermal, a linear and adiabatic model is proposed, in which only the sound intensity field is considered because the sound intensity is generally discussed for the efficiency of energy conversion in the thermoacoustic system. In the numerical model, the divergence of the sound intensity is modeled as the sound source and the dissipation is modeled as loss caused by the specific acoustic impedance of the wall. Some numerical demonstrations are made for the two-dimensional standing wave field in a straight tube with a lossy wall. The influences of the stack and the phase adjuster on the sound field are examined. It is found that the sound field in the thermoacoustic tube can be qualitatively analyzed by introducing the specific acoustic impedance of the wall.

Effect of the relative installation position of two enlarged prime movers on the onset temperature in loop-tube-type multistage thermoacoustic system

Reducing the driving temperature of a thermoacoustic system to effectively utilize the unused low-temperature heat source is important for improving the performance of the system. The driving temperature of the thermoacoustic system was reduced by installing multiple stages of prime movers in series, a heat-to-sound transducer, and a prime mover with an increased cross-sectional area. In this study, the oscillation temperature was investigated both experimentally and by stability analysis in order to verify whether the system is operable, and to determine its operating temperature, when changing the installation position of the two-stage prime mover with increased cross-sectional area.

Fundamental Study for the Solution of Thermoacoustic Phenomenon Using Numerical Calculation: Relationship between the Stack Installation Position and Heat Flow

Thermoacoustic systems have several advantages owing to their simple structure driven by heat energy such as waste heat and solar heat. However, because their energy conversion efficiency is low, they have not been developed for practical use. Therefore, to improve energy conversion efficiency, a method of system design development is examined in this report using numerical calculations. First, calculation results obtained using the transfer-matrix methods are compared with experimentally obtained results, which confirms their good agreement. Secondly, calculated results of each heat flow element in the stack show that heat flow proportional to the temperature gradient QD, which decreases the performance of the system, is dominant. Finally, the installation position of the stack is changed to reduce the ratio that QD occupies in the heat flow. Results show a decrease in the ratio obtained by moving the stack installation position closer to the center of the tube. Energy conversion efficiency was increased eight times.

Control of self-excitation mode in thermoacoustic system using heat phase adjuster

The capability of a heat phase adjuster (HPA) to control the resonance mode in a loop-tube-type thermoacoustic system by locally heating the outside is experimentally investigated. It is shown that the HPA enables the resonance mode of the tube to shift to lower modes with higher thermoacoustic conversion efficiency, thus significantly enhancing the energy conversion efficiency. The transition of the resonance mode due to the stepwise change in the input electric power to the HPA is also investigated. As a result, it is demonstrated that the resonance mode changes with the temperature and a threshold exists for the HPA temperature at which the transition is induced. These results suggest the possibility of externally controlling a loop-tube-type thermoacoustic system.