Thermoacoustic engines and refrigerators utilize acoustically caused pressure and velocity oscillations to perform the energy conversion between heat and work flows. Based on the use of these oscillations, simplicity and high reliability are achieved. Linearized thermoacoustic theory is usually used for designing thermoacoustic energy converters. This commentary describes the assumptions used in the derivation of the basic equations in the theory and shows the method of calculating the performance of the thermoacoustic devices using the theory.
When a narrow duct (i. e. ,regenerator) is installed in a tube while the temperature ratio is higher than some critical value at both ends of the regenerator, the gas inside starts self-excited oscillation. Recently, study on thermoacoustic engines applying thermoacoustic phenomena is being actively conducted. While most waste heat ranges from 400 to 600 K, the critical onset temperature of a thermoacoustic engine is higher, ranging from 600 to 1000 K. In order to solve this problem, a multistage thermoacoustic engine that can lower the critical onset temperature was recently suggested. It was reported that the critical onset temperature was successfully lowered using multistage amplification. However, there is an issue relating multistage engines that realize low-temperature oscillation. A multistage thermoacoustic engines with multiple regenerators needs regenerators installed not only at the peak of acoustic impedance distribution in real part, and therefore is generally inefficient. In this report, we suggest a composition for a multistage thermoacoustic engine using numerical calculation, so that high acoustic impedance and traveling-waves are realized at all regenerator positions, and traveling-waves with acoustic impedance ρc are realized at positions other than the regenerators. Within the suggested composition, the viscous dissipation of the resonators is extremely small. In addition, thermal efficiency of 33% has been achieved with each regenerator when the temperature of the heat exchanger is 600 K. This result suggests that the use of the multistage type enables thermoacoustic engines to have such a composition that both lowtemperature drive and high efficiency.
The oscillatory flow in a thermoacoustic sound wave generator is described. Oscillatory flow plays an important role in thermoacoustic phenomena, in which direct energy conversion from heat to work (oscillatory flow or sound wave) and vice versa are performed with no moving parts. A stack is essential in a thermoacoustic sound wave generator, which is the most important part of thermoacoustic equipment. Clearly, there is an optimum stack size for the oscillatory flow of a thermoacoustic sound wave generator from the viewpoints of both heat transfer and pressure loss between the stack and the working fluid. Five kinds of stainless steel cylindrical tubes with different radii were employed to ascertain the optimum stack size. The results of experiments clarified an optimum stack size of 2.0 mm outer diameter, 1.6 mm inner diameter and 80 mm length. This stack size was verified from the viewpoint of oscillatory flow using the numerical formula calculation software, Mathematica. In addition, the use of a regular polygon instead of a circular cross section for the stack was proposed and analyzed using the hydrodynamic equivalent diameter. It was confirmed that a stack with a square or regular hexagonal cross section is practical for use in thermoacoustic equipment. In particular, regular hexagonal conduits with a side length of 0.924 mm are the most suitable shape for the stack of a thermoacoustic sound wave generator.
In this work, we attempt to understand a thermoacoustic device by dividing it into subsystems and analyzing the acoustic admittance of these subsystems experimentally. It is shown that the temperature difference controls the real part of the admittance, whereas the frequency changes the imaginary part of it. The combined device consisting of these subsystems is found to start working at the temperature difference and the frequency when acoustic admittances of the subsystems agree with each other. On the basis of the result, we propose two new designs for a double-loop thermoacoustic Stirling cooler.
Numerical simulation of heat and fluid flow is performed to estimate the value of the heat transfer coefficient between the working gas and wall in an oscillatory flow. Transient two-dimensional equations of continuity, momentum and energy are solved utilizing a TVD scheme. A physical model of a simple circular tube, in which the wall temperature is constant, is used for the numerical simulation. In this study, the variations in the local heat transfer coefficient in an oscillatory flow are clarified by analyzing the temperature field obtained as a numerical result. The results show that the value of the heat transfer coefficient became considerably high when temperature inversion between the working gas and wall occurred. This high value is theoretically infinite, and this makes it difficult to use the heat transfer coefficient in oscillatory flows.
A thermoacoustic refrigerator consisting of a linear motor, and looped and resonator tubes was constructed. The acoustic field formed in the refrigerator was calculated and analyzed. Further, the performance of the refrigerator was measured as a function of the mean pressure of nitrogen, which was used as a working gas. It was found that although the electroacoustic efficiency of the linear motor only depends on the mean pressure to a limited extent, the coefficient of performance (COP) is highly dependent on it. When the mean pressure was 700 kPa, the highest value used in the present experiment, 11% of the Carnot COP was obtained at -11ºC and the lowest temperature obtained was -51ºC.
This paper reports on acoustic power amplification by a differentially heated regenerator. It is shown that the amplification gain approaches the ideal gain determined by the temperature ratio, and that the efficiency of converting heat power into acoustic power is expected to reach 63% of the Carnot efficiency when the specific acoustic impedance in the regenerator is increased to ten times larger than that of the traveling wave in a free space. When the three regenerators are aligned in series, a total acoustic power gain of ten is achieved with a temperature ratio of 1.95, but the efficiency is reduced to 7%. In order to upgrade the efficiency of three-stage acoustic power amplification, a branch resonator is tested.
We have constructed and tested a travelling-wave thermoacoustic refrigerator using a coaxial configuration with the regenerator positioned in the annulus. The device was tested using three operating frequencies: 100, 140 and 200 Hz. The rise and descent from an initial temperature were recorded at both ends of the regenerator for 60 minutes. At 100 Hz, temperature rise and descent occurred at both ends of the regenerator, and both ends experienced a rise in temperature at 140 and 200 Hz as well. This is due to the fact that the change in frequency caused a change in the direction of the dominant travelling wave at one end of the regenerator. This causes the heat pumping direction within the regenerator to change as it is dependent on the direction of the dominant travelling wave at that end. At 200 Hz, we have seen that the T1 and T2 temperatures of the regenerator have switched roles as the hot section and cold section compared to their previous conditions at 100 and 140 Hz. This is because the heat pumping due to |w +| at the T1 end becomes larger than the heat pumping due to |w - | at the T1 end.
A kind of heat engine called a pulse tube engine has been recently proposed. The engine consists of only a few parts: differentially heated stacked metal meshes in a cylinder and one piston, coupled to a flywheel through a narrow flow passage called orifice. We built the prototype engine and tested its working mechanism from the standpoint of a thermoacoustic framework. We measured the work flux density distribution over the cross-section of the pulse tube to identify the work source of the engine and elucidate the function of the parts used in the engine. This engine belongs to the standing-wave engine group and the work source resides not in the stacked metal meshes but in the pulse tube. It is demonstrated that the stacked metal meshes are installed to break the thermodynamic symmetry, and that the orifice plays the role of extracting a maximum power from the engine.
A novel liquid-piston steam engine, which can achieve high efficiency in the low-temperature region of T < 300°C as well as high reliability and low cost, has been developed. In this study, the steam engine achieved a thermal efficiency of 12.7% at heating and cooling temperatures of 270 and 90°C, respectively, when sintered metal was employed in the heating section. This efficiency value is approximately 40% that of the Carnot cycle. There are expectations for this liquid-piston steam engine to be used as a energy conversion device for waste heat recovery systems.