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.