| Thermoacoustic heat engine and refrigerator use thermoacoustic effect to realize conversion between heat and sound energy. The engine, which can be driven by various grade heat sources, can lessen growing tension of energy problem, and the refrigerator uses natural gases as refrigerant, which are environmentally friendly. Especially, the travelling-wave thermoacoustic heat engine and refrigerator, based on reversible thermoacoustic cycle, ideally have the same efficiency as the Carnot cycle. Thus, it is expected to have widespread applications. Many breakthroughs in travelling-wave thermoacoustic heat engine (TWTAHE), and travelling-wave thermoacoustic refreigerator (TWTAR) have been made in the past decade. However, there is still a long way to go before practical applications. As part of the efforts, the following work has been done in this dissertation:1. Investigation on thermoacoustic performance and working mechanism of travelling-wave thermoacoustic heat engine(TW-TAHE)The performance of a TW-TAHE has been studied using RC load method. The maximum output acoustic power of 492W and the maximum net efficiency of 18.5% have been obtained, with 3.0MPa nitrogen gas at 23.6Hz. The maximum output acoustic power of 801W, the maximum net efficiency of 26.5%, and the total efficiency of abour 32% have been obtained with 3.0MPa helium gas at 67.5Hz. It is the most efficient thermoacoustic heat engine reported so far in the world.In order to improve the capacity of acoustical power ouput, a parallel-plate regenerator was designed to replace the screen-stacked regenerator, and the results have shown that the parallel-plate regenerator could effectively improve the power output of the engine but slightly lower the thermal efficiency.Magnitude of the heat leaks of the engine has been evaluated through experiments and numerical calculations. The results have shown that, the efficiency of the engine could be improved by reducing the thicknesses of regenerator pressure-vessel wall and the thermal buffer tube wall, by decreasing the emissivity of main cold heat exchanger and secondary cold heat exchanger, and by improving insulation quality.Quantitative investigation on the influence of Gedeon streaming revealed that the Gedeon streaming wasted about half of the heat power from hot heat exchanger without creating acoustic power, which would lead to reduction of efficiency of the heat engine. Using the elastic membrane could effectively suppress the Geadeon streaming.2. Study of coaxial travelling-wave thermoacoustic refreigerator (TW-TAR)In order to obtain a more compact TW-TAR, a coaxial travelling-wave thermoacoustic refreigerator (CTW-TAR) was designed to better match the peformance of the TW-TAHE. Influence of the parameters (the paramenters of regenerator screen, working frequency, cooling temperature and phase adjuster) on the performance have been studied numerically and experimentally.A substitution of inertance mass for traditional inertance tube was proposed, which make it possible to decrease the size of the refrigerator. Experimental results have shown that using the inertance mass as phase adjuster could effectively decrease the viscous loss and the loss due to cross section change, and the maximum cooling power increased 43% at -40℃, 40% at -20℃, and 46% at 0℃.The natural heat convection and Gedeon streaming, which are very important to the performance of refrigerator were investigated experimentally. The natural convection made the cooling power of refrigerator decrease 7.13% at 0℃, 20.83%at -20℃, and 44.16% at -40℃, with the inertance mass as phase adjuster. The Gedeon streaming made the cooling power of refrigerator decrease 11.4% at 0℃, and 47.66% at -20℃.3. Investigation on the coupling law between the TW-TAHE and CTW-TARThe output characteristics of the TW-TAHE with different impedence loads and the influence of load on parameters of engine were studied numerically. It has shown that it was better to use the larger-volume reservoir in RC load to reasonably evaluate the power output capacity of the engine.The optimum matching relationship between the CTW-TAR and TW-TAHE has been investigated numerically and experimentally. The results revealed that the real part of refrigerator impedence would increase as the inertance provided by the inertial components in the refrigerator, and the COP of the refrigerator would first increase and then decrease, but the hot end temperature and output efficiency of the engine would increase. Thus, the total efficiency (cooling power divided by input heat power) would increase firstly and then decrease because of the combined effects (COP of refrigerator and output efficiency of engine). The maximum cooling power and the total efficiency was 230W and 0.112 at -40℃; 340W and 0.16 at -20℃; 469W and 0.216 at 0℃, respectively.The impedence match between the TW-TAR and the TW-TAHE was investigated. The analysis has shown that acoustic field of the entrance of the TW-TAR was mainly in the standing wave mode, which was not good for acoustic power input, and the matching relation could be optimized directly (decrease the imagenary part of refrigerator impedence) or indirectly (coupled by acoustic amplifier). Thus, the performance of whole system could still be much improved with appropriate pressure amplifier.
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