Nonlinear Effects In Thermoacoustic Refrigerators

Thermoacoustic refrigeration technology shows great application prospects in military, industry and even daily life by virtue of its high reliability, high stability and no pollution. Some of the nonlinear effects were studied in this thesis, aiming to improve the system optimization of the thermoacoustic refrigerator driven by loudspeakers.Firstly, the discovery of thermoacoustic effects was reviewed, as well as the research and development history of thermoacoustic refrigerator. Besides, research progress on various nonlinear effects of thermoacoustic refrigerator was summarized, including the nonlinear effects of the sound source, the stack, the finite-amplitude acoustic wave in tubes, the acoustic-heat energy conversion and the systematical model.Nonlinear impedance of thermoacoustic stack was studied in chapter 2. A brief introduction was made on the structure, fabrication and basic parameters of thermoacoustic stacks. And the stacks were divided into two parts, namely ordered and disordered ones, according to the tortuosity. Linear impedance theory of thermoacoustic stacks was also introduced, based on which the pressure and impedance distribution in the stack can be obtained. The experimental setup was designed on the analogy of the fluid impedance of porous materials and then the impedance curves of the plate-type, pipe-type and copper mesh stacks were measured. A nonlinear theoretical model was built based on the experimental data, and linear resistance, nonlinear coefficient and effective acoustic mass were extracted as three import parameters. The influence on the stack impedance caused by the volume porosity, length and structure of the stack and the frequency of the acoustic wave were evaluated at the same time. Furthermore, explanations were given based on the "minor loss" theory and the tortuosity for the experimental results, and an agreement was made between our nonlinear theoretical model and the "minor loss" theory. These results may be used as references in the stack choosing and nonlinear impedance match of the thermoacoustic system.In chapter 3, both the nonlinear vibration of the loudspeaker and the nonlinear acoustic field in the thermoacoustic refrigerator were studied. Firstly, a simple thermoacoustic model was built up. The linear acoustic field, particle velocity distribution and the first two resonant frequencies of the model were theoretically calculated, and the first two resonant frequencies were measured experimentally. Then the frequency responses of sound pressure and the diaphragm displacement of the loudspeaker around the first two resonant frequencies were measured. Nonlinear behaviors at different frequencies or under different driven voltages were discussed. These results are helpful deciding both the place to fix the stack in the resonant pipe and the proper working frequency of the system.