Study On The Operating Principle Of Media High Frequency Thermoacoustically Driven Pulse Tube Cooler

A thermoacoustically driven pulse tube cooler is a kind of cryogenic cooler with no any moving mechanical components and uses inert gas as working substance. Thus, it has advantages of structure simplicity, high reliability and environment friendliness, and is expected to have potential application in electronic cooling, nature gases liquefaction, etc.. However, there are still some scientific problems to be solved before extensive application. First, linear thermoacoustic theory is not precise in large pressure ratio cases in which high-power thermoacoustic engines usually operate; second, the large size of the thermoacoustic engine sometimes is an obstacle for some practical applications; third, although the thermal efficiency of a thermoacoustic engine is comparable to that of a internal-combustion engine, the total efficiency of a thermoacoustically driven pulse tube cooler is still low. In order to solve the problems, the following subjects have been studied in this dissertation.1. Study on the operating mechanism, modelization and design methodology of inertance tube phase shifter for pulse tube cooler1.1 As linear thermoacoustic theory has large discrepancy with experimental results in large pressure ratio cases, a simplified thermoacoustic turbulent-flow model with newly defined correction coefficients has been proposed. The comparison of the calculated results using different models and the experimental results shows that this new model has excellent agreement with experimental results in predicting the phase shifting capacity of inertance tube, especially in calculating the inlet phase angle.1.2 A new group of dimensionless parameters has been proposed to better characterize the phase shifting capability of inertance tubes. And these dimensionless parameters are especially suitable for engineering application and inertance tubes design. Based on the turbulent-flow model and the new dimensionless parameters, the phase shifting charts for two typical inertance phase shifter, pure inertance tube and inertance tube with infinite reservoirs, have been given to guide inertance tube design.1.3 Some important conclusions have been obtained through experiments and theories. To meet the phase shifting demand of high efficiency pulse tube cooler, the length of pure inertance tubes is usually in a range of 0.25 to 0.5 wavelength, while the length of inertance tubes with infinitely large reservoir in a range of 0 to 0.25 wavelength. For large cooling power pulse tube cooler, both of the phase shifter can easily meet requirement. But for small cooling power pulse tube cooler, pure inertance tubes with single diameter often can't meet requirement of optimum phase shifting. For this case, we suggest using the combined inertance tubes instead.2. Study on the operating principle and thermodynamic performance of a 300Hz thermoacoustically driven pulse tube cooler2.1 The influence of structure parameters mainly including thermoacoustic stacks, acoustical amplifier and phase shifter has been studied in the 300Hz standing-wave engine driven pulse tube cooler. The following conclusions are obtained by the experimental and numerical results. First, as the thermoacoustic stacks machined by photo-chemical milling method gives much better size of the stack quality than that machined by the traditional electric discharge machining method, the conversion efficiency of thermal energy and acoustical energy is obviously improved. Secondly, an acoustical amplifier tube is still effective in a 300Hz thermoacoustically driven pulse tube cooler, improving the whole efficiency of the system. Finally, compared with the single pure inertance tube, a combined inertance tube phase shifter has a relatively wider phase shifting range, which is consistent with the above theoretical prediction.2.2 The influence of operating parameters including the mean pressure and gravity effect has been studied. The following conclusions are obtained by the experimental and numerical results. First, the cooling performance of the whole system improves as the mean pressure increasing, however, the output acoustical power of thermoacoustic engine doesn't improve much in this process according to the numerical results. Second, the gravity has no obvious effect on the cooling performance of the 300Hz thermoacoustic system.2.3 Based on optimization of structural and operating parameters, a lowest no-load temperature of 69.3K has been achieved with a heating power of 750W. In particular, a cooling power of 0.2W at 80K has been obtained with a heating power of only 500W. Additionally, a new type of insulation with nanometer material has been applied in this system to deduce the heat leak of the hot region, consequently improving the whole efficiency of the thermoacoustically driven pulse tube cooler.3. Study on the operating principle and thermodynamic performance of a 500Hz thermoacoustically driven pulse tube cooler3.1 The performance of a 500Hz thermoacoustically-driven pulse tube cooler has been investigated. During the experiments, we found the acoustical pressure amplifier tube fail to amplify the pressure amplitude as the acoustical impedance connecting to the acoustical amplifier tube is not appropriate. A lowest temperature of 119K has been obtained with a heating power of 2200W after optimizing the structural and working parameters. As the acoustical load does not match the thermoacoustic engine, the frequency of the system can jump up to 918Hz. In this case, a lowest temperature of 179.9K has been obtained with heating power being 1985W.3.2 A new type of regenerator filler, metal fiber felt, has been introduced and studied. Such material might be more suitable for higher frequency pulse tube cooler. A group of linear thermoacoustic equations describing this regenerator has been obtained. Furthermore a pulse tube cooler with this structure has been established and coupled with a 300Hz standing-wave engine. A lowest temperature of 122K has been obtained in preliminary experiments, showing good? feasibility of such a regenerator used for high frequency pulse tube cryocoolers.