Study Of Spontaneous Oscillation And Thermoacoustic Conversion Characteristics Of Thermoacoustic Heat Engines

A thermoacoustic heat engine is a new type of heat engine with remarkable virtues, such as high reliability and environment friendliness. It can be used to drive a thermoacoustic refrigerator or a linear electric generator to construct a totally no-moving-part thermoacoustic refrigerator or a highly reliable thermoacoustic electric generator, respectively, which possess important application prospect. However, as the thermoacoustic heat engine involves complicated process of nonlinear spontaneous oscillation and highly coupled interaction between flow, heat transfer and thermodynamic conversion, its operating mechanisms needs to be further studied. In addition, increasing the working frequency of the thermoacoustic heat engine is helpful to reduce its size and to increase power density, which is one of the developing directions of such a heat engine. The following theoretical and experimental works have been made:1. Study of an experimental thermoacoustic-Stirling heat engine using CFD simulationBased on an experimental thermoacoustic-Stirling heat engine, a 2D numerical model was set up. Various numerical schemes were tested, which indicated that Segregate solver, second-order implicit time discretization and second-order upwind space discretization can eliminate artificial dissipation and false oscillation, and are suitable for the simulation of thermoacoustic heat engines. Two methods of imposing temperature gradient across the regenerator were studied, and the complete evolution of nonlinear spontaneous oscillation was obtained under the thermal boundary condition of imposing heating power for the first time. The nonlinear effects such as critical onset temperature, pressure wave amplification and saturation were successfully captured. Additionally, the Gedeon DC flow that exists in the travelling loop was captured. Based on the 2D simulation, a 3D simulation was performed for the first time and the complicated multi-dimensional effects such as non-uniform distributions of flow and temperature fields were given. 2. Study of a high frequency standing-wave thermoacoustic heat engine using CFD simulationBased on an experimental high frequency standing-wave thermoacoustic heat engine, a 2D numerical model was set up. The calculated model was tested, which indicated that the practical stack model was suitable for the simulation of standing-wave heat engines. Two methods of imposing temperature gradient across the stack were studied, and the process of mean pressure increasing, pressure wave amplification and saturation was obtained under the thermal boundary condition of applying heating power, which was quite different from thermoacoustic heat engines with low working frequency. The acoustic field was given, and the flow vortices and their evolution nearby the ends of the stack and resonator were observed.3. Investigation on thermoacoustic conversion characteristics of a high frequency standing-wave thermoacoustic heat engineThe system performance was greatly improved by introducing a tapered resonator, and a highest pressure ratio of 1.175 was obtained with 4.2MPa helium and 300Hz working frequency.Acoustic amplifier tube was introduced to couple the high frequency standing-wave thermoacoustic heat engine with high frequency pulse tube cooler. The performance of the coupled system was studied and it is shown that the volume flow rate at the coupling position was of great importance. With calculated and experimental optimizations, a lowest temperature of 68.3K was achieved with 63cm i.d. 4.3mm acoustic amplifier tube and 750W heating power, which is the lowest temperature for thermoacoustically driven pulse tube coolers with such high frequency. And with the heating power of 500W, a cooling power of 0.2W at 80K was obtained.Moreover, the coupling mechanism of the high frequency standing-wave thermoacoustic coupled with a RC load was studied numerically and experimentally. Numerical study revealed that with the increase of gas reservoir volume, the pressure amplitude at the inlet of RC load became smaller than the outlet of the engine under certain conditions, which was validated in the experiments. In the experiment, the high frequency standing-wave thermoacoustic heat engine can output acoustic power of about 96W and a thermal efficiency of about 9% at its most powerful point.4. Investigation on thermoacoustic conversion characteristics of a high frequency thermoacoustic-Stirling heat engine The influence of gravity on the performance of the high frequency thermoacoustic-Stirling heat engine was studied experimentally, and the experiment indicated that gravity only affected the onset temperature and had little influence on resonant frequency, pressure amplitude and heating temperature. In addition, the system was studied experimentally in terms of operating parameters, regenerator mesh size and working medium. A high pressure ratio of 1.17 was obtained with 4.0MPa helium and 314Hz working frequency, and a highest pressure ratio of 1.24 for 2.0MPa carbon dioxide and 76Hz working frequency. The preliminary experimental comparison between high frequency thermoacoustic-Stirling heat engine and standing-wave thermoacoustic heat engine indicated that the former has higher thermal efficiency over the latter.