Study On A Miniature Traveling Wave Thermoacoustic System

A pulse tube refrigerator driven by a thermoacoustic engine is a new type of cooler without any moving part. It occupies many advantages over the traditional mechanical compressors, such as simplicity, reliability, longevity, no pollution, which attracts great interest of academic circles and industry. Recently, with the development of the aerospace and defense industry, the demand for miniature cryogenic refrigerator has grown in volume, then how to miniaturize a thermoacoustic driver becomes an urgent problem to be solved. In order to effectively improve the oscillating pressure amplitude and decrease the cooling temperature, the present theoretical and experimental work focuses on the following sections:1. Based on the linear thermoacoustic theory, the relation between the phase difference, impedance and the derivation of the time-averaged acoustic power produced in a length dx of the channel is analyzed qualitatively and the combinative adjustment of the phase difference and the impedance is provided as a design strategy.2. A high frequency thermoacoustic Stirling heat engine (TASHE) is simulated and analyzed. A theoretical linear model of high frequency TASHE was set up, and the oscillation pressure, oscillation flow rate, the phase difference, temperature and the time-averaged energy flux in the engine was analyzed, which make the acoustic distribution clearer. The influence of the radial dimensions on the operating frequency and the performance is also analyzed. The computed results indicate that the resonance tube is a key component which has great influences on the working frequency and performance of the engine. It is an important phase shifting mechanism. The appropriate decrease of the resonance tube diameter may decrease the frequency of the system and hence improve the performance of the thermoacoustic engine, which is useful in the design and miniaturization of the engine. From the angle of the phase lead and the impedance, the influence of the operating parameters including heating power and average pressure on the performance is simulated, and it is concluded that to obtain appropriate impedance and phase difference in the regenerator, the operating parameters must be optimized in company with the structural parameters.3. On the basis of the analysis and consideration, a miniature thermoacoustic Stirling heat engine was built. The overall size is about 690×260mm and the weight is less than 5Kg. By means of regenerator optimization, adjustment of the resonance tube diameter and control of the operating parameters, the pressure ratio and peak to peak pressure amplitude achieved to 1.115 and 2.71Bar respectively, under the condition of 2.5MPa helium gas with 700W heating power. The working frequency was about 282 Hz.4. A simple network model of TASHE with a load was described. The analyses shows that the thermoacoustic engine and the load are related to, restricted and influenced each other. The load will lead to the change of the pressure amplitude and heating temperature in the thermoacoustic engine, reversely the change will influence the performance of the load. The important influence of the connected tube between the engine and load on the coupling system has been discussed. The phase shifting and impedance adjustment function of the inertance tube has been analyzed and the impedance adjustment function has been emphasized.5. Based on the linear thermoacoustic theory, the TASHE and the pulse tube refrigerator has been simulated as an integrated system. The simulation focuses on the length of the regenerator, the diameter and length of the pulse tube, the mesh and length of the heat exchanger, the length and diameter of the inertance tube of the refrigerator and the length of the connected tube. The influences of the structural and operating parameters of the engine on the refrigerator are also investigated.6. Based on the simulation, the thermoacoustically driven pulse tube refrigerator has been experimentally optimized. The optimization centralizes on the diameter and length of the inertance tube and the length of the connected tube, the diameter of the thermoacoustic engine and the average pressure are also optimized. With helium as working fluid, under the conditions of 2.5 MPa average pressure, 600 W heating power, and a no–load cooling temperature as low as 147.3 K has been achieved. It shows feasibility and wide application prospect that hundreds Hz thermoacoustic engine drives refrigerator. In addition, it was found that installation of a ball valve between the engine and the refrigerator may significantly affect the onset temperature of the thermoacoustically driven pulse tube refrigerator. The onset temperature can be greatly decreased with a temporary closure of the ball valve before onset. This simple but effective operation is of benefit to the application of low-grade heat energy.