Development of high efficiency Stirling-type pulse tube cryocoolers

Thermoacoustic theory is a powerful tool to understand the working mechanism of regenerative thermodynamic systems. In this paper, a modified thermoacoustic model is employed to design three single-stage Stirling-type pulse tube cryocoolers. The first one (PTC-10) is designed with in-line configuration and the second one (CPTC-10) is designed with co-axial configuration. Both of them are able to provide about 10 W cooling power at 77 K with a relative Carnot efficiency of about 18.6%. The third one (PTC-20), designed with in-line configuration, has a twice cross section area of the PTC-10. It can provide more than 20 W cooling power at 77 K with a relative Carnot efficiency of 22%.     

Influence of resonance tube length on performance of thermoacoustically driven pulse tube refrigerator

A resonance tube is an important component of a thermoacoustic engine, which has great influence on the performance of the thermoacoustically driven pulse tube refrigerator. A standing wave thermoacoustic engine is simulated with linear thermoacoustics. Computed results show that an appropriate accretion of the resonance tube length may lead to a decrease of the working frequency and an increase of the pressure amplitude, which will improve the match between the thermoacoustic engine and the pulse tube refrigerator. The theoretical prediction is verified by experiments. A refrigeration temperature as low as 88.6 K has been achieved with an optimized length of the resonance tube, helium as working gas, and 2200 W of heating power.     

Investigation on the oscillation modes in a thermoacoustic Stirling prime mover: mode stability and mode transition

The purpose of this paper is to investigate the stability of oscillation modes in a thermoacoustic Stirling prime mover, which is a combination of looped tube and resonator. Two modes, with oscillation frequencies of 76 and 528 Hz, have been observed, stabilities of which are widely different. The stability of the high frequency mode (HFM) is affected by low frequency mode (LFM) strongly. Once the LFM is excited when the HFM is present, the HFM will be gradually slaved and suppressed by the LFM. The details of the transition from HFM to LFM have been described. The two stability curves of the two modes have been measured. Mean pressure P_m is an important control parameter influencing the mode stability in the tested system.     

The frequency dependent regenerator cold section and hot section positional reversal in a coaxial type thermoacoustic Stirling heat pump

We have constructed and tested two travelling wave thermoacoustic heat pumps using a coaxial configuration with the regenerator positioned in the annulus. We discovered a frequency dependent positional reversal of the cold section and hot section of the regenerator within the test frequency range. By decomposing the measured pressure wave within the annulus, we obtained the positive (w₊) and negative (w₋) propagating travelling waves. It has been revealed the change of frequency is accompanied by a change in magnitudes of w₊ and w₋ which is in part influenced by the presence of travelling wave attenuation through the regenerator. The resulting change of dominant travelling wave on a given end of the regenerator will then change the direction of thermoacoustic heat pumping at that end. This will alter the regenerator temperature distribution and may reverse the cold and hot sections of the regenerator. As the reversal does not require additional moving parts, merely a change in frequency, this feature in coaxial travelling wave devices has tremendous potential for applications which require both heating and cooling operation.     

行波热声斯特林机中自激振荡的实验分析--利用相空间重构方法

一个由环形行波圈和谐振管组成的热声斯特林系统巳成功的设计和建造，本文介绍了使用声信号的时间序列来进行相空间重构，用相空间重构方法来描述和分析行波热声系统中的自激振荡，对系统的起振特性以及行波模态到驻波模态的转变进行了实验分析和研究。     

热声制冷技术的研究状况

对国内外热声制冷技术的研究状况进行了综述 ,总结了热声制冷的各种特点与待解决的问题 ,并提出了目前该领域研究的几个热点方向。     

A simple method to determine the frequency of engine-included thermoacoustic systems

Frequency determination is important for the simulation of a thermoacoustic system consisting of a thermoacoustic engine. Based on the characteristics of linear acoustics, this article proposes a simple method for frequency calculation through numeric investigation. According to the method, frequency value can be decided if it leads to an inflexion point of the amplitude of volume flow rate, which is also a local minimum, most close to the volume flow rate node boundary. Compared with experimental data, the method proves to be very reliable. Besides, a concept of virtual tube is also proposed for frequency determination of thermoacoustic systems with no apparent geometrically-closed end.     

Network model approach for calculating oscillating frequency of thermoacoustic prime mover

The network model is given and used to calculate the oscillating frequency of thermoacoustic prime mover. Theoretically calculated frequency by the model is plural, whose real part denotes actual frequency of the system, and imaginary part means attenuation coefficient of acoustic pressure. Numerical calculation is made under different conditions including different acoustic cavities, spacing between each parallel plate, gas pressures and system lengths. Theoretically calculated frequency and experimentally measured frequency are in quite good agreement.     

非对称气体弹簧谐振系统建模与分析

气体弹簧支撑的谐振系统广泛应用于自由活塞斯特林发动机、压缩机等。以非对称气体弹簧谐振系统为研究对象,在直线电机驱动条件下,考虑谐振系统复杂流道内准稳态和间歇瞬态振荡流动、多处活塞-气缸密封间隙内气体流动,通过谐振系统非线性建模与数值计算,得到谐振系统活塞位移、各气腔气体压力变化规律等动态特性。在此基础上,基于时频变换分析谐振系统气体弹簧刚度特性,得到非对称气体弹簧等效刚度计算解析式。数值计算结果与实验结果吻合良好,验证了谐振系统建模与计算的正确性,能够为气体弹簧支撑往复式机器的设计提供参考。     

An innovative configuration for thermoacoustically-driven pulse tube coolers

Obtainable lowest temperature of a thermoacoustically-driven pulse tube cooler is generally limited by the pressure ratio provided by the thermoacoustic engine with helium as working gas. It is also known that a thermoacoustic engine filled with nitrogen can generally provide much larger pressure ratio and lower frequency than the same engine filled with helium. Here we introduce an innovative system configuration which uses an elastic membrane as the interface between the thermoacoustic engine subsystem and the pulse tube cooler subsystem. The membrane can transport acoustic work from the engine to the cooler, and meanwhile separate the working gases used in respective subsystems. Through this way, it is possible for the engine to operate with nitrogen to provide larger pressure ratio and more suitable frequency for the pulse tube cooler which can still use helium as the working gas. To test this idea, a thermoacoustically-driven pulse tube cooler was built. With the innovative configuration, the pulse tube cooler reached a lowest temperature of 139 K. On the other hand, without the membrane, the PTC only achieved a lowest temperature of 186 K when using nitrogen and 145 K with helium for both the PTC and the engine.     

Experimental investigation on a thermoacoustic engine having a looped tube and resonator

The purpose of this paper is to study the impact of regenerator hydraulic radius, resonator length, and mean pressure on the characteristics of the tested thermoacoustic engine, which has a looped tube and resonator. Two different acoustic oscillations are observed in the tested engine [Yu ZB, Li Q, Chen X, Guo FZ, Xie XJ, Wu JH. Investigation on the oscillation modes in a thermoacoustic stirling prime mover: mode stability and mode transition. Cryogenics 2003;43(12):687-91]. In this paper, they are called two acoustic modes, high frequency mode (with a frequency independent of the resonator length) and low frequency mode (with a frequency depending on the resonator length). Experimental results indicate that the relative penetration depth (the ratio of penetration depth over hydraulic radius) plays an important role in the excitation and pressure amplitude of the two acoustic modes. For each tested regenerator hydraulic radius, there is a measured optimal relative penetration depth, which leads to the lowest onset temperature difference. Note that, in the tested engine, the measured optimal relative viscous penetration depths are in the range 3-5 (for low frequency mode). Furthermore, experimental results also show that the resonator length affects the presence of high frequency mode in this engine.     

Stirling engine heat pumps

A Stirling engine is a thermal system that may be used to produce power from a high temperature heat source or as a refrigerator and heat pump to deliver energy at a higher temperature than abstracted from the source. A Stirling engine may therefore be used as the driver for natural gas heated air conditioning/heat pump Rankine cycle vapour compression systems or itself be used as the refrigerating/heat pump system requiring an input of work. Two Stirling systems, one acting as the driver, the other as the heat pump may be combined into the Stirling-Stirling or duplex Stirling arrangement. This paper touches briefly on a number of topics about fundamental aspects and recent developments in this field.     

Thermoacoustic oscillation in channels of the pressurized He II

In channels filled with the pressurized He II at 1 atm, a continuous thermoacoustic oscillation with a clear sound is triggered above the critical power-input q_λ of the phase transition under some conditions. A temperature distribution along the channel length is entirely maintained as long as the thermoacoustic oscillation lasts, i.e. the heat q_λ is kept transporting in the vortex state without drying the heater surface.The characteristic frequency of the thermoacoustic oscillation depends both on the channel length and weakly on the bath temperature, reflecting the property of the 1st sound.     

基于节点分析法斯特林发动机的参数优化

以斯特林发动机为研究对象,将斯特林发动机划分为6个控制容积,采用三阶节点分析法模拟了GPU-3斯特林发动机,得出其内部压力、温度、功率和效率等参数的动态变化规律。在此基础上对影响斯特林发动机工作腔容积与死容积的参数进行了优化分析,得到了发动机输出功达到最大时各参数的最优值。     

Performance comparison of thermoacoustic engines with constant-diameter resonant tube and tapered resonant tube

Two standing-wave thermoacoustic engines with a constant-diameter resonant tube and a tapered one, respectively, are simulated with linear thermoacoustics to explore the reasons for performance improvement of the thermoacoustic engine with the tapered resonant tube substituting for the constant-diameter one. Computed results indicate that the viscous loss in the tapered resonant tube is much lower than that in the constant-diameter one, and the smooth joint between the tapered resonant tube and its resonant cavity may avoid the acoustic power loss derived from sharp variation of flow area. The comparison between the computed results and the experimental data indicates that the simulation can roughly predict the performance of thermoacoustic engines with these two types of resonant tubes.     

Effect of RC load on performance of thermoacoustic engine

Based on linear thermoacoustics, a symmetric standing-wave thermoacoustic engine connected with an acoustic resistance and acoustic compliance (RC) load was simulated to study the effect of the load impedance on the performance of the thermoacoustic engine. Experiments were performed to verify the simulation. Both the simulation and the experimental results show that there is a non-monotonic variation of the acoustic power delivered to the load with the acoustic resistance of the load and a maximal acoustic power may be obtained when the acoustic resistance equals to its compliance impedance. And a lower compliance impedance of RC load may lead to a higher acoustic power delivered to the load.     

热声斯特林发动机的频率计算

利用传输矩阵推导出了一种计算行波形热声发动机的谐振频率的方法,得到了其基频和高阶谐波.对处于不同结构参数和运行参数情况下的系统频率进行了比较计算,得到了各参数对系统谐振频率的影响规律.     

Potential performance of real gas Stirling cycle heat pumps

Heat pumps based on the reversed Stirling cycle are shown to be positively influenced by real gas effects, provided they are designed to operate in a proper region of the fluid state diagram. A simplified model of a Stirling heat pump, aimed at understanding the basic cycle thermodynamics is presented, which allows a first optimization of real gas cycles. Provided the expansion process takes place in a proper narrow region close to the critical point, efficiencies much higher than those achievable with an ideal gas and similar to those of vaporization-compression cycles are obtained. A number of zero ODP, safe fluids are considered (Xe, CHF₃, C₂F₆, CHF₃ + CF₄ mixtures) allowing optimum operation in a wide range of heat source and heat production temperatures. Only mixtures, however, are recognized to permit a fine adjustment of the fluid properties to the heat source characteristics and to the user's temperature requirements. In order to reach good energy performance, high-pressure operation (around 200 bar) and an efficient internal regeneration of heat are needed. Graphs are supplied that reveal the heat pump cycle performance for each fluid at a wide range of temperatures, pressures and cycle compression volume ratios. Loss analysis shows that fluids having a simple molecule yield the best efficiency and the minimum amount of heat regeneration. Stirling power cycles are also shown to benefit from real gas effects, with the result that at top temperatures around 400–450°C, which are probably acceptable for a number of organic fluids, a fuel to work conversion efficiency around 25–30% seems possible for a cogenerative prime mover. The performance of such motors, intended for heat pump drives, are given for C₂HF₅ and C₃F₈ fluids. Very high pressures are required to optimize the cycle performance. Preliminary information on the prospective characteristics of a fuel powered Stirling-Stirling low-grade heat generator is given.