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.     

Numerical investigation on a 300 Hz pulse tube cryocooler driven by a three-stage traveling-wave thermoacoustic heat engine

A 300 Hz pulse tube cryocooler (PTC) driven by a three-stage traveling-wave thermoacoustic heat engine (TSTHE) has been proposed and studied in this paper. In the configuration, three identical thermoacoustic heat engine units are evenly incorporated in a closed traveling-wave loop, in which three pulse tube cryocoolers are connected to the branch of each thermoacoustic heat engine. Compared with the conventional thermoacoustic heat engine which involves a traveling-wave loop and a long resonator, it has advantages of compact size and potentially high thermal efficiency. A TSTHE–PTC system was designed, optimized and studied in detail based on the thermoacoustic theory. Firstly, numerical simulation was conducted to design the system thus the optimum structure parameters of the system were obtained. With the operating condition of 4 MPa mean pressure and high working frequency, a cooling power of 7.75 W at 77 K and an overall relative Carnot efficiency of 11.78% were achieved. In order to better understand the energy conversion characteristics of the system, distributions of key parameters such as acoustic work, phase difference, dynamic pressure, volume flow rate and exergy loss were presented and discussed. Then, the coupling mechanism of the system was investigated. In addition, influence of coupling position on the system performance was further studied.     

A high frequency cascade thermoacoustic engine

A miniature cascade thermoacoustic engine, which consisted of one standing-wave stage and one traveling-wave stage in series, was built and tested, which length was about 1.2 m, operating at 470 Hz using helium as working gas. The cascade modeling, the simulation and the primary experimental results are described in this paper. Four different configurations of the miniature cascade thermoacoustic engines had been designed and compared. According to the analysis, the diameter ratio of stages was designed to extend the traveling-wave region, which optimized value was about 1.69. The peak-to-peak value of the acoustic pressure was predicted to arrive to 3 bar at the 3 MPa mean pressure of helium when 300 W heating power was the input. The features of the engine were predicted delivering 68 W acoustic power with a thermal efficiency of up to 22.74% (the ratio of acoustic power to heater power). Due to careful designing, the engine self-excited the oscillation smoothly from the first experiment. An onset temperature gradient of about 4.5 K/mm was achieved, and the peak-to-peak acoustic pressure was 48 KPa at the 2 MPa mean pressure when 200 W heating power was the input. The design computation and experimental results showed a rather good agreement between the measured and calculated pressure phasor and temperatures distributions in the cascade thermoaoustic engine.     

Experimental investigation of transient nonlinear phenomena in an annular thermoacoustic prime-mover: observation of a double-threshold effect

Thermoacoustic engines have been subjected to numerous studies for the past 10 years. Our current research is focused on the transient regime in an annular thermoacoustic prime-mover. It appears that several nonlinear phenomena can play a role in the amplification and saturation regimes. Indeed, acoustically induced conductivity, forced convection due to acoustic streaming, minor loss phenomenon, and saturation due to harmonic generation can be quoted among the others. The experiments presented here show for the first time a double-threshold phenomenon during the amplification regime. The first threshold, which corresponds to the setting of the thermoacoustic instability, is followed by a saturation regime. Then after a time delay, without any changes in the control parameters, a second threshold corresponding to an additional amplification has been observed.     

环路声学共振多级行波热声发动机的机理研究

针对环路声学共振多级行波热声发动机的工作机理进行了研究,重点分析了环路声学共振4级行波热声发动机无负载工作情况,并比较了环路声学共振4级、8级、16级行波热声发动机的工作性能。计算结果表明,这一行波热声发动机具有较好的声场分布并通过增大回热器横截面积有效降低了回热器内的粘性流动损失。增加环路声学共振多级行波热声发动机的级数仍能获得较为理想的工作性能,并能够增加整机产生净声功率,降低谐振管消耗声功率的比例,相对传统带驻波谐振管的行波热声发动机更为紧凑。     

Experimental study of quasi-periodic on-off phenomena in a small-scale traveling wave thermoacoustic heat engine

Periodic and spontaneous on-off oscillation belongs to the onset and damping behaviors of thermoacoustic engines, and investigations on this phenomenon lead to better operation of the thermoacoustic engines with stable performances. In this paper, the quasi- periodic on-off oscillation in a small-scale traveling wave thermoacoustic heat engine with a resonator length of only 1 m was experimentally investigated. The type of working media, mean pressure and the input heating power are the main operating parameters, which significantly affect the formation of the periodic on-off oscillation. The experimental results demonstrated there was a critical charge pressure over which the periodic on-off oscillation could happen. For the small- scale engine with helium gas as the working media, the mean pressure threshold value was about 1.4 MPa and the on-off oscillation occurred with a single frequency. Using nitrogen and argon gas as the working media, the on-off oscillation was not observed. The reason was qualitatively analyzed as well.     

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.     

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.     

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.     

Effect of different working gases on the performance of a small thermoacoustic Stirling engine

The performance of a small thermoacoustic Stirling heat engine (TASHE) was investigated with three kinds of working gases experimentally and numerically. The examined performances focused on the operating frequency, onset temperature, pressure amplitude and some temperature characteristics after onset. The working frequency with nitrogen, argon and helium as the working gas was 45 Hz, 42 Hz and 130 Hz, respectively. The engine worked with helium in a much wider range of mean pressure than with nitrogen and argon. There was an optimal mean pressure for the minimum onset temperature for each working media. Using nitrogen and argon as working gas rather than helium, another optimal mean pressure for the highest pressure ratio was obtained in the experiment. The loop dimension was indispensable in determining the frequency and the highest pressure ratio was observed in the resonator cavity.     

采用正弦型谐振管的高频驻波热声发动机

设计了一截面积正弦变化的对称管道作为高频驻波发动机谐振管,实验研究了该发动机的工作性能与声功输出能力,并与采用等直径谐振管的实验结果进行对比.在一定加热功率下,正弦管能有效抑制系统产生的二阶谐波,提高一阶压力波动幅值并降低加热块温度.由于存在显著的非线性效应,基于经典线性热声理论的计算无法预测两种谐振管实验性能的不同,并且与实验结果偏差很大.     

Construction and performance of a thermoacoustic refrigerator

This paper deals with the construction and performance of a thermoacoustic refrigerator. The manufacturing of the different components of the apparatus will be explained along with the reasons for using specific materials. The setup consists of three major parts: The refrigerator which is contained in a vacuum vessel, the electronic apparatus necessary for the measurements and acquisition of the experimental data, and the gas-control panel which is used to fill and purge the system and to prepare gas mixtures. The system is assembled and the first measurements show a good behavior. A low temperature of −65 °C is achieved which is one of the lowest reported temperatures up to date.     

Experimental investigation of a thermoacoustic refrigerator driven by a standing wave twin thermoacoustic prime mover

The objective of this study is to analyse the performance of thermoacoustic refrigerator (TAR) measured in terms of hot end temperature and temperature difference across refrigerator stack with two different spacing namely 0.4 mm and 0.8 mm and stack used in refrigerating section was made of low thermal conductivity materials namely Mylar sheet and photographic film & the experiments were carried out at 1 MPa pressure using helium as working fluid. High powered acoustic wave with frequency of 460 Hz and pressure amplitude of $?$0.07 MPa was obtained from twin thermoacoustic prime mover (TAPM) and this acoustic wave produced temperature difference of 16 °C across the Mylar sheet stack made of 0.4 mm spacing in refrigerator section. From this study, it has been inferred that twin TAPM can act as efficient drive for TAR.     

行波热声发动机中平板与丝网回热器的对比实验研究

制作出了一个平板回热器,将其置入行波热声发动机中与丝网回热器进行对比实验研究.实验结果表明,平板回热器能够有效提高行波热声发动机的输出声功,但是其热效率有所降低.     

斯特林热声发动机的直流研究

具有环路的斯特林热声发动机引入了反馈回路,实现了行波相位的热声转换,获得了较高的热声转换效率.然而,环路的拓扑结构使得声场中直流的产生成为可能,直流的出现将严重制约热声发动机的性能.本文以温度和压力测量作为重要实验手段,研究直流与热声发动机温度分布、压比、热声转化效率之间的关系,考察了直流抑制前后系统的性能,指出有效抑制直流能大幅度提高热声发动机的性能.     

Numerical investigation of thermoacoustic refrigerator at weak and large amplitudes considering cooling effect

In this paper, OpenFOAM package is used for the first time to simulate the thermoacoustic refrigerator. For simulating oscillating inlet pressure, we implemented cosine boundary condition into the OpenFOAM. The governing equations are the unsteady compressible Navier–Stokes equations and the equation of state. The computational domain consists of one plate of the stack, heat exchangers, and resonator. The main result of this paper includes the analysis of the position of the cold heat exchanger versus the displacement of the pressure node at large amplitude for successful operation of the refrigerator. In addition, the effect of the input power on the successful operation of the apparatus has been investigated. It is observed that for higher temperature difference between heat exchangers, the time of steady state solution is longer. We show that to analyze and optimize the thermoacoustic devices, both heat exchangers should be considered, coefficient of performance (COP) should be checked, and the successful operation of the refrigerator should be evaluated.     

燃气驱动的热声发动机初步实验

在原有电驱动混合型热声发动机的基础上,设计建造了燃气驱动的混合型热声发动机,并进行了初步实验.以氮气为工质,在充气压力为1.2 MPa时发动机压比达到了1.29,在充气压力为2.4 MPa时得到了250.3 W的声功.通过采用弹性膜片阻断直流,在充气压力1.2 MPa时,系统最高压比达到1.34.     

回热器参数与热声系统动态特性的实验研究

利用沿轴向均匀布置在回热器内部的4个温度传感器,采集了不同工况条件下的回热器内部温度,并在现有的渗透深度等回热器参数模型和实验获得的系统谐振频率的基础上,计算了回热器的相关参数,进而分析了这些参数的变化规律,验证了热声系统谐振模态的动态演化过程.     

丝网板叠型驻波热声发动机的工作特性研究

基于线性热声理论设计并搭建了一台丝网型驻波热声发动机,采用不同目数的不锈钢丝网进行试验,考察了丝网板叠几何参数和工作压力对热声发动机工作特性的影响,实验结果和线性热声理论计算结果吻合度较好。实验发现,高目数丝网板叠更容易使系统起振,在实验范围内热声发动机采用不同板叠时的最小起振温度均发生在小充气压力下;在不同的充气压力下,存在最优目数的丝网使得发动机性能最佳。对该台热声发动机而言,最佳的丝网水利半径应为热渗透深度的4.5倍左右。     

Design and construction of a traveling wave thermoacoustic refrigerator

A traveling wave thermoacoustic refrigerator was designed, constructed, and tested. This refrigerator was composed of a linear motor, a branched tube, and a looped tube. A regenerator, composed of many screen meshes, was installed inside the looped tube. When the linear motor delivers an acoustic wave to the branched and looped tubes, heat pumping occurs along the regenerator. In this work, the regenerator radius and position were numerically optimized and the performance of the optimized refrigerator was measured. At the optimized values of regenerator radius and position, the refrigerator achieves a minimum cold temperature of 232 K and about 20% of the Carnot coefficient of performance at 265 K.