Adaptive tuning of an electrodynamically driven thermoacoustic cooler

The commercial development of thermoacoustic coolers has been hampered in part by their low efficiencies compared to vapor compression systems. A key component of electrodynamically driven coolers is the electromechanical transducer, or driver. The driver's electroacoustic transduction efficiency, defined as the ratio of the acoustic power delivered to the working gas by the moving piston and the electrical power supplied, must be maintained near its maximum value if a high overall system efficiency is to be achieved. Modeling and experiments have shown that the electroacoustic efficiency peaks sharply near the resonance frequency of the electro-mechano-acoustic system. The optimal operating frequency changes as the loading condition changes, and as the properties of the working gas vary. The driver efficiency may thus drop significantly during continuous operation at a fixed frequency. In this study, an on-line driver efficiency measurement scheme was implemented. It was found that the frequency for maximum electroacoustic efficiency does not precisely match any particular resonance frequency, and that the efficiency at resonance can be significantly lower than the highest achievable efficiency. Therefore, a direct efficiency measurement scheme was implemented and validated using a functional thermoacoustic cooler. An adaptive frequency-tuning scheme was then implemented. Experiments were performed to investigate the effectiveness of the control scheme to maintain the maximum achievable driver efficiency for varying operating conditions.     

扬声器驱动热声制冷机设计研究

热声制冷机主要由声驱动器、共振管、板叠、高低温端换热器、声容等部件组成。它具有运动部件少、运行可靠、振动小和寿命长等优点 ,在军事、航天、微电子、低温物理等领域有着十分诱人的应用前景。文中运用经典线性热声理论对热声制冷机各个组成部件设计进行了介绍 ,在此基础上设计了一台扬声器驱动热声制冷机 ,并用专业计算软件进行了数值模拟验证 ,为热声制冷机进一步优化与发展提供了一个解决方案。     

Compact standing wave thermoacoustic generator for power conversion applications

In the past decade, a variety of thermoacoustic engines (TAEs) were devised to convert thermal energy to acoustic power. In this paper, we optimized the design of a standing wave thermoacoustic generator that can provide high intensity acoustic pressure and convert it into electrical power output using a low cost alternator. Three prototypes of standing wave thermoacoustic generator (TAG) were designed to optimize the overall efficiency. The first prototype of standing wave TAG could produce an acoustic pressure of 0.9 kPa (153 dB) with an input thermal power of 210 W. Further, the maximum heat to electrical conversion efficiency was 0.045% with an input thermal power of 250 W. However, the performance of this system was not fully optimized. The performance of TAE depends upon various parameters including stack position, stack length and resonator length. Hence, a new second prototype of tunable TAG was developed to tune these critical parameters in order to improve the overall efficiency. A compact third prototype of TAG was successfully built with optimized parameters and has been tested. In the improved design, high intensity acoustic pressure of 2.9 kPa (163.5 dB) was observed for the same 210 W input thermal power. The maximum heat to electrical energy conversion efficiency was 0.084% with an input of 250 W which is 87% higher as compared to the first prototype. The major reason for the lower conversion efficiency is due to the low efficiency of the alternator. In future, high efficiency alternator designs can be employed along with careful impedance matching to obtain higher conversion efficiencies. The results described in this paper demonstrate the potential of developing compact portable acoustic power and electricity generators for decentralized power applications.     

Thermoelectric power generation in a thermoacoustic refrigerator

Commercially available solid-state thermoelectric devices may be used for their electrical power generation capabilities when coupled to a thermoacoustic refrigerator or heat pump. General performance characteristics as well as bulk thermal conductivity for a selection of thermoelectric elements was first found by using a two-plate apparatus to maintain a constant temperature difference across the element. Further studies of an element’s performance when placed in series with the thermoacoustic refrigerator’s heat exchangers will be presented. Design considerations for using thermoelectric elements in a no-moving parts electrical power generation scheme will be discussed.     

热声斯特林发电系统的实验研究

本文介绍了一台由热声斯特林发动机及其驱动的直线发电机组成的热声斯特林发电系统原理样机.一方面为满足发电机与发动机间体积流率和相位的匹配要求,另一方面为了能在直线发电机活塞处获得较好的压力波与体积流率间相位关系、提高直线发电机的电功输出能力,装置保留了发动机原有的锥形谐振管.初步实验以氦气为工质,在2.5MPa平均压力、64Hz工作频率下,获得了97W的电功.本文还分析了该热声发电系统的效率,得出直线发电机声电转换效率超过了0.8.然而由于谐振管耗散了大量的声功,目前整机的热电转换效率还较低.     

Synthetical optimization of the structure dimension for the thermoacoustic regenerator

The quantitative investigation of parameters in the renegerator is essential for the optimization of thermoacoustic devices, while the majority of the previous research only considered parameters of the working field, working gas and the hydraulic radius. Based on the linear thermoacoustic theory, this paper extracts a normalized parameter for low-amplitude conditions, which is called the regenerator operation factor. By extracting the regenerator operation factor and relative hydraulic radius, the influence of frequency on the efficiency can be controlled and offset. It can be found that thermoacoustic devices with different frequencies can perform the same efficiency by adjusting the radius in proportion to the axial length. Finally, this paper synthetically optimizes the dimension of the thermoacoustic regenerator by taking the regenerator operation factor, relative hydraulic radius and acoustic field parameter as variables. Conclusions in this paper are of great significance for explaining the best working conditions of engines and directing the miniaturization and optimal design of thermoacoustic devices.     

Investigation of azimuthal staging concepts in annular gas turbines

In this work, the influence of azimuthal staging concepts on the thermoacoustic behavior of annular combustion chambers is assessed theoretically and numerically. Staging is a well-known and effective method to abate thermoacoustic pulsations in combustion chambers. However, in the case of, for example, fuel staging the associated inhomogeneity of equivalence ratio may result in increased levels of NO_x emissions. In order to minimize this unwanted effect a staging concept is required in which the transfer functions of the burners are changed while affecting the equivalence ratio as little as possible. In order to achieve this goal, a theoretical framework for predicting the influence of staging concepts on pulsations has been developed. Both linear and nonlinear analytical approaches are presented and it is shown that the dynamics of azimuthal modes can be described by coupled Van der Pol oscillators. A criterion based on the thermoacoustic coupling strength and on the asymmetry degree provides the modal behavior in the annular combustor, i.e. standing or traveling waves. The model predictions have been verified by numerical simulations of a heavy-duty gas turbine using an in-house thermoacoustic network-modeling tool. The interaction between the heat release of the flame and the acoustic field was modeled using measured transfer functions and source terms. These numerical simulations confirmed the original theoretical considerations.     

Influence of stack geometry and resonator length on the performance of thermoacoustic engine

Thermoacoustic engines convert heat energy into high amplitude sound waves, which is used to drive thermoacoustic refrigerator or pulse tube cryocoolers by replacing the mechanical pistons such as compressors. The increasing interest in thermoacoustic technology is of its potentiality of no exotic materials, low cost and high reliability compared to vapor compression refrigeration systems. The experimental setup has been built based on the linear thermoacoustic model and some simple design parameters. The engines produce acoustic energy at the temperature difference of 325–450 K imposed along the stack of the system. This work illustrates the influence of stack parameters such as plate thickness (PT) and plate spacing (PS) with resonator length on the performance of thermoacoustic engine, which are measured in terms of onset temperature difference, resonance frequency and pressure amplitude using air as a working fluid. The results obtained from the experiments are in good agreement with the theoretical results from DeltaEc.     

基于DeltaE的驻波型热声发动机设计方法

在消化和吸收目前普遍接受的热声系统计算软件DeltaE(Design Environment for Low Amplitude Thermoacoustic Engines)的基础上,发展了驻波型热声发动机的结构设计方法;提出引入双参数评估法对发动机性能进行评估,以获取较高的热声转化效率。     

Use of electrodynamic drivers in thermoacoustic refrigerators

Some issues involved in matching electrodynamic drivers to thermoacoustic refrigerators are examined using an equivalent circuit model. Conclusions are that the driver should be chosen to have a large product (Bl)2/(ReRm); the suspension stiffness should be chosen to make the combined impedance of the mechanical and acoustical parts of the system entirely real at the operating frequency; and the piston area should be selected to maximize electroacoustic efficiency, or other desired parameter, by matching the acoustic load to the optimum mechanical load for the particular driver. Alternately, if the piston area is fixed, the operating frequency can be adjusted to make this same match.     

二维谐振腔中声驻波的格子气仿真

引入带温度的9_Bit正方形格子气自动机模型,利用Monte Carlo方法模拟声源并激发平面波,在二维谐振腔中建立了声驻波场。该模型能够动态地反映声波的建立和演化过程,为进一步模拟分析热声效应等物理过程提供了可视化的有力工具     

A method of characterising performance of audio loudspeakers for linear alternator applications in low-cost thermoacoustic electricity generators

This paper investigates the feasibility of using commercially available loudspeakers as low-cost linear alternators for thermoacoustic applications, to convert acoustic power to electricity. The design of a purpose built experimental apparatus, in which a high intensity acoustic wave is induced by using a high power woofer, is described. The rig is used to excite loudspeakers (referred here as “alternators”) under test, while a pair of microphones and a laser displacement sensor are used to enable acoustic power measurements. The paper presents a case study in which characteristics of acoustic-to-electric energy conversion of a candidate loudspeaker (alternator) – selected from the viewpoint of general performance, as well as parameters such as: high force factor, low electrical resistance and low mechanical loss – are measured. The measurements of acoustic power absorbed by the alternator and the electric power extracted from it by the load resistor, which allow estimating acoustic-to-electric efficiencies, are presented. The alternator has been tested at different operating frequencies, cone displacements and load resistance values. The measurement results are discussed and compared in detail with the calculations based on the linear acoustics model.     

弱非线性热声理论模型的建立及简化

本文指出非零的周期平均热声效应在本质上是非线性的,应保留到二阶精度。在小振幅条件下,利用摄动方法,以无限大平板流道为例,建立了二阶精度的弱非线性热声理论模型,并在不同条件下对模型做了进一步简化。这一理论为理解热声系统的工作机制以及设计优化热声系统提供了强有力的理论工具。     

80 K温区的小型行波低温热声制冷机

热声制冷机由于结构简单、无运动部件,具有广阔的应用前景。本文在已有的小型行波热声发动机的基础上,开展了热声制冷的工作。利用线性热声理论对制冷机进行数值模拟,并对制冷机的各热声元件优化。优化后,系统整体装配横向尺寸仅0.5 m,在充气压力3 MPa,发动机输入功率384 W的条件下,达到了80 K的无负载温度。由于本制冷机由行波型热声发动机驱动,并且是通过线性热声理论优化,因此称之为小型行波低温热声制冷机。     

大功率RF 离子源阻抗匹配电路的设计与实现

在大功率RF 离子源中,激励器的作用是产生等离子体,阻抗匹配电路是激励器吸收RF 功率的关键。将激励器等效阻抗视为一个电阻和一个电感的串联,采用了一种由一个RF 变压器并和两个可调电容组成的阻抗匹配电路,给出了视RF 变压器为理想变压器时阻抗匹配电路的分析模型,推导了阻抗匹配时两个可调电容的电气参数。当激励器实验装置使用一个石英玻璃激励器时,搭建了一个阻抗匹配电路,成功地将RF 功率耦合进激励器并产生了等离子体。     

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For a RF ion source test facility, the driver is used to produce plasma, and the matching unit is the key for the driver to absorb RF power. The driver’s impedance could be equivalent to a resistance and an inductance in series. A matching unit which consisted of an RF transformer and two variable capacitances was chosen, it was analyzed by an ideal transformer model and its electrical parameters were derived under the condition of matching. When a quartz glass driver was mounted onto the test facility at HUST, a matching unit was implemented, it transmitted RF power into the driver and then plasma was successfully produced.     

基于传递矩阵法的热声发动机频率设计计算

介绍利用传递矩阵法计算热声发动机的工作频率的思路和方法。理论分析表明该方法是切实可行的 ,和其他方法得到了相互的印证。利用该方法的计算结果和已发表的试验结果进行了对比分析。研究结果表明 ,相比已有的计算方法而言 ,传递矩阵法更加简单直观 ,方便于采用计算机编程计算复杂结构的热声系统的工作频率。     

高效室温行波热声制冷的研究 第一部分 方案及热力分析

文中对简化的热声网络法、T型传输线网络法以及传输矩阵法进行了比较讨论。采用简化的热声网络法对几种可能的制冷流程进行了分析 ,并确定出了可能高效工作的制冷方案。最后基于传输矩阵法对一种行波热声制冷机进行了分析和热力设计 ,得到该制冷机较优的结构参数。当制冷温度为 - 2 3℃时 ,其 COP可达 2 .86 ,相对卡诺热效率为 5 7%。     

Validation of estimated isotropic viscoelastic material properties and vibration response prediction

This paper is concerned with finite element (FE) prediction of forced vibrations using a linear viscoelastic constitutive vibration damping modelling technique. A combined numerical and experimental investigation was performed on two bonded aluminium-PMMA (polymethyl methacrylate) plates with different geometry. Three-dimensional FE models were established using experimentally estimated PMMA material properties (elastic and damping) from previously published procedures. The viscoelastic material damping parameters are here validated from the perspective of accurate estimation of constitutive material properties. Vibration responses were predicted from the FE models and measured on the two composite plate structures at a large number of points. Comparisons between the numerical FE simulations and corresponding measured responses show that the estimated material damping properties used as input to the computations are very accurate and may be treated as independent of the geometry and boundary conditions of the plate structures, i.e., as constitutive damping parameters.     

1kW碟式太阳能行波热声发电系统

碟式太阳能行波热声发电是近年来兴起的新型热发电技术,具有可靠性好、潜在效率高、分布灵活等优点。本文介绍了正在研制的一套1 kW碟式太阳能行波热声发电系统。该系统利用碟式集热器收集太阳辐射热量,通过高温热管将热量传输到发动机热端,再采用行波热声发电机进行热-电转换。初步调试采用高频加热模拟太阳能,以3.5 MPa氦气为工质、加热温度为751℃和798℃时分别实现了116 W和255 W的电功输出。实验验证了系统的可行性。目前系统的安装调试仍在进行中,相关的实验结果将在后续的文章中进行报道。