Evanescent modes and anomalous streaming in a thermoacoustic device

An oil-heated thermoacoustic refrigerator was constructed in order to investigate the use of waste-heat sources to operate a refrigerator. Fluid flows within the resonator in the vicinity of the stack/heat exchanger assemblies were measured through optical means. During the course of the experiment, anomalous centerline steady flows were observed at magnitudes of up to three times the acoustic amplitudes within the resonator of the thermoacoustic device. An evanescent component of the acoustic field was also measured at the same location. An order of magnitude calculation indicates that the body force induced by the evanescent mode is of sufficient magnitude and structure to be the source of the streaming.     

Acoustic streaming in closed thermoacoustic devices.

A derivation of acoustic streaming in a steady-state thermoacoustic device is presented in the case of zero second-order time-averaged mass flux across the resonator section (nonlooped device). This yields analytical expressions for the time-independent second-order velocity, pressure gradient, and time-averaged mass flux in a fluid supporting a temperature gradient and confined between widely to closely separated solid boundaries, both in the parallel plate and in the cylindrical tube geometries (two-dimensional problem). From this, streaming can be evaluated in a thermoacoustic stack, regenerator, pulse tube, main resonator of a thermoacoustic device, or in any closed tube that supports a mean temperature gradient, providing only that the acoustic pressure, the longitudinal derivative of the pressure, and the mean temperature variation are known.     

The optimal stack spacing for thermoacoustic refrigeration

The characteristic pore dimension in the stack is an important parameter in the design of thermoacoustic refrigerators. A quantitative experimental investigation into the effect of the pore dimensions on the performance of thermoacoustic devices is reported. Parallel-plate stacks with a plate spacing varying between 0.15 and 0.7 mm are manufactured and measured. The performance measurements show that a plate spacing in the stack of 0.25 mm (2.5 deltak) is optimum for the cooling power. A spacing of 0.4 mm (4 deltak) leads to the lowest temperature. The optimum spacing for the performance is about 0.3 mm (3 deltak). It is concluded that a plate spacing in the stack of about three times the penetration depth should be optimal (3 deltak) for thermoacoustic refrigeration.     

电动声源热声致冷机声学和计算实例

我们将各种热声致机简化为一包括声学终端在内的声管道系统，并通过实例讨论了致冷机的声学特性，该管道系统与一般声管道不同：１．在热声堆中热波和粘滞波不可不计。２．在热声堆与声管连接时，必需考虑合成波的体积流；而热声堆内只需考虑传播波的体积流。本文对此提出了阻抗连接条件的修正。实例使用电动扬声器为声源，给出了热声行波和驻波致冷的声学计算方法以及它们的声学特性，所用扬声器的标称伏安为１００ＶＡ，可为热声致     

An aeroacoustically driven thermoacoustic heat pump

The mean flow of gas in a pipe past a cavity can excite the resonant acoustic modes of the cavity--much like blowing across the top of a bottle. The periodic shedding of vortices from the leading edge of the mouth of the cavity feeds energy into the acoustic modes which, in turn, affect the shedding of the next vortex. This so-called aeroacoustic whistle can excite very high amplitude acoustic standing waves within a cavity defined by coaxial side branches closed at their ends. The amplitude of these standing waves can easily be 20% of the ambient pressure at optimal gas flow rates and ambient pressures within the main pipe. A standing wave thermoacoustic heat pump is a device which utilizes the in-phase pressure and displacement oscillations to pump heat across a porous medium thereby establishing, or maintaining, a temperature gradient. Experimental results of a combined system of aeroacoustic sound source and a simple thermoacoustic stack will be presented.     

Optimal acoustic fields in compact thermoacoustic refrigerators

Thermoacoustic refrigerators have been developed during the last 15 years, employing quasi-standing resonant acoustic waves inside fluid-filled cavities to transfer heat along a stack region. Because higher efficiency can be reached when a significant travelling wave component exists in the resonator, specific resonant thermoacoustic devices have been designed allowing to adjust more or less the ratio of travelling and standing wave components. However, the acoustic pressure field and the particle velocity field do not appear to be the optimal ones, for the thermal quantities of interest. Thus, it is the aim of the paper to present a new kind of thermoacoustic standing wave-like device which allows to control easily and independently the pressure field and the particle velocity field, after investigating the optimal acoustic field, in the stack region, for the main parameters of interest, i.e. the temperature gradient, the thermoacoustic heat flow and the coefficient of performance.     

热声发动机的格子气模拟

在传统的9-bit格子气模型中引入了温度区的概念,并应用于热声发动机的模拟研究.利用改进后的格子气模型,模拟了热声谐振管中的自激振荡和二维温度场的分布及温度随时间的演化过程,同时,对热声板叠的长度和在声场中的位置对声幅的影响也进行了数值研究,所得结果对板叠的优化设计是有价值的.通过对模拟和实验结果的比较,验证了热声机格子气模型的有效性,表明格子气方法适用于热声模拟.     

Studies on Performance of Thermoacoustic Prime Mover

Thermoacoustic engines are the devices that convert thermal energy into acoustic energy without moving parts. The main objective of this study is to analyze the performance of a thermoacoustic prime mover measured in terms of onset temperature difference, frequency, and pressure amplitude by varying resonator, stack length, and plate thickness. From the experiments, it is observed that onset temperature difference and pressure amplitude increases with increase in resonator and stack length with minimum plate thickness, whereas the frequency increases with decrease in resonator and stack length with higher plate thickness. The experimental results are compared with simulated results via Design Environment for Low Amplitude Thermoacoustic Energy Conversion software (Los Alamos National Laboratory, Los Alamos, New Mexico, USA).     

回热器的热声直流模型及其效应研究

本文由基本的守恒方程出发,获得了能描述回热器存在声直流时的非线性动力学模型及由此而导致的非线性时均热力学效应。根据所得到的模型,考察了声直流对临界声功产生率温度梯度、回热器温度分布以及脉冲管制冷性能的影响。     

Piezo-driven thermoacoustic refrigerators with dynamic magnifiers

Thermoacoustic refrigeration is an emerging cooling technology which does not rely for in its operation on the use of any moving parts or harmful refrigerants. This technology uses acoustic waves to pump heat across a temperature gradient. The temperature gradient forms across the ends of a porous body, called the stack, enclosed in a resonator. The vast majority of thermoacoustic refrigerators to date have used electromagnetic loudspeakers to generate the acoustic input. In this paper, the design, construction, operation, and modeling of a piezo-driven thermoacoustic refrigerator are detailed. The performance of the refrigerator is significantly enhanced by coupling the acoustic driver with an elastic structure, referred to as a dynamic magnifier. Proper selection of the magnifier parameters can increase the magnitude of the pressure oscillations across the stack, and consequently the temperature difference. The magnified refrigerator demonstrates the effectiveness of piezoelectric actuation in moving 0.3 W of heat across a 10 °C temperature difference with an input power of 7 W. All the theoretical predictions are validated against data from experimental prototypes. The developed theoretical and experimental tools can serve as invaluable means for the design and testing of piezo-driven thermoacoustic refrigerator configurations.     

Thermoacoustics with idealized heat exchangers and no stack

A model is developed for thermoacoustic devices that have neither stack nor regenerator. These "no-stack" devices have heat exchangers placed close together in an acoustic standing wave of sufficient amplitude to allow individual parcels of gas to enter both exchangers. The assumption of perfect heat transfer in the exchangers facilitates the construction of a simple model similar to the "moving parcel picture" that is used as a first approach to stack-based engines and refrigerators. The model no-stack cycle is shown to have potentially greater inviscid efficiency than a comparable stack model. However, losses from flow through the heat exchangers and on the walls of the enclosure are greater than those in a stack-based device due to the increased acoustic pressure amplitude. Estimates of these losses in refrigerators are used to compare the possible efficiencies of real refrigerators made with or without a stack. The model predicts that no-stack refrigerators can exceed stack-based refrigerators in efficiency, but only for particular enclosure geometries.     

Condensation in a steady-flow thermoacoustic refrigerator

Condensation may occur in an open-flow thermoacoustic cooler with stack temperatures below the saturation temperature of the flowing gas. In the experimental device described here the flowing gas, which is also the acoustic medium, is humid air, so the device acts as a flow-through dehumidifier. The humid air stream flows through an acoustic resonator. Sound energy generated by electrodynamic drivers produces a high-amplitude standing wave inside of the resonator, which causes cooling on a thermoacoustic stack. Condensation of water occurs as the humid air passes through the stack and is cooled below its dew point, with the condensate appearing on the walls of the stack. The dry, cool air passes out of the resonator, while the condensate is wicked away from the end of the stack. Thermoacoustic heat pumping is strongly affected by the form of the condensate inside of the stack, whether condensed mostly on the stack plates, or largely in the form of droplets in the gas stream. Two simple models of the effect of the condensate are matched to a measured stack temperature profile; the results suggest that the thermoacoustic effect of droplets inside the stack is small.     

渐变截面热声波导管内的声流解析模型

建立了二维渐变截面热声波导管内的声流模型,分别考虑了封闭直管和环形回路两种不同结构,获得了更为普适的解析结果。封闭直管结构的声流结果可应用于任意宽度的波导管,环路结构的结果考虑了渐变截面管段宽度远大于热、黏穿透深度的情形。研究结果表明,渐变截面热声波导管内的声流主要受声场结构、截面变化及轴向时均温度分布的影响,在其它参量不变时声流量值及分布随波导管特征尺度的不同而变化。该解析模型可应用于热声及其它物理背景下的声流分析。      

多段式回热器的斯特林热声发动机CFD仿真

热声回热器的温度梯度使得轴向气体存在不同粘性,因此单一水力直径的丝网会引起回热器的直流损失。为了减小该损失,文中提出了多段式回热器结构,建立了适用于大振幅下斯特林热声发动机的三维CFD计算模型。计算结果验证了多段式回热器能有效提高热声发动机的整机性能。该研究有助于增加对回热器的进一步认识和理解。     

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.     

Saturation of thermoacoustic mixture separation

The theory for thermoacoustic mixture separation is extended to include the effects of a nonzero concentration gradient. New data are presented, which are in excellent agreement with this theory. The maximum concentration gradient which may be achieved in a binary mixture of gases through this separation process is intrinsically limited by the fractional pressure amplitude, by the tidal displacement, and by the size of the thermal diffusion ratio. Ordinary diffusion further detracts from the attainable final concentration gradient and can become the dominant remixing process as the cross section of the duct is increased. Rayleigh streaming also works against thermoacoustic separation, and an estimate of the molar flux from streaming is given.     

Nonperiodicity of the flow within the gap of a thermoacoustic couple at high amplitudes

The flow inside a thermoacoustic couple is investigated experimentally using particle image velocimetry. Measurements show the oscillation of the shear layers flowing out of a single stack, thus forming an asymmetric vortex street at high driving amplitudes. Development of vortices is also observed within the gap of a thermoacoustic couple. It causes the flow not to repeat from one acoustic period to another. The nonperiodicity of the flow will lead to unsteady heat transfer between the stack and heat exchangers and to the oscillation of the cooling load.     

An open-air infrasonic thermoacoustic engine

A low-frequency open-air thermoacoustic engine in a Helmholtz resonator has been constructed. Tests indicate that the system resonates in the Helmholtz mode for modest thermoacoustic stack temperature differences using stacks of varying type and pore size located within the neck of the Helmholtz resonator. The maximum acoustic pressure radiated from the open end of the resonator corresponds to 81 dB-SPL ref 20 μPa at a stack temperature difference of 185 K and an input electric power of 276 W. The system is well characterized by a numerical model of a representative stack.     

热声致冷效应演示实验

基于热声致冷原理,利用自制的热声堆,采用常见的材料和仪器,设计了扬声器驱动热声致冷实验演示装置.以空气作工质,在无冷却措施的情况下,系统运行200 s后,实现了13℃的降温及25℃的温跨.     

Investigation on influencing factors of acoustic streaming in thermoacoustic waveguides with slowly varying cross-section

The influencing factors of acoustic streaming in thermoacoustic waveguides with slowly varying cross-section are analyzed based on theoretical analysis and numerical simulation. The distribution curves of acoustic streaming velocity in waveguides with different characteristic scales are presented in several specific cases.The results show that appropriate forms of varying cross-section can strengthen or weaken acoustic streaming for specific acoustic fields and the thermophysical parameters have no effect on this part.In addition,the influence of time-average temperature distribution on acoustic streaming is substantial in tubes with a width of the order of the thermal penetration depth.Without time-average temperature distribution,the effect of heat conduction on acoustic streaming is great in tubes whose width is an order of about 10 to 20 times the viscous penetration depth.