Numerical simulation of the onset characteristics in a standing wave thermoacoustic engine based on thermodynamic analysis

In this paper, a simplified physical model of standing wave thermoacoustic engines (SWTE) is developed based on thermodynamic analysis. Transient pressure drop and heat transfer data are first calculated based on linear thermoacoustic theory. The effects of stack spacing, charge pressure, and resonator length on onset temperature were investigated and compared with experimental results. The calculations agree well with the experimental results, which validates the model for calculating the onset conditions.     

Performance of solar powered thermoacoustic engine at different tilted angles

In this study, the thermodynamic performance of a thermoacoustic engine charged with different working fluids were examined at different tilted angles ranging from ?90° to 90° with 45° angular interval. The results suggest that the influence of the tilted angle on the onset temperature of the engine depends on the viscidity of the working gas. The lower the viscidity is, the more obvious the influence is. The difference between the maximum and the minimum onset temperature of the engine charged with nitrogen could be as high as 52 °C, but the difference for system charged with helium is only about 1.5 °C. The tilted angle has little or no effect on the pressure oscillation amplitude, pressure ratio, resonance frequency and the relation of the temperature versus heat power. They are mainly affected by the properties of the working gas. Furthermore, the interactions of the oscillatory motion and the natural convection of the working gas within the thermoacoustic core were also examined. The properties are of importance for the thermoacoustic engine driven by two-axis solar collector, for the tilted angle of the engine varies with the sun position.     

Experimental Investigation of Self—sustained Oscillation in a Traveling Wave Thermoacoustic System

A traveling wave thermoacoustic engine consisting of a loop tube with a resonator has been tested. The onset characteristic together with the transition of oscillation mode from traveling wave to standing wave and the periodic shifting between modes in this system are investigated experimentally. The process of self-sustained thermoacoustic oscillation in this heat engine is described and analyzed through phase space distribution reconstructed from the time series of acoustic signal.     

Performance of an air-cooled looped thermoacoustic engine capable of recovering low-grade thermal energy

An air-cooled looped thermoacoustic engine is designed and constructed, where an air-cooled cold heat exchanger (consisting of copper heat transfer block, aluminum flange, and aluminum fin plate) is adopted to extract heat and the resonant tube is spiraled and shaped to fit to the available space. Experiments have been conducted to observe how onset temperature difference and resonant frequency are affected by mean pressure, working fluid, and diameter of compliance tube. Besides, the influences of temperature difference, mean pressure, working fluid and diameter of compliance tube on pressure amplitude, output acoustic power, and thermal efficiency of the system have been investigated. The air-cooled looped thermoacoustic engine can start to oscillate at a lowest temperature difference of 46°C, with the working fluid of carbon dioxide at 2.34 MPa. A highest output acoustic power obtained is 6.65 W at a temperature difference of 199°C, with the working gas of helium at 2.58 MPa, and the thermal efficiency is 2.21%. This work verifies the feasibility of utilizing low-grade thermal energy to drive an air-cooled looped thermoacoustic engine and extends its application in the water deficient areas.     

非对称双级环路行波热声热机的实验研究*

对于双级环路行波热声热机,两个热声核的相对位置直接影响到其起振温度,而热声热机的起振温差决定了其可利用的热源品位。基于线性热声理论分析,通过改变两个热声核的相对位置,研究了两个热声核的相对位置改变对其起振温差、压力振幅和压比等的影响。结果表明,双级环路行波热声热机的起振温度随着两个热声核从中心对称位置逐步靠近时先下降再上升,当两个热声核之间的谐振管长度比例为1:3.5时,系统获得最小的起振温差为59.6℃（工质为N₂,充气压力为2.5 MPa）。在相同温差下,该系统在谐振管长度比例为1:3.5的位置相较于其他位置具有较大的压力振幅和压比。     

以氦气为工质的行波热声发动机研究

随着对热声热机研究的深入，特别是行波热声发动机概念的提出，热声发动机效率得到了质的提高。为了实现热声发动机与制冷机的良好匹配，以氦气为工质时热声发动机需具有较低的起振温度、较大的压力波强度、较好的单频率特性。本文对自行研制的新型热声发动机进行了深入研究，以氦气为工质，在充气压力为2．0MPa时获得了1．19的压比，系统频率稳定在约73Hz，为利用新型热声发动机驱动脉管制冷机或其它热声制冷机创造了有利条件。此外，该热声发动机起振温度较低，初步具备了利用工业废热等低品位能源驱动的条件。     

Novel Helmholtz resonator used to focus acoustic energy of thermoacoustic engine

A thermoacoustic engine (TE) converts thermal energy into acoustic power without any mechanical moving parts. It shows several advantages over traditional engines, such as simple configuration, stable operation, and environment-friendly working gas. In order to further improve the performance of a thermoacoustically driven system, methods are needed to focus the acoustic energy of a TE to its load. By theoretical analysis based on linear thermoacoustics, a novel Helmholtz resonator is proposed to increase the transmission ability of a TE, which makes full use of the interaction between inertance and compliance effects. With this configuration, the output pressure amplitude of a TE is amplified and the maximal pressure amplitude can occur at the end of the Helmholtz resonator tube with a length much shorter than 1/4 wavelength. Furthermore, the Helmholtz resonator has shown remarkably increased volume flow rates at both ends. In experiments, a Helmholtz resonator amplifies the pressure ratio from 1.22 to 1.49 and produces pressure amplitude of 0.44 MPa with nitrogen of 2.2 MPa as working gas. Relatively good agreements are obtained between computational and experimental results. This research is instructive for comprehensively understanding the transmission characteristics of acoustic components.     

Numerical oscillatory on the simulation of thermoacoustic waves

The SIMPLE algorithm for compressible flows is introduced to predict the thermoacoustic wave in a one‐dimensional closed region. The thermoacoustic waves are generated by an impulsive rise of the temperature on the left wall, while the other walls are kept at the initial temperature. Four different schemes are employed to deal with the convection‐diffusion terms, i.e., CD, FUD, QUICK, and MUSCL. The calculations and analysis show that numerical oscillation occurs under all four schemes, affected by the intensity of the waves, type of schemes, and other factors. The results are beneficial for the further investigation of the thermoacoustic waves and high efficiency scheme development. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(5): 265– 275, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20162     

Optimum packing factor of the stack in a standing‐wave thermoacoustic prime mover

A bench consisting of a pulse tube refrigerator driven by a standing‐wave thermoacoustic prime mover has been set up to study the relationship among stack, regenerator and working fluids. The stack of the thermoacoustic prime mover is packed with dense‐mesh wire screens because of their low cost and easy manufacture. The effect of the packing factor in the stack on onset temperature, refrigeration temperature and input power is explored. The optimum packing factor of 1.15 pieces per millimeter has been found experimentally, which supplies an empirical value to satisfy a compromise for enhancing thermoacoustic effect, decreasing heat conduction and fluid‐friction losses along the stack. The pulse tube cooler driven by the thermoacoustic prime mover is able to obtain refrigeration temperatures as low as 138 and 196K with helium and nitrogen, respectively. Copyright © 2002 John Wiley & Sons, Ltd.     

Temperature separation produced by a Hartmann-Sprenger tube coupling a secondary resonator

Temperature separation of a gas is produced by the Hartmann-Sprenger (HS) tube worked by jet now issuing from a convergent nozzle. The effect of temperature separation depends on thermal effects of the HS tube.

The coupling of a secondary resonator to the HS tube or the use of a convergent nozzle with thin rod laid along its axis changes operation mode of the HS tube and these can improve strength of oscillation in the HS tube and the thermal effect of it.

In the present paper, effects of the secondary resonator and of the use of the nozzle with rod on the thermal effects are observed. Considering these effects, an apparatus of temperature separation is composed and its separation characteristics are investigated for various operation modes of the HS tube and for various sizes of hot-gas outlet.

From experimental results, optimum operation mode and optimum size of hot-gas outlet for the efficiency of temperature separation are found.     

丝网热声板叠的最佳填充率

自行研制了热声驱动脉管制冷机实验台,着重研究了热声机械中热声转换的关键部件丝网板叠的填充率对热声驱动脉管制冷机起振温度,制冷温度和加热功率等的影响,并通过实验发现了丝网板叠的最佳填充率,以氮和氮作工质,分别获得了196K和138K的无负荷制冷制度,达到国际先进水平,为热声机械的实用化奠定了基础。     

Oscillatory Flow in Thermoacoustic Sound Wave Generator

Oscillatory flow in a thermoacoustic sound wave generator is described. The thermoacoustic sound wave generator plays an important role in thermoacoustic equipment. The heat exchange between the working fluid and the stack, the acceleration and deceleration of the working fluid and viscous friction loss both in the stack and in the resonance tube influence the performance of the thermoacoustic sound wave generator. Particularly, oscillatory flow significantly influences the heat exchange mechanism between the working fluid and the stack. Temporal changes in pressure and velocity are sinusoidal inside the resonance tube. Flow forms an oscillatory jet just behind the tube outlet, and becomes intermittent far downstream outside the resonance tube. The open-end corrections of 0.63R, that is, the region where oscillatory flow characteristics are maintained downstream in spite of being outside the tube outlet, are confirmed by velocity measurements and flow visualization. Also, they are almost equal to acoustical theoretical results.     

Visualization investigation of the flow and heat transfer in thermoacoustic engine driven by loudspeaker

Heat transfer process in thermoacoustic engine is affected by acoustic oscillation which makes it different from the heat transfer in steady flow. This study pays attention to the flow and heat transfer characteristics of thermoacoustic engine driven by loudspeaker. Thermal infrared imager and particle image velocimetry (PIV) were used to investigate the temperature and flow fields under two heat levels (150 °C and 200 °C). The radial and axial temperature distribution was analyzed through dimensionless temperature. To explore the appropriate working frequency, resonance characteristic was discussed. The experimental results illustrated that the first resonance frequency is the most effective driving frequency where thermoacoustic system shows the best performance. Heat transfer mode changed from natural convection to forced convection with the addition of acoustic oscillation. Original temperature field induced by heat convection was destroyed and temperature gradient redistributed as parabolic after sound addition.     

热声热机的热力学分析

回顾了热声现象的研究历史,介绍了热声热机的结构和分类,基于气体微团的热力学循环和能量转换,分析了热与声功的转换原理、转换条件以及热声热机的工作机理,并指出了该研究领域的工业应用前景。     

A miniature thermoacoustic stirling engine

A miniature thermoacoustic stirling engine was simulated and designed, having overall size of length 0.65 m and height of 0.22 m. The acoustic field generated in this miniature system has been described and analyzed. Some efforts had been paid to coupling and matching, and a miniature thermoacoustic engine and some extra experimental components have been constructed. Analysis and experimental results showed that to obtain better performance of the engine, the diameter of the resonance tube must be chosen appropriately according to the looped tube dimension and the input heating power. It provided an effective way to miniaturize the thermoacoustic stirling heat engine. The experimental results showed that the engine had low onset temperature and high pressure amplitude and ratio. With the filling helium gas of 2 MPa and heating power of 637 W, the maximal peak to peak pressure amplitude and pressure ratio reached 2.2 bar and 1.116, respectively, which was able to drive a refrigerator, a heat pump or a linear electrical generator. The operating frequency of the engine was steady at 282 Hz.     

CFD simulation on the performance of twin thermoacoustic prime mover for various resonator lengths and operating pressures

The main objective of the present investigation is to analyze the effect of resonator length and different working pressures on the performance of a twin thermoacoustic prime mover, which was measured in terms of frequency and amplitude of the oscillations using the computational fluid dynamics tool FLUENT 6.3. The simulation was carried out for different resonator lengths such as 0.5, 0.6, 0.8, 1.1, and 1.4 m using nitrogen as a working fluid for various operating pressures such as 1, 2, 3, 4, and 5 bar. It was observed from the results that the pressure amplitude increases with an increase in resonator length and that the output frequency decreases with an increase in resonator length for a fixed operating pressure. For a constant resonator length, the increase in operating pressure has little influence on output frequency, and the pressure amplitude also increases. The results obtained were found to be in accordance with experimental works published by Hariharan and colleagues. 1     

Performance analysis of a solar glass tube collector

The thermal performance and thermal processes of a glass tube collector have been analysed in this paper. Its thermal performance can be improved by changing the thermal processes to take advantage of the glass tube's ability to transmit sunlight; that makes it possible for the working fluid to directly absorb part of solar radiation. Its thermal performance is even better in most parts of the working region than that of a steel tube collector, even when the structure, meteorological conditions and thermodynamic properties of the working fluid are exactly the same.An equation of steady-state instantaneous efficiency of a glass tube collector has been derived in the paper. Calculations of various operating conditions have been made with a computer, and the calculated results are quite agreeable with the experimental results. Thus the equation and the calculation method can be used in the design of glass tube collectors and for comparison calculations. The calculations also show some other important features of a glass tube collector.     

124K热声驱动的脉管制冷机

热声压缩机是一种利用热能（如太阳能集热、废热等）进行驱动的新型驱动器。自行研制的驻波型热声压缩机驱动脉管经过改进后，以氦为工质，取得了124.3K的制冷温度。此外，该文还讨论了热声压机系统中的水冷却存在的问题以及它对系统整体性能的影响，并对热声机的应用前景进行了展望。     

Brinkman–Forchheimer modeling for porous media thermoacoustic system

In this paper, analytical studies have been conducted on the flow and thermal fields of unsteady compressible viscous oscillating flow through channels filled with porous media representing stacks in thermoacoustic systems. The flow in the porous material is described by the Brinkman–Forchheimer–extended Darcy model. Analytical expressions for oscillating velocity, temperature, and energy flux density are obtained after linearizing and solving the governing differential equations with long wave, short stack, and small amplitude oscillation approximations. Experimental work is also conducted to verify the temperature difference obtained across the porous stack ends. To produce the experimental results, a thermoacoustic heat pump is designed and constructed where reticulated vitreous carbon (RVC) is used as the stack material. A very good agreement is obtained between the modeling and the experimental results. The expression of temperature difference across the stack ends obtained in the present study is also compared with the existing thermoacoustic literature. The proposed expression surpasses the existing expression available in the literature. The system of equations developed in the present study is a helpful tool for thermal engineers and physicist to design porous stacks for thermoacoustic devices.     

卧式燃油燃气炉炉胆传热计算方法的比较研究

对卧式燃油燃气锅炉或加热炉,炉胆传热计算是热力计算中的重要计算,其主要目的是确定炉胆出口烟温。本文对已有的卧式内燃燃油燃气锅炉炉胆传热计算公式进行了比较,提出了适合加热炉炉胆传热计算的方法和简化算法。