Numerical Simulation of Heat Exchanger＇s Length＇s Effect in a Thermoacoustic Engine

A numerical study of a standing-wave thermoacoustic engine is presented. The aim of this work is to study the effect of increasing the heat exchangers length on the acoustic power. The analysis of the flow and the prediction of the heat transfer are performed by solving the non linear unsteady Navier-Stocks equations using the finite volume method implemented in -ANSYS CFX- CFD code. The results show an increase in the limit cycle acoustic pressure and power as well as the specific work per cycle with the increase of heat exchangers length.

Development and Assessment of Thermoacoustic Generators Operating by Waste Heat from Cooking Stove

This paper presents the development and assessment of two low-cost, travelling wave, thermoacoustic generators operating by waste heat energy from cooking stove. One powered by waste heat from a propane-driven stove, the other powered by waste heat from a wood-burning stove. The propane-driven thermoacoustic generator was successfully demonstrated to produce approximately 15 watts of electricity using a commercial audio loudspeaker. The wood-burning thermoacoustic generator was successfully constructed and tested to generate a maximum of 22.7 watts of electricity under a pressurised condition. The latter has a high potential to be used by over 1.5 billion people in rural communities for applications such as LED lighting, charging mobile phones or charging a 12V battery. A comprehensive power assessment of the propane-driving generator as well as the development and performance assessment of the wood-burning generator are described throughout this article.

Temperature Difference and Stack Plate Spacing Effects on Thermodynamic Performances of Standing-Wave Thermoacoustic Engines Driven by Cryogenic Liquids and Waste Heat

The standing-wave thermoacoustic engines(TAE)are applied in practice to convert thermal power into acoustic one to generate electricity or to drive cooling devices.Although there is a number of existing numerical researches that provides a design tool for predicting standing-wave TAE performances,few existing works that compare TAE driven by cryogenic liquids and waste heat,and optimize its performance by varying the stack plate spacing.This present work is primarily concerned with the numerical investigation of the performance of TAEs driven by cryogenic liquids and waste heat.For this,three-dimensional(3-D)standing-wave TAE models are developed.Mesh-and time-independence studies are conducted first.Model validations are then performed by comparing with the numerical results available in the literature.The validated model is then applied to simulate the standing-wave TAEs driven by the cryogenic liquids and the waste heat,as the temperature gradientΔT is varied.It is found that limit cycle oscillations in both systems are successfully generated and the oscillations amplitude is increased with increasedΔT.Nonlinearity is identified with acoustic streaming and the flow reversal occurring through the stack.Comparison studied are then conducted between the cryogenic liquid-driven TAE and that driven by waste heat in the presence of the same temperature gradientΔT.It is shown that the limit cycle frequency of the cryogenic liquid system is 4.72%smaller and the critical temperatureΔT_(cri)=131 K is lower than that of the waste heat system(ΔT_(cri)=187 K).Furthermore,the acoustic power is increased by 31%and the energy conversion efficiency is found to increase by 0.42%.Finally,optimization studies on the stack plate spacing are conducted in TAE system driven by cryogenic liquids.It is found that the limit cycle oscillation frequency is increased with the decreased ratio between the stack plate spacing and the heat penetration depth.When the ratio is set to between 2 and 3,the overall performance of the cryogenic liquid-driven TAE has been greatly improved.In summary,the present model can be used as a design tool to evaluate standing-wave TAE performances with detailed thermodynamics and acoustics characteristics.The present findings provide useful guidance for the design and optimization of high-efficiency standing-wave TAE for recovering low-temperature fluids or heat sources.

Study on the onset temperature of a standing-wave thermoacoustic engine based on circuit network theory

Onset mechanism is one of the most fundamental issues in thermoacoustic field.However,the onset conditions and the phenomena happening in the onset process have not been well explained theoretically.In this paper,a novel model based on the circuit network analogy is proposed to predict the onset temperature of a standing-wave thermoacoustic engine.The activity and instability criteria are proposed to be the onset criteria in the model.The influences of the porosity of the heat exchanger and the stack,and the length of the resonant tube on the onset temperature are analyzed.The calculated results are in agreement with the experimental results,which indicates that the activity and instability criteria can be used to predict the onset conditions of a thermoacoustic engine.

The Myth of the High-Efficiency External-Combustion Stirling Engine

The reported discrepancy between theory and experiment for external combustion Stirling engines is explained by the addition of thermal resistance of the combustion gasses to the standard Carnot model. In these cases, the Stirling engine ideal efficiency is not as is normally reported equal to the Carnot cycle efficiency but is significantly lower. A new equation for ideal Stirling engine efficiency when the heat is obtained through external combustion without pre-heating the air, is presented and results for various fuels tabulated. The results show that petrol and diesel, internal combustion engines (Otto cycle) have a higher ideal efficiency than the Stirling engine. When comparing thermoacoustic engines heated by wood, efficiency should not be quoted as a percentage of the Carnot efficiency, but against a figure 48% lower than Carnot. The effect is not seen with electrically heated rigs, solar or nuclear fission heated engines.

热声发动机中的交变流动换热器特性研究

为阐明热声发动机交变流动换热器内基本换热与阻力特性,以10 kW直线热管耦合自由活塞斯特林发电机的发动机核心为例,对三维模型进行等效,分别利用Sage一维和CFD二维模型两种方法,对交变流动换热单元在额定工况下的沿程基本状态参数、时均能量的分布特征进行计算并比较。结果表明:Sage一维模型中的换热分布特征及换热总量均与CFD二维模型计算结果相近,即Sage一维模型可以较好地捕捉换热器内的瞬态与时均特征;同时,Sage一维模型对换热器界面的突变截面损失则与CFD二维模型计算差异较大,可利用CFD二维模型的阻力计算结果,提出采用有效压降与有效速度获得突变截面处的局部阻力系数修正,并添加修正因子后代入Sage一维模型,可使Sage一维模型的阻力特性与CFD二维模型的阻力特性基本吻合。所得热声热机中换热器特性规律及分析方法同样可用于指导其他热声热机的相关研究与工程设计,具有一定的普适性。

热声斯特林发动机热动力学特性的CFD研究——第一部分:热声自激振荡演化过程

采用商业计算流体动力学(CFD)软件Fluent6.0对热声斯特林发动机热动力学特性进行了热声自激振荡演化过程的数值模拟研究。主要研究了边界条件、初始条件和数值离散方法对模拟瞬态、可压缩、非线性的热声系统的重要影响,同时给出了两种在回热器内部建立温度梯度的方法,比较了它们对应的不同自激振荡演化过程。模拟结果表明,在高于功产生的临界温度梯度时,波动压力振幅被显著放大。本研究初步验证了该CFD研究的有效性。

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。初步实验以氮气为工质,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.

热声斯特林发动机热动力学特性的CFD研究 第二部分:热声转换特性及热声声流的研究

研究了热声斯特林发动机的声场特性和热力特性,计算了波动压力和体积流率振幅及两者的相位差、截面平均温度以及声功流在系统内的沿程分布,与实验及预期的趋势相符合。对热声斯特林发动机内部的Gedeon声直流进行了研究。通过监测系统内截面的质量流率周期分布特性观测到Gedeon直流的存在,计算了直流量的大小及在系统内的分布特性,并采用风扇模型抑制了直流,给出了直流抑制过程中的系统性能变化。进一步验证了CFD研究的有效性。

高频热声斯特林发动机的实验研究

高频热声斯特林发动机具有体积小,热效率高和无机械运动部件等特点,是驱动脉冲管制冷机等回热式制冷机的理想的压力波发生器。本文设计并研制了一高频热声斯特林发动机系统,实验研究了系统布置方位和工作介质对发动机性能的影响。实验结果发现布置方位对起振温度的影响较大,对稳定振荡的波动压力振幅和加热温度影响较小。采用氦气、氮气、氩气和二氧化碳作为工质,工作频率分别是314 Hz、108 Hz、98 Hz和76 Hz时,其压比分别达到了1.17、1.23、1.22和1.24。

热声热机中回热器的数值计算与分析

针对热声热机系统回热器 (或板叠 )的数值计算 ,讨论了简单实用的简谐分析方法 ,并对热声制冷机中的热声板叠进行了数值计算与分析 .计算结果为回热器 (或板叠 )的设计与优化奠定基础 。

回热器参数对热声发动机性能影响的理论及实验研究

针对本实验室的行波热声发动机系统,研究了回热器对系统的重要作用。利用建立在线性热声理论基础上的模块化程序预测了不同回热器下系统的工作性能,并结合实验进一步分析比较了不同回热器对系统性能的影响。实验结果表明,低目数丝网填充的回热器更容易使系统起振,同时,起振温度随着工作压力的增加而降低;相同的输入功率和充气压力下,压比和加热块温度都随着目数的增大而升高;相同输入功率和目数下,压比和加热块温度随充气压力的增大而降低;丝网目数对系统频率的影响很小。

斯特林型热声发动机驱动直线发电机的工作特性

为探索发电机负载与热声发动机之间的耦合规律,开展发电机负载和声学传输管对斯特林型热声发动机工作特性影响的理论与实验研究.自行搭建一套斯特林型热声发动机驱动单直线发电机的实验系统,通过控制发动机与发电机间球阀的开关考察发动机在有无负载情况下的工作特性,通过改变声学传输管长度考察声学传输部件对发动机输出特性的影响.研究表明,发电机负载使系统起消振温度提高,而接入发电机负载前后系统工作频率基本不变;通过优化声学传输部件可有效改善发动机的输出特性,提高发动机的输出压力振幅和发电机输出电功;计算结果表明,改善频率匹配可大幅提高热声发动机驱动发电机系统的性能.

热声发动机径向尺寸对谐振频率的影响

频率是热声系统中最为关键的参数,小型化面临的问题是系统运行频率升高,粘性阻力损失增加。对频率展开研究的结果是谐振管的径向尺寸对热声发动机的谐振频率影响很大,通过调节谐振管的径向尺寸,可以大大降低整机的谐振频率,对于指导热声斯特林发动机的小型化具有重要的指导意义。据此得到的计算结果与实验研究的结果验证了这种方法是十分有效的,可以达到降低系统谐振频率30%以上。

热声热机技术的研究进展

首先简要地介绍热声热机(包括热声发动机和热声制冷机)的基本工作原理和热声技术特点,指出热声热机是基于热致声效应和声制冷效应两类热声效应而工作的,它完全没有机械运动,采用惰性气体工作,是一种可靠性高和环保的新型的能源转换机械。然后重点介绍近十几年来国内外在热声技术研究方面所取得的重要进展,主要包括在热声发动机和热声制冷机两个方面。在热声发动机研究方面,简要回顾热声效应的发现和驻波热声发动机的研究进展,接着介绍行波热声发动机的提出和迅速发展历程,重点介绍热声转换效率达到30%的热声斯特林发动机和压比达到1.3(采用氦气)、效率达到32%的聚能型热声发动机研究工作;在热声制冷技术方面,分别介绍热声驱动脉冲管制冷获取低温的研究工作以及热声驱动室温制冷机获得高效率、大冷量等方面的研究工作,并重点介绍采用声压放大器获得液氢温度以下的完全无运动部件热声驱动脉冲管制冷机(低于20K),以及在?20℃获得了300W以上的制冷量的双行波流程的热声驱动室温热声制冷机。最后,扼要地介绍热声理论方面的研究进展,并对热声技术的问题和该研究领域的发展方向进行展望。

回热器的热声网络模型

从基本守恒方程出发，以多孔介质毛细管束模型为基础，建立了回热式热机回热器的有源热声网络模型，并且根据网络理论及热声理论对有源热声网络模型中源项的特性进行了讨论。

液氮温区高频热驱动脉冲管制冷机实验研究

优化设计并研制了一小冷量液氮温区高频热驱动脉冲管制冷机,系统轴向长度仅为1.2 m。针对之前系统热效率较低问题,在数值计算的指导下,对高频驻波热声发动机的板叠流道尺寸和热腔进行了优化,并对声压放大器进行了调整。采用4.0 MPa氦气为工质,在发动机端加入750 W加热量获得了68.3 K的最低制冷温度,这是目前国际上报道的高频热驱动脉冲管制冷机的最低制冷温度。加入500 W加热量时,制冷机获得了76.9 K的无负荷最低制冷温度,在80 K时有0.2 W的制冷量。

行波型热声热机的研究进展

Ceperley首先提出了行波型热声热机的概念 ,此后许多研究者对此类型的热机进行了理论及实验研究。文中对行波型热声热机的发展历史。

可用于热声发动机的加热方式

加热装置在热声发动机中起着举足轻重的作用。文中首先介绍了目前热声发动机已经采用的几种加热方式并给出了应用范例 ,同时还对其它可能的加热方式及其工作原理作了简要介绍 。

液氮温区热声驱动脉管制冷机的研究

热声发动机驱动的脉管制冷机是一种完全无运动部件的低温制冷机,具有非常好的应用前景,本文介绍了本实验室在这方面取得的最新进展。首先我们对驻波热声发动机进行了改进设计,提高了其驱动压比,用氦气作为工质最大压比达到了1.15。在此基础上我们用其驱动同轴双向进气小孔型脉管制冷机,通过调整热声发动机的振荡频率,使之与脉管达到匹配,最终达到了84.3K的最低制冷温度,这也是目前用驻波热声发动机驱动脉管所达到的最低制冷温度。同时,在此实验过程中,一些抑制跳频的方法也得到了实验验证。