The performance improvement of a cascade thermoacoustic engine by adjusting the acoustic impedance in the regenerator

A novel cascade configuration consisting of one standing wave unit and one travelling wave unit arranged in series is studied in this paper. Theoretically, a straight‐line cascade engine provides an efficient energy conversion, reduces the difficulties of fabrication and allows no Gedeon streaming. In order to achieve such a powerful cascade thermoacoustic engine, the regenerator of the travelling wave unit must be operated in high impedance and travelling wave phasing region. Various techniques of phase adjustment by modifying the configurations and geometrical dimensions of the system are investigated both numerically and experimentally in order to adjust the position of the sweet spot as well as to promote the acoustic impedance in the regenerator. It is found that the effective tuning methods with less modification here are accomplished by changing the volume of down‐cavity and reducing the flow area of down‐resonator by inserting the pencil. The exploration also shows that the acoustic field in the system is quite sensitive to the effect of down‐resonator length. The performance of the proposed system is clearly improved after the phase‐adjustment schemes are completely implemented, in which the regenerator works within the sweep spot zone with high acoustic impedance. Copyright © 2016 John Wiley & Sons, Ltd.     

Performance comparison of looped thermoacoustic electric generators with various thermoacoustic stages

Thermoacoustic engine is a kind of novel heat engine based on thermoacoustic effect, with the merits of environmental benignity, simplicity, and reliability. In this work, looped travelling-wave thermoacoustic electric generators (LTTEGs) with one to four thermoacoustic stages have been developed and experimentally studied. It is observed that adding thermoacoustic stages can improve the thermal-electric efficiency of LTTEGs, while whether the extra stages lead to efficiency gain depends on the number of existing stages and other operating parameters (hot temperature, for instance). One main reason is that the Gedeon streaming, which might cause severe heat loss, can be enhanced by adding thermoacoustic stages and increasing hot temperature. The results suggest that the suppression of streaming in the looped thermoacoustic engine with multiple stages is even more urgent than in the traditional travelling-wave engine with only one stage.     

一种蓄热室性能的诊断方法

提出了一种通过测量和分析蓄热室外表面的温度分布,来研究蓄热室性能的方法,并结合实际测试说明了该方法的应用。根据表面温度的不同变化,可以得到蓄热室内温度和气流的分布情况。该方法对于分析蓄热室在生产应用中出现问题的原因,很有帮助。     

Linear analysis of rapidly switched heat regenerators in counterflow

This paper is intended to provide an accurate analytical solution to the 1D differential equations modelling cyclic steady heat transfer processes in rapidly switched heat regenerators for any value of the flush ratio. The temperature solution for the fluid is initially given in an integral form along the path of a gas particle as a function of the matrix temperature for different space and time intervals. In particular, as a Lagrange system of reference is assumed, the above solution deals separately with gas particles of three possible types (‘cold’, ‘hot’ and ‘internal’) according to Organ’s concept of independent flow regimes. Also, it accounts for the possible superposition of the so-called hot and cold zones of the regenerative matrix depending on the value of the flush ratio. Then, assuming a linear distribution for the matrix temperature, the fluid temperature may analytically be calculated. A closed-form expression for the regenerator effectiveness as a function of NTU and flush ratio is given. It provides a simple but accurate tool to estimate the regenerator effectiveness in rapid cyclic flow situations and the deriving results indicate that it is underestimated by the conventional regenerator theory.     

Temperature Distribution and Heat Saturating Time of Regenerative Heat Transfer

In this paper, heat transfer of the ceramic honeycomb regenerator was numerically simulated based on the computational fluid dynamics numerical analysis software CFX5. The longitudinal temperature distribution of regenerator and gas were obtained. The variation of temperature with time was discussed. In addition, the effects of some parameters such as switching time, gas temperature at the inlet of regenerator, height of regenerator and specific heat of the regenerative materials on heat saturating time were discussed. It provided primarily theoretic basis for further study of regenerative heat transfer mechanism.     

600 MW回转式空气预热器传热模型的解析-数值法

传统的数值迭代法只给出了两分仓回转式空气预热器稳态时端面的二维温度变化曲线,并不能直观地反映空气预热器从启停到正常运行时真实的温度分布.对回转式空气预热器的换热机理进行详细分析,提出采用解析-数值混合计算方法求解换热模型的数学方程,并用该方法模拟600 MW回转式空气预热器金属板和介质温度分布场,结果表明,解析-数值混合计算法,不仅克服了传统迭代法的精度不高、不易收敛的缺点,而且它可以真实地反映出空气预热器整个运行工况金属板和介质的温度分布情况,容易发现引起低温腐蚀的金属最低温度位置,因此对回转式空气预热器安全经济运行提供了更加科学的理论依据.     

提高蓄热室内气流均匀度的研究

阐述热风炉鼓风室内气流的流动规律,论述气流分布不均的主要原因,介绍均匀配气原理和冷风均匀配气技术。测试结果表明,冷风均匀配气技术动力消耗低,可显著提高蓄热室横截面上的送风均匀度,提高送风温度。     

回转式空气预热器传热模型动态仿真

根据回转式空气预热器的传热机理进行了理论分析,给出了相应的传热计算模型.并根据空气预热器的动态平衡过程和结构特性,结合金属蓄热板径向温度场的二维分布假设,提出采用解析-数值混合计算方法求解换热模型的数学方程,并将解析-数值方法与传统的矩阵迭代法分别计算某电厂的实际空气预热器从启动到正常运行阶段蓄热板和工质温度分布,结果表明传统的矩阵迭代法计算对特征值的要求很严格,而解析-数值方法对任意特征值均可以收敛,并且计算结果的工程精度也很高,因此对电厂安全经济运行提供了可靠的理论依据.     

Experimental investigations of a gamma Stirling engine

The present work deals with the measurement and performance of a gamma Stirling engine of 500 W of mechanical shaft power and 600 rpm of maximal revolutions per minute. Series of measurements concerning the pressure distribution, temperature evolution, and brake power were performed. The study of the different functioning parameters such as initial charge pressure, engine velocity, cooling water flowrate, and temperature gradient (between the sources of heat) has been analyzed. The engine brake power increases with the initial charge pressure, with the cooling water flow, and with the engine revolutions per minute. The working fluid temperature measurements have been recorded in different locations symmetrically along both regenerator sides. The recorded temperature in regenerator side one is about 252 °C and about 174 °C in the opposite side (side two). It shows an asymmetric temperature distribution in the Stirling engine regenerator; consequently, heat transfer inside this porous medium is deteriorated. Copyright © 2011 John Wiley & Sons, Ltd.     

新型旋流热风炉填充球蓄热室实验研究及效率分析

提出了向小型化，高效化改造热风炉的方案。建立了新型旋流式热风炉填充球蓄热室温度分布的混合扩散数学模型，通过模型的数值解讨论了影响蓄热室热效率和温度效率的主要因素，并通过实验检测验证了新型旋流式热风炉蓄热室能更好的满足数学模型的假设条件。     

Thermal-fluid-solid coupling of electric heat storage device based on wind power absorption

电网调峰能力不足，弃风问题严重，已成为我国风力发电规模进一步扩大的瓶颈。通过配置蓄热装置参与风电调峰，改变传统“以热定电”的约束模式，是解决我国大量弃风问题的一个趋势。提高电热蓄能装置效率及优化蓄热装置分布对于实现有效调峰具有重要意义。本文针对固体电蓄热装置内流动、传热、应力等现象，建立了热-流-固多场三维耦合传热数学模型，采用流-固耦合传热模型将难以确定的热流边界转化为系统内部边界，分析固体电蓄热装置温度场及应力场分布，并对比三种不同孔隙率、电热丝排布方式对蓄热装置温度分布均匀性及热膨胀量的影响。研究结果对提高固体电蓄热装置的效率以及电网调峰具有一定的参考价值。     

基于FLUENT的热风炉蓄热室传热及操作制度研究

为获得热风炉蓄热室内气体及格砖温度随时间的变化规律,用FLUENT软件建立了热风炉蓄热室温度场计算的简化模型,并进行了蓄放热过程的模拟计算.在分析不同操作制度下热风炉工作特点的基础上,比较计算了具代表性的三座热风炉在二烧一送、一烧二送和半交叉并联三种操作制度下的风温变化状况和热效率.模拟计算结果表明:该方法可以得到符合设计计算精度要求的模拟结果,可以方便地进行蓄热室传热特征分析,预测热风炉在不同操作制度下的工作特点,为热风炉生产选择优化操作制度提供了便捷有效的方法.     

新型档位蓄热器的开发与研究

研究开发了一种可应用在高温空气燃烧（HTAC）技术中的新型蓄热器--档位蓄热器.档位蓄热器吸收了回转式蓄热器的思想,结合了切换式蓄热器的特点,将二者的优势融合在一起,其外观类似于回转式蓄热器,但运动型式却由回转式蓄热器的连续运动变为间歇转动,且具有了新型的密封结构.对档位蓄热器的基本工作原理、具体设计方案、实验研究及应用前景进行了详细介绍.研究结果表明档位蓄热器具有以下特点：可实现对炉窑的连续供气,保证了炉内火焰的稳定性,从而使炉内工况不易波动;另外系统的漏风率较低,阻力损失也较低,并且设备紧凑、操作安全可靠,易于实现标准化生产.实验研究的结果证明该蓄热器的温度效率可达88%,热效率可达77%.     

再生式换热器的实验研究与数值模拟

为了解再生式换热器工作特性,采用Matlab软件计算简化后的模型,对换热器内流体和固体的温度分布进行了数值模拟,并进行了实验验证.研究了蓄热体长度、比表面积、速度和蓄热体材质对换热效果的影响.结果表明,面积和流速对换热效果影响明显,而改变蓄热体材质基本无影响.     

A thermoacoustic engine capable of utilizing multi-temperature heat sources

Low-grade energy is widespread. However, it cannot be utilized with high thermal efficiency directly. Following the principle of thermal energy cascade utilization, a thermoacoustic engine (TE) with a new regenerator that can be driven by multiple heat sources at different temperature levels is proposed. Taking a regenerator that utilizes heat sources at two temperatures as an example, theoretical research has been conducted on a traveling-wave TE with the new regenerator to predict its performance. Experimental verification is also done to demonstrate the benefits of the new regenerator. Results indicate that a TE with the new regenerator utilizing additional heat at a lower temperature experiences an increase in pressure ratio, acoustic power, efficiency, and exergy efficiency with proper heat input at an appropriate temperature at the mid-heater. A regenerator that uses multi-temperature heat sources can provide a means of recovering lower grade heat.     

新型回热器结构设计及换热特性研究

回热器作为斯特林热机的关键部件,对于太阳能斯特林热机整机性能有着重要影响。为克服传统金属丝网回热器结构存在的填料单一,制造成本较高,工艺复杂问题,采用实用等温分析法,以回热器的长径比、通流面积、填料种类以及孔隙率各项回热器参数为基础,设计了一种新型斯特林热机回热器,该回热器具有轴向压降小,换热性能高,结构稳定,加工制造简单的特点。开展了新型回热器和传统金属丝网回热器的换热性能对比研究,采用振荡条件下的局部热平衡方法研究回热器的传热过程,对比传统金属丝网回热器和新型回热器的温度变化,速度变化以及压力变化。结果表明:在整体孔隙率相同的条件下,新型回热器和传统金属丝网回热器相比,整体启动速率相似,但新型回热器压降减少0.04 MPa,速度出现分段式变化,有利于回热器的换热和结构稳定。因此,新型回热器不但在结构上优于传统金属丝网回热器,在换热特性上也优于传统金属丝网回热器。     

Thermal performance analysis on a two composite material honeycomb heat regenerators used for HiTAC burners

Honeycomb heat regenerators do not only reduce the fuel consumption in a high temperature air combustion (HiTAC) burning system but also provide the necessary high temperature of combustion air. A two-dimensional simulation model was developed to numerically determine the dynamic temperature and velocity profiles of gases and solid heat-storing materials in a composite material honeycomb regenerator. Consequently, the energy storage and the pressure drop are calculated and the thermal performance of honeycomb heat regenerator is evaluated at different switching times and loading. The model takes into account the thermal conductivity parallel and perpendicular to flow direction of solid and flowing gases. It considers the variation of all thermal properties of solid material and gases with temperature. Moreover, the radiation from combustion flue gases to the storage materials was considered in the analysis. The results are presented in a non-dimensional form in order to be a design tool as well. These analyses were applied on a regenerator made of two layers of ceramic materials, one is pure alumina and other is cordierite. This regenerator is contained in a 100 kW twin-type regenerative-burning system used for HiTAC. The effectiveness and the energy recovery rate were 88% and 72% respectively at nominal operating range of the regenerator and the pressure drop across the twin regenerator system was 1.16 kPa. The periodic steady state condition is reached after about 11 min and it takes only 2 min of operation until the temperature of combustion air remains above the self-ignition temperature that is required for HiTAC. Furthermore, these mathematical analyses show good agreement with experiments made on the same regenerator. In the experiments, the dynamic behavior of the heat regenerator operation was considered in order to compensate measurement readings for this effect.     

Model validation and case study on internally cooled/heated dehumidifier/regenerator of liquid desiccant systems

Traditional dehumidifiers and regenerators of liquid desiccant systems often use packed columns supporting adiabatic heat and mass transfer between air and liquid desiccant. As new-style equipment, internally-cooled dehumidifiers can improve dehumidification performance due to restraining temperature increase of the desiccant. Similar to internally-cooled dehumidifiers, an idea of internally heating is imitated to put forward internally-heated regenerators. The uniform mathematical model for an internally cooled dehumidifier and internally heated regenerator was presented and validated by comparison of computation results with experimental data in this study. The case study focused on the parameters distribution comparisons of the internally cooled/heated dehumidifier/regenerator with adiabatic ones and demonstrated coupled heat and mass transfer behavior. The results show that the internally-heated regenerator can produce higher regeneration efficiency than the adiabatic one to produce better energy efficiency and eliminate the dehumidification possibility which would happen in adiabatic regenerators. The internally-cooled dehumidifier can also provide better dehumidification performance comparing with the adiabatic one; however its benefit would be not as good as the internally-heated regenerator. In addition, effect of the width of the air channel on internally cooled/heated dehumidifier/regenerator was discussed and the results can help the optimal design of this kind of dehumidifiers and regenerators.     

Empirical estimation of regenerator efficiency for a low temperature differential Stirling engine

An experimental method of regenerator evaluation is proposed in this paper. The configuration of the experimental equipment used in the method is similar to that of an alpha-configuration Stirling engine with a phase angle of 180°. The temperature of the hot side heat exchanger is controlled by an electric heater, and the heat sink was room air. An air conditioner controlled the temperature of the room air. The temperature and pressure of the working fluid were measured during the piston motion. A #18 stainless steel mesh was used as a regenerator matrix for a low temperature differential Stirling engine (LTDSE). The regenerator efficiency can be calculated based on the measurement results. The product of the swept volume, the density of the working fluid, the specific heat and the difference in the working fluid temperatures between the hot side and the cold side is greater than the amount of the internal energy fluctuation. The reason for this is assumed to be the temperature fluctuation in the region between the two heat exchangers. The walls of the region are made of acrylic resin. The amount of the temperature fluctuation in the region is assumed to be uniform. The regenerator efficiency is calculated as a function of the temperature fluctuation in the region. The evaluation method does not require a fast-response thermocouple. The prediction of the regenerator efficiency is possible basted on some experimental results of same matrix. Polyurethane foam and #18 stainless steel mesh, layered parallel to the stream line of the working fluid, were each tested. These materials can realize a non-rectangular regenerator without the generation of waste. Non-rectangular regenerator includes regenerator that can be installed into narrow gaps. The regenerator efficiency of the stainless steel mesh layered parallel to the stream line of the working fluid was significantly less in comparison to that of the normal mesh layers. In the polyurethane foam case, a pressure loss was observed.     

Thermodynamic analysis of a Stirling engine including dead volumes of hot space, cold space and regenerator

This paper provides a theoretical investigation on the thermodynamic analysis of a Stirling engine. An isothermal model is developed for an imperfect regeneration Stirling engine with dead volumes of hot space, cold space and regenerator that the regenerator effective temperature is an arithmetic mean of the heater and cooler temperature. Numerical simulation is performed and the effects of the regenerator effectiveness and dead volumes are studied. Results from this study indicate that the engine net work is affected by only the dead volumes while the heat input and engine efficiency are affected by both the regenerator effectiveness and dead volumes. The engine net work decreases with increasing dead volume. The heat input increases with increasing dead volume and decreasing regenerator effectiveness. The engine efficiency decreases with increasing dead volume and decreasing regenerator effectiveness.