NUMERICAL STUDY ON FLOW DISTRIBUTION IN PLATE-FIN HEAT EXCHANGERS

Objective To investigate the flow distribution in plate fin heat exchangers and optimize the design of header configuration for plate fin heat exchangers. Methods A mathematical model of header was proposed. The effects of the header configuration on the flow distribution in plate fin heat exchangers were investigated by CFD. The second header configuration with a two stage distributing structure was brought forward to improve the performance of flow distribution. Results It is found that the flow maldistribution is very serious in the direction of header length for the conventional header used in industry. The numerical predictions indicate that the improved header configurations can effectively improve the performance of flow distribution in plate fin heat exchangers. Conclusion The numerical simulation confirms that CFD should be a suitable tool for predicting the flow distribution. The method has a wide variety of applications in the design of plate fin heat exchangers.

Performance Evaluation Methods for Multi-stream Plate-Fin Heat Exchanger

Mathematical model of cross type multi-stream plate-fin heat exchanger is established.Meanwhile,mean square error of accumulative heat load is normalized by dimensionless,and the equations of temperature-difference uniformity factor are improved.Evaluation factors above and performance of heat exchanger are compared and analyzed by taking aircraft three-stream condenser as an example.The results demonstrate that the mean square error of accumulative heat load is common result of total heat load and excess heat load between passages.So it can be influenced by passage arrangement,flow inlet parameters as well as flow patterns.Dimensionless parameter of mean square error of accumulative heat load can reflect the influence of passage arrangement to heat exchange performance and will not change dramatically with the variation of flow inlet parameters and flow patterns.Temperature-difference uniformity factor is influenced by passage arrangement and flow patterns.It remains basically unchanged under a certain range of flow inlet parameters.

Experimental Analysis of Performance of Heat Exchanger with Plate Fins and Parallel Flow of Working Fluids

Heat exchangers are devices in which heat is transferred from one fluid to another fluid as a result of temperature difference. Heat exchanger presented in the current paper in which inside the tubes flows water, but outside the tubes flows air aims to enable cooling of circulating water, which serves to cool the engine of a machine. Such exchangers find application in the automotive industry as well as heating and cooling equipment and HVAC systems etc. The surface of the heat exchanger by the air side always tends to be much larger using surface fins in order to facilitate equalization of thermal resistance for both sides of the heat exchanger, because the rate of transmission of heat from the water side is much greater. Furthermore, the paper will present analytical and experimental studies involved for determination of performance of plate-fin heat exchanger for various flows of working fluids in order to get the highest values of performances i.e.: overall heat transfer coefficient U, efficiency of heat exchanger ε, maximal and real heat transferred, pressure drop, air velocity and Reynolds number from the air side of heat exchanger etc. The present scientific paper is based on the fact that from the experimental model made for laboratory conditions, conclusions are derived that can be used during installation of such heat exchanger on certain machines in order to predict their performance.

Optimal design of the first stage of the platefin heat exchanger for the EAST cryogenic system

The size of the heat exchanger is an important factor determining the dimensions of the cold box in helium cryogenic systems. In this paper, a counter-flow multi-stream plate-fin heat exchanger is optimized by means of a spatial interpolation method coupled with a hybrid genetic algorithm.Compared with empirical correlations, this spatial interpolation algorithm based on a kriging model can be adopted to more precisely predict the Colburn heat transfer factors and Fanning friction factors of offset-strip fins. Moreover, strict computational fluid dynamics simulations can be carried out to predict the heat transfer and friction performance in the absence of reliable experimental data. Within the constraints of heat exchange requirements, maximum allowable pressure drop, existing manufacturing techniques and structural strength, a mathematical model of an optimized design with discrete and continuous variables based on a hybrid genetic algorithm is established in order to minimize the volume. The results show that for the first-stage heat exchanger in the EAST refrigerator, the structural size could be decreased from the original2.200？×？0.600？×？0.627（m^3） to the optimized 1.854？×？0.420？×？0.340（m3）, with a large reduction in volume. The current work demonstrates that the proposed method could be a useful tool to achieve optimization in an actual engineering project during the practical design process.

保温时间对钛合金板翅式换热器真空钎焊过程温度场及残余应力的影响

钛合金板翅式换热器成品率较低、残余应力较大,该文针对钛合金板翅结构真空钎焊过程开展热-固耦合建模与仿真研究,阐述了钎焊过程温度均匀性及残余应力分布特征,探明保温时间对钛合金板翅式换热器真空钎焊过程温度场及残余应力的影响机理.结果表明,板翅结构两侧温度较高,中部温度较低,延长保温时间可有效改善板翅结构的温度均匀性.残余应力均主要集中于翅片上表面与隔板背侧中部,在钎缝及夹持点处存在明显的应力集中现象,保温时间越长翅片钎缝的残余应力越小.模拟得到钛合金板翅式换热器真空钎焊中残余应力与试验结果相吻合,相对误差为5.3%,验证了该模型的准确性.

Experimental Study on Heat Transfer and Pressure Drop Characteristics of Four Types of Plate Fin－and－TUbe Heat Exchanger Surfaces

In this paper, air side heat transfer and pressure drop characteristics of twelve three-row plate finandtube heat exchanger cores of four types of fin configurations have been experimentally investigated.The heat transfer and friction factor correlations for'the twelve cores are provided in a wide range ofReynolds number. It is found that in the range of Reynolds number tested, the Nusselt number of theslotted fin surface is the largest and that of the plain plate fin is the lowest while the Nusselt numbersof two types of wavy fins are somewhere in between.

钛合金板翅式换热器钎焊工艺仿真分析

针对钛合金板翅式换热器结构特点,确定了加压钎焊的工艺方法,开展了单层钛合金板翅式换热器钎焊仿真分析,并进行了工艺验证。根据换热器结构的受力特点,设计了加压后挡板变形和波纹板变形两种钎焊方式,并设置了不同变形下压量,采用ABAQUS有限元仿真软件对焊接过程进行了分析。结果表明,在单层挡板下压量为0.1 mm时,换热器整体受力均匀,最大应力为170MPa;在单层波纹板下压量为0.3 mm时,波纹板变形均匀,未出现畸变扭曲等缺陷,最大应力为165.8 MPa。设计了单层挡板总高度低于波纹板总高度0.3 mm,先将波纹板下压0.3 mm,再将挡板下压0.1 mm的工艺方案,并进行了验证,制备得到了质量良好、焊接紧密的换热器样件。

锯齿型翅片内不同温区氦气流动与传热性能对比及节距对翅片性能影响的研究

为分析板翅式换热器在低温工况下的流动换热性能与常温工况下的差异和翅片节距对其性能影响,建立氦气在锯齿翅片通道内流动与传热的数值模型,开展数值模拟分析。结果表明:低温氦气在锯齿型翅片通道内的传热性能优于常温氦气。在氦气入口温度为20 K时,j因子比300 K时提升1.1%~55.9%,比77 K时提升0.41%~13.01%;在氦气入口温度为77 K时,j因子比300 K时提升0.53%~38.47%。低温氦气在锯齿型翅片通道内的流动性能比常温氦气差。在氦气入口温度为20 K时,f因子比300 K时高16.84%~28.87%,比77 K时高3.98%~23.19%。翅片节距大小在不同温区下对翅片流动与传热性能均有显著影响。在不同温区下,j因子随着翅片节距增大而减小,f因子随着翅片节距减小而增大。低雷诺数时,JF因子随着翅片节距减小而增大,而高雷诺数时则出现相反的趋势。

Al-1.4Mn-0.5Si-0.8Fe合金铸锭均匀化工艺研究

为满足板翅式换热器用铝合金翅片箔承压能力的要求,设计开发了Al-1. 4Mn-0. 5Si-0. 8Fe合金。该合金是在3003铝合金的基础上加入了少量的Zr、Cr、V、Ti等微合金元素;试验研究了生产翅片箔用的该合金铸锭的均匀化退火工艺,该合金铸锭适宜的均匀化退火工艺为610℃7 h。

用CO_2-空气换热器冷却燃机进口空气的实验研究

对CO2-空气换热器进行了研究和试验，得出了该类型换热器的传热和阻力特性曲线以及温度场分布规律。实验发现空气经冷却后压力损失比未冷却时有5％～10％的减小，而传热因子j和阻力因子f，随换热条件的改变而基本保持不变。将实验结果与参考文献数据相比较，发现阻力因子f与参考文献基本吻合，而传热因子j有所减小。

LNG铝制板翅式结构的常/低温断裂机理研究

为避免LNG铝制板翅式换热器的强度破坏并提高其使用寿命,基于轴向拉伸试验与扫描电子显微镜观测方法,研究了其铝制板翅式结构在常/低温下的拉伸断裂机理.结果表明,相比常温,低温下板翅试样的屈服极限增大了42.6%,抗拉强度增大了48.9%,破坏时的应变增大了27.8%.常/低温下板翅试样的断裂趋势主要为翅片根部中心朝钎焊区发展断裂以及钎焊区朝翅片根部发展断裂;断裂整体以解理断面及韧窝等形貌为主,但低温下翅片的解理断面密集程度更高,韧窝等塑性形貌更少.板翅试样常温下的断裂位置主要位于钎焊区,低温下的断裂位置仅位于钎焊区.因此,为避免铝制板翅式结构在钎焊区处发生断裂,可采取改善铝制板翅式结构的钎焊工艺以及增加钎焊区焊料等方式.

板翅式换热器内气液两相流分配特性数值分析

板翅式换热器是海上天然气开采后进行液化处理的主换热设备,由于其内部结构的复杂性,容易引起各通道流量分配不均匀,会严重影响换热器效率。对换热器封头简化后进行建模,构建了板翅式换热器封头内两相流流动三维非稳态模型,以两相天然气为主要介质,利用计算流体力学(CFD)的方法,分析了不同气相体积分数的两相天然气在板翅式换热器封头内的分配特性。研究结果表明,当入口处气相天然气体积分数为50%时,气相整体分配不均匀度为0.50,液相整体分配不均匀度为0.03,各通道气相天然气的质量流量分布呈现中间低两端高的特点,液相天然气则相反,验证了两相流分配性能比单相流更加复杂的正确性。当入口处气相天然气体积分数逐渐增加时,中间通道和两端通道的两相流质量流量差逐渐变大,这表明流体的整体分配均匀性逐渐恶化。

多场协同下的空调换热器主流翅片性能CFD仿真分析

空调换热器翅片是空调四大部件主要换热装置中进行热传递的金属片,是空调中最主要的组成部分。对于翅片式换热器的理论研究,往往涉及对流换热和传质换热等多种方式,本文借助仿真软件进行翅片多场协同换热性能进行研究。单一变量下仿真对比研究发现,四行桥片与五行桥片换热性能对比,五行桥片减少了尾流区,更有利于换热器翅片的换热,换热量性能更好。

基于热-应力耦合场有限元分析的钎焊板翅结构性能研究

采用有限元分析中的热-应力(Thermal-Structure)耦合场分析法,对不锈钢板翅式换热器高温运行工况产生的热应力与热变形展开研究。建立板翅式换热结构单元二维模型,对高温流体换热工况进行数值模拟计算,并将计算结果作为初始条件导入结构分析中。考虑钎焊材料与母材的差异和焊缝结构的影响,确定了最大应力的位置和数值,并分析不同工况下翅片结构的应力、应变的影响因素。

碱性燃料电池-温差热电制冷混合系统的性能分析

建立了由碱性燃料电池(alkaline fuel cell,AFC)、热电发电器、热电制冷器和一组板翅式热交换器组成的混合系统模型。考虑系统的电化学和热力学不可逆性损失,导出了混合系统的等效输出功率和效率的表达式,确定了燃料电池电流密度的优化区间,讨论了工作压力、工作温度、热电元件数量、热电元件传热系数、板翅式热交换器传热系数对混合系统的等效输出功率和效率的影响。结果表明,混合系统的最大输出功率密度及对应的效率,相比单独的碱性燃料电池分别提高了7.56%和6.40%,相比无板翅式热交换器时分别提高了7.55%和8.04%。所得结果可为碱性燃料电池温差热电制冷混合系统的优化设计提供理论依据。

基于热-结构耦合的LNG板翅结构热应力模拟

为保证大型天然气液化(LNG)用板翅式换热器冷箱安全、可靠运行,建立了大型LNG冷箱内板翅式换热器的板翅结构热应力有限元分析数学物理模型,基于热弹性理论采用热-结构直接耦合方法分析了运行参数(流体温度、外载荷以及操作压力)对LNG板翅结构等效热应力的影响规律,模拟结果表明:天然气和混合冷剂之间温差越大板翅结构承受最大等效热应力越大,混合冷剂和天然气温度越低其最大等效热应力越大;外载荷为压力时压力越大其最大等效热应力越小,外载荷为拉力时拉力越大其最大等效热应力越大;此外,天然气侧压力对热应力的影响大于混合冷剂侧压力对其影响,但均表现出增大趋势。上述研究成果将为大型LNG冷箱内板翅式换热器结构以及运行参数设计和安全可靠运行提供重要参考依据。

板翅式气-水换热器传热与阻力性能试验研究

利用风洞试验台,针对某工程机械用铝制板翅式气-水换热器进行传热和流动阻力性能试验,获得层流状态下的传热因子与阻力因子,并与通用关联式进行比较。在实验基础上,修正得到新的传热因子,阻力因子与雷诺数的关联式。

石墨烯板翅式全热换热器性能试验研究

针对空调系统运行时存在新风能耗大的问题,对小型热回收新风机组用石墨烯板翅式全热换热器性能进行了试验研究。搭建了全热回收新风机组试验平台,以层高为2.0 mm的石墨烯芯体为试验研究主体,常规高效的瓦楞纸芯体和层高为3.0 mm的石墨烯芯体为试验对照组,进行了冬、夏季标准工况下不同迎面风速下热湿交换效率和阻力试验研究,并对石墨烯芯体进行了室外新风干球温度和相对湿度变化对热湿交换效率影响的试验研究。结果表明,石墨烯芯体单位风量和面积下全热换热量比瓦楞纸芯体高50%,并且瓦楞纸芯体的流道摩擦系数要比石墨烯芯体大21.5%;在环境温湿度提高时,夏季工况下,石墨烯芯体的温湿度效率分别提高了4.1%和6.7%,冬季工况下,湿度效率提高了8.9%。综上所述,相同的热回收新风机组尺寸和风量情况下,采用石墨烯芯体替代目前高效的瓦楞纸芯体,风机运行能耗更低,风量调节性能更好,有更为突出的热湿交换性能。

板翅式换热器的强化传热数值模拟

为开发效率更高的板翅式换热器,对锯齿形板翅式换热器性能的影响因素进行深入研究。采用数值模拟的方法,建立锯齿形板翅式换热器模型后进行仿真计算,采用单一变量法分别对比分析翅片厚度、间距及高度3种结构参数下板翅式换热器的换热性能和流动特性。分析得出:在研究范围内,换热器综合性能随翅片厚度的降低小幅度增加,翅片间距的增大则会造成换热器中整体翅片数量的减少,减小对流体的扰动,降低换热器总体换热量,相比之下翅片间距为18 mm的翅片综合性能最优。降低翅片高度会增大流道内阻力,高度为48 mm时的翅片综合性能要优于其他尺寸。综合考虑间距较小、翅片高度偏低且翅片厚度更薄时翅片的综合性能最佳,然而翅片厚度越薄往往对制造工艺和经济成本的要求越高,可能得不偿失。

波纹翅片板翅式换热器换热与阻力性能

为研究翅片结构对波纹翅片板翅式换热器换热与阻力性能的影响规律,在模化实验验证的基础上,对21种波纹翅片的性能进行数值模拟。结果表明:翅高h≤10 mm时,其对换热与阻力性能影响较小;翅高h>10 mm时,随h增大,换热性能先提高(Re=1500、5000时分别提高3.54%、5.98%)后降低(Re=1500、5000时分别降低4.2%、5.95%),阻力性能先降低(Re=1500、5000时分别降低2.83%、5.65%)后提高(Re=1500、5000时分别提高2.99%、3.98%)。随翅片间距s增大,换热性能在低雷诺数时降低(Re=1500时降低16.02%),在高雷诺数时提高(Re=5000时提高6.83%),阻力性能降低(Re=1500、5000时分别降低151.6%、73.87%)。随翅片振幅2 A增大,换热性能提高(Re=1500、5000时分别提高49.29%、34.43%),阻力性能降低(Re=1500、5000时分别降低201.18%、146.2%)。随波长L增大,换热性能降低(Re=1500、5000时分别降低45.92%、51.73%),阻力性能提高(Re=1500、5000时分别提高78.51%、85.3%)。获得了传热因子j、摩擦因子f与翅片结构参数及雷诺数Re的关联式。