Experimental Research on Flow Maldistribution in Plate-Fin Heat Exchangers

The flow maldistribution and the effect of different inlet configuration on the flow distribution in platefin heat exchangers were studied experimentally. It is found that the flow maldistribution is serious because of the defects of inlet configurations, while the inlet configuration and Reynolds number are the main factors affecting the flow distribution. The improved inlet configurations, which are the header with a two-stage distributing configuration and the guide vane with a fluid complementary cavity were proposed and tested in this paper. The experimental results show that the improved inlet configurations can effectively improve the performance of flow distribution in heat exchangers.

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.

PIV Investigations of Flow Patterns in the Entrance Configuration of Plate-fin Heat Exchanger

Flow characteristics in the entrance of plate-fin heat exchanger have been investigated by means of particle image velocimetry (PIV). The flow field was measured using the two-frame cross-correlation technique. Streamline and velocity contour graphs at different cross-sections were obtained in the experiment. The experimental results indicate that flow maldistribution in the conventional header is very serious, while the improved header configuration with punched baffle can effectively improve the uniformity. The flow maldistribution parameter in plate-fin heat exchanger has been reduced from 1.21 to 0.21, and the ratio of the maximum velocity to the minimum is reduced from 23.2 to 1.8 by install-ing the punched baffle. The results suggest room for the optimum design of plate-fin heat exchanger.

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.

Refrigerant Distribution Characteristics in Vertical Header of Flat-Tube Heat Exchanger without Internal Protrusion

A heat exchanger that arranges flat tubes horizontally has a vertical header that distributes the refrigerant to each tube. When the heat exchanger works as an evaporator, differences in flow conditions at each branch, such as the ratio and distribution of vapor and liquid, due to the differences in densities and momentums of vapor and liquid in the two-phase flow make equal distribution difficult. This paper describes the distribution characteristics of a four-branch header that has a rectangular cross-section without the internal protrusion of flat tubes in the case of the inflow of the refrigerant R32 from the bottom of the header by using an equipment that can estimate the distribution ratio of the liquid and vapor phase to each branch. This paper also discusses the distribution characteristics on the basis of the flow visualization in the header. The flow visualization shows that a liquid level that contains vapor phase exists in the header and affects the distribution greatly.

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.

Exploration and numerical study of new structures for high-efficiency and low-resistance vehicle-mounted plate-fin heat exchanger based on the three-field synergy principle

As the power density of the power transmission device increases,heat exchangers are required to dissipate more heat and provide better flow resistance in the limited space of the vehicle.In this study,the synergy mechanism of the temperature field,pressure field,and velocity field in the serrated fin channel of the plate-fin heat exchanger(PFHX)was thoroughly analyzed under the guidance of the three-field synergy principle.This study also quantitatively revealed the distribution of the angleθbetween the temperature gradient and velocity,and the angleαbetween the pressure gradient and velocity.For the regions in the channel whereθwas too large(i.e.,poor synergy between temperature and velocity fields)andαwas too small(i.e.,poor synergy between pressure and velocity fields),high-efficiency and low-resistance fin structures were proposed.The performance improvement of the new structures was quantified using the comprehensive heat transfer and flow resistance performance evaluation plot in the three-field synergy standard.The results indicate that the new structures improve the synergy of the three fields in the channel.When the air velocity is 15 m s^(-1),the average synergy angleθmbetween the temperature gradient and the velocity of the two structures,changing the inlet flow direction and the slotted fin,decreases from 83.4° to 80.3° and 82.8°,respectively.The outlet temperature increases by 2.3 and 1.6 K,respectively,compared to the basic structure,indicating enhanced the heat transfer of the PFHX.By changing the shape of the fin cross-section,the average synergy angleαmbetween the pressure gradient and the velocity increased from 143.6°to 150.8°,whileθmincreased by only 0.6°.The pressure loss was reduced by 15.2% compared to the basic structure,resulting in a significant decrease in pressure drop while maintaining essentially the same heat transfer performance.Meanwhile,the optimized PFHX can increase the heat transfer rate by 0.2%–8% under identical pump power.This work provides guidance on selecting high-efficiency and low-resistance vehicle-mounted PFHXs.

EXPERIMENTAL AND NUMERICAL ANALYSIS OF LAMINAR AND LOW TURBULENT FLOW DISTRIBUTIONS IN INLET DIVIDING HEADER OF SHELL AND TUBE HEAT EXCHANGER

Flow distribution headers play a major role in heat exchangers.The selection of header diameter,branch pipe diameter,branch pipe spacing etc.is based on the designer＇s experience and general guide lines.The proper selection of the header dimensions will yield uniform flow distribution in heat exchangers,which in turn will enhance the heat exchanger efficiency.In this work,the flow distribution in branch pipes and the pressure variation across the branch pipes in laminar and low turbulence region is studied with two models of the inlet dividing headers.When the numerical analysis has been applied,its inability to predict the no flow condition through the branch pipes is revealed.The results are presented in the form of flow rate ratio through branch pipes and nondimensional coefficients across branch pipes which are useful to apply the existing mathematical models for the present experimental setup.

Numerical simulation of heat transfer and flow of cooling air in triangular wavy fin channels

Numerical computation models of air cooling heat transfer and flow behaviors in triangular wavy fin channels(TWFC) were established with structural parameters of fins considered.The air side properties of heat transfer coefficient and pressure drop are displayed with variable structural parameters of fins and inlet velocities of cooling air.Within the range of simulation,TWFC has the best comprehensive performance when inlet velocity vin=4-10 m/s.Compared with those of straight fins,the simulation results reveal that the triangular wavy fin channels are of higher heat transfer performances especially with the fin structural parameters of fin-height Fh=9.0 mm,fin-pitch Fp=2.5-3.0 mm,fin-wavelength λ=14.0-17.5 mm and fin-wave-amplitude A=1.0-1.2 mm.The correlations of both heat transfer factor and friction factor are presented,and the deviations from the experimental measurements are within 20%.

Computer Implementation of the Passage Arrangement for Plate-Fin Heat Exchangers According to Local Balance Principle

A two-step method for stacking arrangement of passages in multistream plate-fin heat exchanger is proposed. The first step (predict step) is to initialize the arrangement according to a local-balance principle, and the second step (correct step) is to resadust the arrangement according to the results of differential computation of the temperature distribution. The computer implementation process of the local balance principle is described in detail and three examples are illustrated to show the feasibility of this principle. A subroutine program is provided to facilitate the reader to adopt this principle.

微通道换热器两级均液竖直集管设计及两相分配特性

制冷剂两相分配均匀性是影响换热器性能的重要因素。针对微通道换热器采用竖直集管分液效果较差的问题,提出两级均液集管的设计方案,通过两相制冷剂分配测试,对比了不同干度和总流量下,各支路的气液相流量比率。实验结果表明:空集管的顶部支路缺液,底部过液,各支路液相流量的相对标准差(RSD)大于95%;多股流集管通过底部进液腔分配,在多股流通道中向上流动并从侧面出流孔进入环形横隔腔,各支路液相RSD大于60%;在多股流集管中加入芯部整流腔和顶部分配腔后,液相RSD在5%以内,气相RSD在15%以内,分配效果良好。

封头结构对板翅式换热器温度分布特性的影响

在对板翅式换热器内部物流分配不均匀性研究的基础上,研究了各种封头结构对换热器换热性能的影响以及由于换热器内部物流分配不均匀引起换热器效能下降的问题.实验结果表明:不同封头结构对换热器流量分配和温度分布影响很大,改进型的封头结构不仅缩短了换热器从开启到稳定工况的时间,而且使得板束内部截面物流分配和温度分布更加均匀,并且通过实验研究得出了物流分配不均匀度与换热器整体效能变化的关系曲线和关系式.

带有导流翼的板翅式换热器封头结构优化研究

针对板翅式换热器封头内部流场分布不合理和流道流量不均匀分配的现象,提出了一种新型导流翼封头结构的改造方案。采用CFD方法对该封头结构改进前后的流场进行数值计算与分析,证明了导流翼的应用使换热器封头流道的流量分配的均匀性得到很大改善,使得封头内部流场分布更加合理。模拟研究还发现通过改变导流翼片的安装参数可有效地改善换热器封头内部物流分配特性,当导流翼角为70°、翼片顶端安装距离为6 mm时为最佳导流翼片结构安装参数组合,文中原始模型的模拟结果与相关文献中封头的实验结果数据吻合很好,从而验证了该数值模拟研究中物理模型与数值模型的合理性与准确性,此封头结构的研究结论对板翅式换热器的优化设计具有重要意义。

基于模块化的氟塑料换热器优化布置研究

对1 000MW火力发电机组中间热媒体烟气换热器(media gas-gas heater,MGGH)系统中脱硫塔前的氟塑料换热器展开设计计算,从交叉次数的角度对氟塑料换热器模块化以及优化布置展开研究.结果表明:水侧阻力与交叉次数的平方成正比;随着介质交叉次数的增加烟气侧压降降低管外总换热面积、联箱总开孔数和模块单元联箱开孔数均减少,且当交叉次数由2增加到4时上述参数均大幅降低,当交叉次数大于4以后降幅均相对较小,即氟塑料换热器的换热性能、阻力性能及联箱结构在交叉次数为4时得到优化,结合热膨胀因素建议在工程应用中尽量采用2个U形换热模块单元布置成W形.

板翅式换热器封头内部流场的测量与改进研究

利用粒子图像测速仪(PIV),对板翅式换热器封头结构改进前、后的流场进行了可视化研究分析.实验结果表明,原始封头内部的物流分配存在严重的不均匀性,采用添加打孔挡板之后的改进型封头,流场的不均匀性有了很大的改善.从实验对比分析中得出,当挡板小孔呈错排分布、开孔率沿挡板的中轴线向两侧逐渐增加、且总开孔率达到50%左右的时候,在相同的入口条件下,不均匀系数由原始封头的1.210降至0.209,最大流速与最小流速之比由23.163降至1.756.此研究结果对于板翅式换热器的优化设计具有重要意义.

倾斜状态下板翅式换热器封头工质分配特性的实验研究

海上平台受到海浪的作用会发生晃动和倾斜,从而影响板翅式换热器的入口工质分配特性。通过空气—水两相工质分配实验系统,分别进行封头结构水平状态和倾斜状态下的单相以及气液两相实验,获得不同实验工况下的封头工质分配特性,研究倾斜对封头工质分配的影响,从而为换热器的安装及工艺的改进提供依据。结果表明:水平状态下工质分配受惯性力的影响,不均匀度随雷诺数和气液比的增加而增大;倾斜状态下流体受到惯性力和重力的双重影响,工质分配均匀程度显著低于水平状态,倾斜角越大工质分配越不均匀,气液比越大工质分配受倾斜影响程度越大;固定封头或安装换热器时应严格保证其水平/竖直状态,或者通过液化工艺及换热器设计的改进消除倾斜的影响。

错排孔板封头对板翅式换热器流动与换热的影响

在对板翅式换热器内部物流分配研究的基础上,研究了错排孔板型结构封头对板翅式换热器出口流动与换热的影响。通过实验与工业用基本型封头对比,结果表明错排孔板型封头能够明显地改善换热器出口截面流量和温度分布的不均匀性。在Re=2 000时换热器的出口流量分布不均匀度大幅度地下降;而在Re数较大情况下,能更加显著地改善温度分布的不均匀性。

板翅式换热器封头对其流体分配及换热的影响

实验研究了板翅式换热器封头结构对其内部流体分配及换热特性的影响,实验结果表明,目前工业应用的板翅式换热器内部存在流体分配极为不均匀的严重问题,造成了其换热效能的严重下降。研制了各种新型孔板封头结构,并且实验研究了新型封头的各种参数,如孔板长度、开孔的分布规律和小孔直径对板翅式换热器流体分配及换热的影响。研究结果表明,孔板封头从根本上改善了换热器内部流体的分配问题,流体不均匀度从0.208减小到0.035,最大与最小流速比由改进前的2—3倍降低到1.1—1.2倍,温度分布不均匀度从0.826减小到0.601。孔板型封头结构增加了换热器的流动阻力,错排孔板型封头在改善换热器流体分配的同时,也较好地抑制了阻力损失的增加,得到了不同封头结构的流动阻力与雷诺数之间的关系。

封头结构对板翅式换热器流体分配性能的影响

研究了板翅式换热器封头高度与流体分配之间的关系,实验结果说明板翅式换热器的流体分配与封头高度有很大关系,封头高度越高流体分配就越均匀。引入不均匀参数,以便可以对流体在出口截面的分配特性进行研究,并进一步研究封头结构型式不同对于流体分配的影响,研究表明:换热器内部流体的流速的最大值与最小值的比值由3—4减少到1.1—1.2,其不均匀度也从原来的0.331减少到0.046。

不同封头结构对板翅式换热器换热性能的影响

研究了不同封头结构对换热器换热性能参数的影响情况,计算了雷诺数及封头结构不同时换热器的传热系数K、摩擦因数f和火用效率ηe,结果表明,采用孔板型封头可提高传热系数,改善换热效果,进、出口阻力的增加幅度在工业允许范围内。进一步的热力过程分析发现,在实验雷诺数范围内,错排孔板型封头的火用效率提高了近3%。