Enhanced static evaporation heat transfer surfaces for compact two-phase heat exchangers

In the frame of the JOULE 1 R&D programme of the Commission of the European Communities this project has been carried out on enhanced evaporation heat transfer surfaces. The main goal of the project was to develop and investigate enhanced evaporation heat transfer surfaces for industrial compact two-phase heat exchangers. Planar and tubular surfaces have been investigated by IKE (structured planar and tubular surfaces), GRETh (covered planar surfaces) and NEL (covered tubular surfaces).     

紧凑式换热器均布性研究进展

紧凑式换热器具有结构紧凑、换热效率高的特点,广泛地应用于多种工业场合。综述了不同换热器内流体和温度的不均匀分布、不均匀性对不同传热设备的影响、换热器入口处流体的分布和流型对换热器性能的影响。指出入口封头和分布器的设计对换热器性能具有重要影响,可通过CFD和实验相结合对其进行模拟计算。     

紧凑式低温换热器的发展概况

介绍了一些目前低温与制冷领域中使用的紧凑式换热器,对其特点、性能以及发展动态和研究方向进行了细致的分析和综述。同时也给出了一些观点,为低温换热器的设计及以后的深入研究提供参考。     

紧凑式低温换热器研究进展

介绍了紧凑式低温换热器的研究进展,对低温换热器传热机理研究中所存在的问题以及新型低温换热器的研制进行了详细分析,对现有换热器优化设计方法进行了比较,给出了作者的一些观点,为低温换热器的设计及以后的进一步研究提供参考。     

板翅式换热器物流分配研究综述

板翅式换热器由于具有紧凑高效等突出优点,在航空、化工、HVAC等领域具有重要地位。由多种因素引起换热器内部流动的不均匀分配,是造成换热效能的下降最主要原因之一。文中总结了国内外学者对于板翅式换热器物流分配不均匀的研究,详细阐述了各种流动分配模型及其对换热器效能的影响。     

Experimental studies of influence of the turbulent flow structure on temperature distribution in a compact heat exchanger

Results of experimental investigation of temperature distribution over the surface of a complex heat exchanger (the Frenkel packing type) are presented. Measurements were carried out in the air flow between two sheets with triangular corrugations directed at 90° to each other. Measurements were carried out by the microthermocouples glued on the heated outer surface. The effect of Reynolds numbers, a gap between corrugated sheets, and substitution of one corrugated sheet by the smooth one on temperature distribution over the heat exchanger surface in the turbulent air flow is analysed. According to the performed experiments, there is a significant effect of a gap and applied perturbations on the type of temperature distribution over the perimeter of a heated cell.     

换热器对氦液化流程的影响及其热力学分析

采用Matlab编程实现对氦液化流程的模拟,在验证模型准确性的基础上,针对流程中换热器的有效度对流程性能的影响进行了详尽的研究和分析。此外还分析得到了所有换热器大小对液化率的不同影响作用,并且从热力学角度探讨了其原因,所得结果对大型氦液化器中换热器的选择和面积分配具有指导意义。     

Performance of shell-and-dimpled-tube heat exchangers for waste heat recovery

This paper presents an experimental study of waste heat recovery shell-and-tube heat exchangers. The exchanger heat duty, overall heat transfer coefficient, effectiveness and tubeside friction factor are investigated as functions of the tube surface geometry (plain or dimpled), the flow type (counter or parallel), the tube Reynolds number and the shellside heat capacity rate. Water and the exhaust gases of a Diesel engine are passed inside the tube and the shell, respectively.The heat transfer characteristics increase with an increase in tube Reynolds number and the shellside heat capacity rate, for all the flow types and the surface geometries examined. The counter-flow, shell-and-dimpled-tube heat exchanger, compared with that exchanger having a plain tube, increases the heat duty and the overall heat transfer coefficient by 80%, and the heat exchanger -effectiveness increases by 35%. For the parallel-flow, shell-and-dimpled-tube heat exchanger, the heat duty, the overall heat transfer coefficient and the effectiveness increase by 30, 55, and 25%, respectively. At the same time the dimpled tube increases the tubeside friction factor by 600% over that of the plain tube. The rate of waste heat recovered from the exhaust gases of the Diesel engine by the counter-flow, shell-and-dimpled-tube heat exchanger is equal to 10% of the maximum brake power of the engine running at 1500 rpm, and the tube Reynolds number equal to 8875.     

Numerical research of stream analysis on helical baffles heat exchangers

In this paper, the numerical simulations for a helical baffles heat exchanger and a segmental baffles heat exchanger with component clearance are performed to reveal the features of leakage streams and their effect on heat exchanger performance. Helical baffles heat exchanger models with four different angles and segmental baffles heat exchanger model were established and calculated via Gambit and Fluent software. The results reveal that the heat exchanger with a 40° helix angle shows the best comprehensive heat transfer performance in turbulent state, and the heat exchanger with a 50° helix angle shows better comprehensive heat transfer performance in laminar flow state. The leakage streams proportion of the helical baffles heat exchanger varies from 5.5% to 6.1%, compared with the leakage streams proportion changes from 16.6% to 21.0% in the segmental baffles heat exchanger. In both turbulent flow state and laminar flow state, with the rise of shell-side Reynolds number, the main spiral stream B proportion decreases and the leakage streams proportion increases in the segmental baffles heat exchanger, while the stream B proportion increases and the leakage streams proportion decreases in helical baffles heat exchanger. The proportion of stream B increases with the increase of the helix angle β and the proportions of leakage streams decrease with the increase of β. The proportion of the tube-baffle leakage stream A increases in radial direction. Both the proportion of stream A and that of the baffle-shell leakage stream E fluctuate in the intermediate zone in axial direction; the stream A proportion decreases at the inlet and outlet zone, while the stream E proportion increases at the inlet and decreases at the outlet zone. The results of this paper could be of great significance in the optimal design and manufacture of the helical baffles heat exchanger.     

Effect of inlet flow distortions on fan performance in forced draught air-cooled heat exchangers

Forced flow air-cooled heat exchangers (ACHEs) as found in the petro-chemical, process and power industries use fans arranged in single or multiple fan rows to force air over finned tube bundles. Any flow disturvances or distortions experienced at the inlets of these fans tend to reduce the effectiveness of the ACHE.The reduction of the air flow rate through ACHE models having different numbers of fan rows is investigated experimentally. By varying the distance between the fan platform and the ground level, significant changes in air flow rate are observed and the results are correlated by an empirical relation which can be applied to the design of ACHEs. Fans along the edge of an ACHE are most affected by the distorted inlet air flow pattern. Furthermore, the influence of a walkway along the edge of the fan platform on the air flow rate through an ACHE is also investigated.     

基于有限时间热力学的换热器构型优化研究

针对逆流换热器的换热过程,以(火积)耗散最小为优化目标,推导出冷、热流体温度的最佳分布;进而在满足最佳温度分布的前提下,得到了最小(火积)耗散、最小热阻、(火积)传递效率与总传热单元数的关系.引入数值算例,结果显示随着总传热单元数的增加,换热过程不可逆损耗减小,但减小速率也越来越小.同时也证明了(火积)理论比熵理论更适...     

The use of polymers in heat exchangers

A variety of heat exchangers constructed using polymers are available. They can offer good corrosion resistance, and in some cases cost benefits. This paper discusses the types and properties of polymer materials available, types of heat exchangers in which they are used, and possible future trends.     

Determination of temperature distribution in laser welded heat exchangers with the aid of finite element methods

The determination of the thermal histories in a tube-plate combination when welded with a high intensity heat source is discussed in this paper. The influence of the geometry of the plate and the use of nitrogen gas during the welding process on the thermal histories is shown. With the thermal histories known, the distortion due to thermal stresses could be determined.     

Relationships between temperature differences in heat exchangers of heat transformation devices

The temperatures which are present in a heat transformation device play a very important part: at first, the temperatures determine the maximum performance or efficiency of the cycle via the first and second laws of thermodynamics. Secondly, the temperatures determine the heat transfer area which is required to put a given heat flux through the system. Consequently, they relate power to investment cost. In order to elaborate further on these interdependencies, in this paper basic relationships between technically and thermodynamically relevant temperatures, as they are present in the heat exchangers, are being derived. To this end, we will define several temperature differences as usual: the temperature glide, the driving mean temperature difference and the thermodynamic or entropic mean temperature difference. The logarithmic temperature mean is significant for determining the heat transfer. It will be shown that, as long as the temperature gradient between external and internal fluids is larger than the difference in glide of both fluids, the log-mean can be substituted by the difference of the arithmetic mean temperatures. Consequently, it is almost identical to the entropic temperature difference. The entropic temperature difference is a measure of efficiency whereas the logarithmic temperature difference is a measure of first cost. As both temperature differences deviate only marginally from each other in most technical applications it will easily be possible to establish a relationship between performance and investment.     

Thermal behavior of heat exchangers in off-design conditions

An analytical method has been developed to predict the thermal behavior of an exchanger when a deviation occurs between the design point and the actual working conditions. As long as the P_R, P_ν curves of a particular exchanger are available, the method makes it possible to estimate the exchanger characteristic parameters at the modified working state. Moreover, at the design stage, the use of exchanger effectiveness gradient vector concept provides a basis for comparing the stability of several proposed exchangers under variable load conditions. Sample problems illustrate the application of the method and the use of presented charts.     

Performance of a compact,spiral coil heat exchanger

A spiral coil heat exchanger consists of a number of horizontal layers of spirally wound, finned tubes connected to vertical manifolds at the inner and outermost turns of each coil. This design has advantages in heat recovery and air-conditioning applications. Two theoretical models to predict the performance of this compact heat exchanger are presented based upon unmixed and mixed air-flow considerations. Results of experimental studies on a laboratory model of the spiral coil heat exchanger agree well with the predicted performance of the theoretical models. Charts of effectiveness vs NTU suitable for design are presented.