Advanced Micro-Heat Exchangers for High Heat Flux

Three micro-heat exchangers for use in a liquid cooling system with a long offset strip, short offset strip, and chevron flow path based on the traditional heat transfer enhancement concepts were designed and tested. A straight channel heat exchanger was also made for comparison. The liquid crystal thermography method described by Lin and Yang (2005) was used to observe the flow and temperature distributions in the micro-heat exchangers. The test results show that the chevron channel heat exchanger provides the lowest thermal resistance. However, its pressure drop is also the highest, approximately five times higher than that for other three heat exchangers. The offset strip heat exchangers provide better thermal performance than does the straight channel heat exchanger. The performance of the heat exchanger with the shorter strip is better than that of heat exchanger with longer strip. From the above results, all of the three micro-heat exchangers with conventional heat transfer enhancement showed less thermal resistance than the straight channel heat exchanger. The conventional heat transfer techniques may be effectively applied in the high-flux micro-heat exchanger design.     

A General Matrix Approach to Model Steady-State Performance of Cross-Flow Heat Exchangers

A steady-state performance model of multirow multipass cross-flow tubular heat exchangers is developed. The proposed matrix approach uses the concepts of local effectiveness, energy balance, and number of transfer units (NTU) applied to every pass/row in the cross-flow heat exchanger to predict thermal performance. The method can predict the total effectiveness of assemblies of heat exchangers. Several circuiting configurations, such as overall counter-cross-flow, overall parallel cross-flow, and fluids in parallel in one of the streams, were considered. Predictions of the steady heat transfer performance of selected multirow multipass cross-flow heat exchangers are obtained by applying the general matrix approach. The heat exchanger geometries selected for the comparative study represent common cross-flow heat exchanger configurations used in industry. For these heat exchangers the overall heat exchanger effectiveness values were computed for various capacity rate ratios and NTU values. The validity of the matrix approach was then verified by comparing the resulting predictions with those obtained using the P-NTU approach and the Domingos method for the selected complex cross-flow heat exchanger configurations.     

Numerical model and effectiveness correlations for a run-around heat recovery system with combined counter and cross flow exchangers

A two-dimensional steady-state numerical model is developed to study the heat transfer in a run-around heat recovery system with two exchangers each with a combination of counter and cross (counter/cross) flow between parallel plates or membranes. A finite difference method is used to solve the steady-state equations of continuity, momentum and heat transfer. The simulated values for the effectiveness of each counter/cross flow heat exchanger and the overall run-around system are used to develop effectiveness correlations which agree within ±2% of the simulated effectiveness of individual heat exchangers and overall system. It is shown that the effectiveness of this new run-around heat exchanger (RAHE) falls between the effectiveness of similar run-around systems with either two cross-flow exchangers or two counter-flow exchangers. For a given total surface area of the exchangers, the highest overall sensible effectiveness is achieved with exchangers which have a small exchanger aspect ratio and relatively small solution flow inlet and outlet lengths.     

Compact potential of exhaust heat exchangers for engine waste heat recovery using metal foams

To improve the practicability of the waste heat recovery system for internal combustion engines, the compact potential of exhaust heat exchangers using metal foams is investigated. In the present study, the performance of compact exhaust heat exchangers is compared with that of a conventional shell and tube heat exchanger in a real test bench. Both heat transfer and pressure drop performance are considered when assessing the performance of heat exchangers because these two factors normally show a trade‐off relationship when designing exhaust heat exchangers. Compared with the conventional heat exchanger, the compact heat exchanger can achieve a similar pressure drop, and at the same time the heat transfer is increased by 30%, whereas the volume and the weight are each reduced by 2/3. The performance of compact heat exchangers with six types of Ni metal foams is also investigated under different mass flow rates and thicknesses of the porous layer. Results show that the optimum compact heat exchanger enhances the comprehensive performance 1.9 times compared with original one. This study shows that metal foams have great potential in realizing a compact exhaust heat exchanger for engine waste heat recovery.     

基于Visual Basic 6.O的高温空气换热器热力参数计算软件的开发

基于面向对象和模块化编程思想,以Microsoft Visual Basic 6．0语言为开发平台,结合Access数据库,开发出高温空气换热器热力参数计算软件。该软件涵盖了辐射式、对流式、喷流式三种常用的空气换热器,及上述三种换热器的组合式设计,具有操作简便、适应性广、界面友好的优点,对指导空气换热器的设计具有重要的实际应用价值。     

Online Fouling Detection of Domestic Hot Water Heat Exchangers

Due to hardness of cold water supply in many countries, there is a risk of fouling in domestic hot water (DHW) counterflow plate heat exchangers. The scaling will result in increased resistance to heat transfer, which has negative effects on the economics of the district heating network. A common approach is to clean or change the heat exchanger periodically, which can be expensive if only limited fouling has occurred (unnecessary) or if a higher than expected scaling layer has formed (inefficiency). A better approach is to monitor the state of the heat exchangers and clean them when actually required. This would result in more energy-efficient operation and provide an optimum schedule for heat exchanger cleaning. This can be simple if the heat exchangers are operating under steady-state conditions; however, if large variations in the inlets are experienced, as is the case with the mass flows in DHW heat exchangers, it quickly becomes impossible with standard methods. In this paper it is proposed to monitor the state of the heat exchanger online by using measurements that are easily obtainable under normal operation and applying fast mathematical models to estimate the overall heat transfer coefficient of the heat exchanger. The results show that the methods proposed can be used to detect fouling in DHW heat exchangers.     

Detailed equipment design in heat exchanger networks synthesis and optimisation

In heat exchanger network synthesis, important features like pressure drop and fouling effects are usually neglected. In this work a new methodology is proposed to include these effects in grassroots as in retrofit designs. Heat exchangers are detailed designed during the heat exchanger network synthesis. Pinch analysis is used to obtain the heat exchangers network with the maximum energy recovery, and a new systematic procedure is proposed to the identification and loop breaking. Bell–Delaware method for the shell side is used to design the heat exchangers. An example of the literature was studied and the results show differences between heat exchangers, with and without the detailed design, relative to heat transfer area, fouling and pressure drop. The great contribution of this work is that individual and global heat transfer coefficients are always calculated, in despite of the current literature, where these value are assumed in the design step. Moreover, the methodology proposed to the heat exchangers design assures the minor heat exchanger according to TEMA standards, contributing to the minimisation of the heat exchanger network global annual cost. Finely, the new heat exchanger network considering pressure drops and fouling effects presents values more realistic then those one neglecting the equipment detailed design.     

Different Strategies to Improve Industrial Heat Exchange

A variety of different strategies are available to process and equipment designers to improve industrial heat transfer. These range from the use of efficient forms of individual heat exchangers to the optimum utilization of the individual units in a network, generally referred to as "process integration." This article attempts to review these strategies with reference to the conventional and more recent forms of shell-and-tube heat exchangers. In the context of a heat exchanger network, process heat transfer intensification ( global intensification ), i.e., network design for maximum energy recovery, is a first important step. This needs to be then combined with heat transfer intensification in individual units/shells ( local intensification ). The benefits of global and local intensification are illustrated with examples of a helically baffled heat exchanger ( Helixchanger ), as a representative of a more recent form of shell-and-tube exchanger. Some aspects concerning the use of multistream heat exchangers are discussed, and finally, an example for optimization of a plate-type heat exchanger is presented.     

Effectiveness–NTU Relations for Counterflow Heat Exchangers: The Effect of Kinetic Energy Variation and Heat Leak From Outside

The effectiveness–number of transfer units (NTU) relations are useful data for designing and performance evaluation of heat exchangers with fluids having considerable variation in velocities in the presence of heat leak. In this article, the closed-form (benchmark) solutions for counterflow heat exchangers, when the heat leak is either on the hot or cold side of the heat exchanger in the presence of kinetic energy variation, are presented. It was found that the effectiveness depends on NTU and fluid capacity ratio along with six other dimensionless variables that reflect the magnitude and axial distribution of the kinetic energy and heat leak on the hot and cold sides of the heat exchanger. The results are also presented in a graphical form exhibiting the variation of effectiveness of the heat exchanger with the already-mentioned parameters. It was demonstrated that when the dimensionless heat leak and kinetic energy terms approach zero, the solution reduces to the classical effectiveness–NTU relations for counterflow heat exchangers.     

沉浸式污水换热器的传热模型与验证

沉浸式污水源热泵系统可避免机组或换热器的腐蚀,便于换热器的清洗,已在工程中应用.文章建立了沉浸式污水换热器的传热模型,并通过实验验证了模型的准确性;在污水流量变化的情况下,分别测试了沉浸式换热器在冬、夏季的传热系数.实测结果表明,采用高密度聚乙烯管的沉浸式污水换热器单位长度的传热量约为100 W/m.     

Heat transfer and entropy analysis of three different types of heat exchangers operated with nanofluids

The development of nanotechnology has witnessed an emergence of new generation of heat transfer fluids known as nanofluids. Nanofluids are used as coolants which provide excellent thermal performance in shell and tube heat exchangers. However, the viscosity of these fluids increases with the addition of nanoparticles. Furthermore, the performance of these heat exchangers is influenced by the arrangement of baffles. Thus, in this paper, the study focuses on the heat transfer and entropy analysis of segmental, 25° and 50 helical baffles shell and tube heat exchangers. Heat transfer rate of the 25 helical baffles heat exchanger found to be the highest among the three heat exchangers studied in this research. Study indicates that shell and tube heat exchanger with 50° helical baffles exhibits lowest entropy generation among three different heat exchangers.     

Perfluoropolyethers Coatings Design for Fouling Reduction on Heat Transfer Stainless-Steel Surfaces

The scope of this research is to obtain a film coating on stainless-steel surfaces in order to reduce the interaction between the metal surface and the precipitates, so as to mitigate fouling in heat exchangers. Perfuoropolyethers were used to obtain nano-range fluorinated layers in order to make hydrophobic the stainless-steel surfaces. A pilot plant with two identical heat exchangers was built to investigate the ability of the hydrophobic coating of preventing fouling. The heat exchangers, installed in parallel, operated at the same temperature and pressure conditions, namely, laminar flow regime and inlet flow temperatures of 291–293 K for cold streams and 313–333 K for hot streams. We compared the heat transfer performance of the two heat exchangers. After a 5-month operation, the decrease in the heat transferred was 56% for the coated heat exchanger and 62% for the uncoated heat exchanger. Moreover, the increase of heat transfer resistance due to scale on the uncoated heat exchanger, with respect to the coated one, was three times higher.     

埋地换热器放热工况的现场运行实验研究

结合一地源热泵示范工程，建立了垂直竖井与地基桩单U型管埋地换热器联合使用的地源热泵系统的综合性能实验装置。通过对一个夏季长期间歇运行测试数据的分析，发现地下各个测点的温度上升幅度各不相同；桩埋管的热作用半径至少为2．5m，井埋管的热作用半径至少为2m；在相同的工况条件下，桩埋管的放热率大于井埋管。     

内燃机车热交换器传热强化的研究

对国内现有内燃机车用热交换器的传热机理和结构进行了分析、比较和研究,指出热交换器传热强化应采取的措施,并针对光管热交换器、螺旋扁管热交换器、螺旋隔板热交换器作了性能对比研究,指出螺旋隔板热交换器具有较高的冷却能力。     

A Comparison of Metal-Foam Heat Exchangers to Compact Multilouver Designs for Air-Side Heat Transfer Applications

High-porosity metal foams, with novel thermal, mechanical, electrical, and acoustic properties, are being more widely used in various industrial applications. In this paper, open-cell aluminum foam is considered as a highly compact replacement for conventional louver fins in brazed aluminum heat exchangers. A model based on the ?-N_TU method is developed to compare the flat-tube, serpentine louver-fin heat exchanger to the flat-tube metal-foam heat exchanger. The two heat exchangers are subjected to identical thermal-hydraulic requirements, and volume, mass, and cost of the metal-foam and louver-fin designs are compared. The results show that the same performance is achieved using the metal-foam heat exchanger but a lighter and smaller heat exchanger is required. However, the cost of the metal-foam heat exchanger is currently much higher than that of the louver-fin heat exchanger, because of the high price of metal foams. If the price of metal foam falls to equal that of louver-fin stock (per unit mass), then the metal-foam heat exchanger will be less expensive, smaller, and lighter than the louver-fin heat exchanger, with identical thermal performance.     

Application of parametric identification methods for the analysis of the heat exchanger dynamics

The investigation of heat exchanger dynamics has been carried out. The three kinds of exchangers have been taken into consideration: the low power plate heat exchanger, the high power industrial heat exchanger and the model of heat exchanger. The dynamics of the examined exchangers has been analyzed using the parametric identification methods and their implementation in MATLAB. The character of observed phenomenon is not unique. In some of the cases it shows inertial character, in the others oscillatory character. It might indicate an appearing of the kind of a thermal oscillation during the non-stationary states of the heat exchanger.     

Comparing exergy losses and evaluating the potential of catalyst-filled plate-fin and spiral-wound heat exchangers in a large-scale Claude hydrogen liquefaction process

Detailed heat exchanger designs are determined by matching intermediate temperatures in a large-scale Claude refrigeration process for liquefaction of hydrogen with a capacity of 125 tons/day. A comparison is made of catalyst filled plate-fin and spiral-wound heat exchangers by use of a flexible and robust modeling framework for multi-stream heat exchangers that incorporates conversion of ortho-to para-hydrogen in the hydrogen feed stream, accurate thermophysical models and a distributed resolution of all streams and wall temperatures. Maps of the local exergy destruction in the heat exchangers are presented, which enable the identification of several avenues to improve their performances.The heat exchanger duties vary between 1 and 31 MW and their second law energy efficiencies vary between 72.3% and 96.6%. Due to geometrical constraints imposed by the heat exchanger manufacturers, it is necessary to employ between one to four parallel plate-fin heat exchanger modules, while it is possible to use single modules in series for the spiral-wound heat exchangers. Due to the lower surface density and heat transfer coefficients in the spiral-wound heat exchangers, their weights are 2–14 times higher than those of the plate-fin heat exchangers.In the first heat exchanger, hydrogen feed gas is cooled from ambient temperature to about 120 K by use of a single mixed refrigerant cycle. Here, most of the exergy destruction occurs when the high-pressure mixed refrigerant enters the single-phase regime. A dual mixed refrigerant or a cascade process holds the potential to remove a large part of this exergy destruction and improve the efficiency. In many of the heat exchangers, uneven local exergy destruction reveals a potential for further optimization of geometrical parameters, in combination with process parameters and constraints.The framework presented makes it possible to compare different sources of exergy destruction on equal terms and enables a qualified specification on the maximum allowed pressure drops in the streams. The mole fraction of para-hydrogen is significantly closer to the equilibrium composition through the entire process for the spiral-wound heat exchangers due to the longer residence time. This reduces the exergy destruction from the conversion of ortho-hydrogen and results in a higher outlet mole fraction of para-hydrogen from the process.Because of the higher surface densities of the plate-fin heat exchangers, they are the preferred technology for hydrogen liquefaction, unless a higher conversion to heat exchange ratio is desired.     

换热技术从大型化向微小化的发展

在分析传统换热器结构特点的基础上,具体分析了目前常用紧凑式换热器的结构、性能及应用情况,阐述了微型化学（化工）机械系统的发展及应用情况。以氨冷器为例,分别进行了板翅式换热器和另一种高效的小型换热装置——热管换热器的设计,表明板翅式换热器具有更加高效的换热效率和紧凑的结构,由此综合说明了换热技术从大型化向紧凑化、微小化发展的必然趋势。进一步以典型的微型化换热器装置——微通道换热器、微通道蒸发器和微通道加热器为例,具体分析了微型换热技术广阔的应用前号。     

Heat exchanger optimization for geothermal district heating systems: A fuel saving approach

One of the most commonly used heating devices in geothermal systems is the heat exchanger. The output conditions of heat exchangers are based on several parameters. The heat transfer area is one of the most important parameters for heat exchangers in terms of economics. Although there are a lot of methods to optimize heat exchangers, the method described here is a fairly easy approach. In this paper, a counter flow heat exchanger of geothermal district heating system is considered and optimum design values, which provide maximum annual net profit, for the considered heating system are found according to fuel savings. Performance of the heat exchanger is also calculated. In the analysis, since some values are affected by local conditions, Turkey's conditions are considered.     

Performance Comparison of Rectangular and Air-Foil Shapes Offset Strip Flow Paths Micro-Heat Exchangers

This study designed and tested eight micro-heat exchangers with rectangle-, air-foil-, and shuttle-type strips for use in the liquid cooling system. The effects of strip length, strip type, and strip arrangement were considered for heat transfer performance comparison. The test results show that the heat exchangers with shorter strip length and narrower strip space provide better heat transfer performance. The short air-foil strips heat exchanger with 1.0 mm strip length performed the lowest thermal resistance among all types of heat exchangers. Because of its narrow flow paths, the performance of the overlapped shuttle strip heat exchanger is better than that of the offset shuttle, long air-foil, and rectangle strip heat exchangers. However, the space between strips is limited by the fabrication techniques and is difficult to be made narrower by the method of chemical etching.