Heat transfer analysis of fin-and-tube heat exchangers with flat and louvered fin geometries

This paper analyzes the fluid flow and heat exchange on the air side of a multi-row fin-and-tube heat exchanger. A comparison is given between fin-and-tube heat exchanger characteristics with flat and louvered fins in a wider range of operating conditions defined by Reynolds number (based on fin spacing and air frontal velocities). The detailed representation of calculated data for the louvered heat exchanger shows significantly better heat transfer characteristics and a slightly higher pressure drop. The CFD procedure was validated by comparing the numerical simulation results with the experimental results showing the minimal average Nusselt number deviation and an almost perfectly corresponding pressure drop.     

Heat transfer characteristics of flat plate finned-tube heat exchangers with large fin pitch

The objective of this study is to provide experimental data that can be used in the optimal design of flat plate finned-tube heat exchangers with large fin pitch. In this study, 22 heat exchangers were tested with a variation of fin pitch, number of tube row, and tube alignment. The air-side heat transfer coefficient decreased with a reduction of the fin pitch and an increase of the number of tube row. The reduction in the heat transfer coefficient of the four-row heat exchanger coil was approximately 10% as the fin pitch decreased from 15.0 to 7.5 mm over the Reynolds number range of 500–900 that was calculated based on the tube diameter. For all fin pitches, the heat transfer coefficient decreased as the number of tube row increased from 1 to 4. The staggered tube alignment improved heat transfer performance more than 10% compared to the inline tube alignment. A heat transfer correlation was developed from the measured data for flat plate finned-tubes with large fin pitch. The correlation yielded good predictions of the measured data with mean deviations of 3.8 and 6.2% for the inline and staggered tube alignment, respectively.     

小管径椭圆管开缝翅片换热器的数值模拟

对小管径椭圆管开缝翅片换热器空气侧的流动与传热特性进行数值模拟,对影响其换热性能的2个主要参数椭圆管偏心率和开缝翅片开缝错列高度分布进行优化,与传统管翅式换热器换热性能进行比较。模拟结果表明:当椭圆管两轴之比Rx:Ry=2:3(偏心率),开缝高度分布为0.8 mm,0.6 mm和0.4 mm时,换热效果最好。与传统管翅式换热器相比,小管径椭圆管开缝翅片换热器换热系数提高10%~20%,而压降几乎相等,总体换热性能提高。     

Transition from nucleate boiling to convective boiling in compact heat exchangers

The boiling of pure fluids has been experimentally studied in several types of compact heat exchanger channels. Plate fin and corrugated heat exchangers have been studied (seven geometries). Controlling the flow parameters (mass flux and vapour quality), the heat flux and measuring the wall temperature, have allowed characterization of the local heat transfer coefficient.

The results clearly show that the dominant mechanisms occurring could be nucleate or convective boiling. The transition between these two mechanisms depends on the flow characteristics and also on the channel geometry. Based on these measurements, an objective criteria can be established to identify the flow boiling regime. The knowledge of such a criteria is useful if we want to extend the use of compact heat exchanger to boiling of mixtures.     

Evaporation heat transfer and pressure drop of ammonia in a mixed configuration chevron plate heat exchanger

Ammonia is a naturally occurring environment friendly refrigerant with attractive thermo-physical properties. Experimental investigation of heat transfer and pressure drop during steady state evaporation of ammonia in a commercial plate heat exchanger has been carried out for an un-symmetric 30°/60° chevron plate configuration. Experiments were conducted for saturation temperatures ranging from −25 °C to −2 °C. The heat flux was varied between 21 kW m⁻² and 44 kW m⁻². Experimental results show significant effect of saturation temperature, heat flux and exit vapor quality on heat transfer coefficient and pressure drop. Current mixed plate configuration data are compared with previous studies on the same heat exchanger with symmetric plate configurations. This comparison highlighted importance of optimization in selection of the heat exchangers. Correlations for two phase Nusselt number and friction factor for each chevron plate configuration considered are developed. A Nusselt number correlation generalized for a range of chevron angles is also proposed.     

Air-side thermal hydraulic performance of multi-louvered fin aluminum heat exchangers

An experimental study on the air-side heat transfer and pressure drop characteristics for multi-louvered fin and flat tube heat exchangers has been performed. For 45 heat exchangers with different louver angles (15–29°), fin pitches (1.0, 1.2, 1.4 mm) and flow depths (16, 20, 24 mm), a series of tests were conducted for the air-side Reynolds numbers of 100–600, at a constant tube-side water flow rate of 0.32 m³/h. The inlet temperatures of the air and water for heat exchangers were 21 and 45°C, respectively. The air-side thermal performance data were analyzed using effectiveness-NTU method for cross-flow heat exchanger with both fluid unmixed conditions. The heat transfer coefficient and pressure drop data for heat exchangers with different geometrical configurations were reported in terms of Colburn j-factor and Fanning friction factor f, as functions of Reynolds number based on louver pitch. The general correlations for j and f factors are developed and compared to other correlations. The f correlation indicates that the flow depth is one of the important parameters for the pressure drop.     

An experimental investigation of heat transfer and friction losses of interrupted and wavy fins for fin-and-tube heat exchangers

The present paper discusses the results of an extensive investigation about the performance of various fin configurations, carried out in the Luve Contardo experimental facilities and aimed to enhance the heat transfer capabilities of air-cooled condensers and liquid coolers. Test results here discussed are relative to 15 coil prototypes, having the same tube and fin geometry (25×21.65 mm staggered 5/8” tube banks, 2 mm fin spacing) but different fin surface geometry, from flat to wavy to louvered to “winglet”. Different rates of heat transfer and pressure loss enhancement were obtained, also depending on the quality of the pressing process. General approaches to evaluate the “goodness” of one fin design with respect to another one provided questionable results: pressure loss influence on the air flow cannot be properly evaluated unless the actual fan head curve and the coil dimensions (front area and rows number) are stipulated. The performance of air-cooled condensers was therefore predicted and compared, for various fin design and for coil arrangements of practical interest. The type of fin adopted strongly influences the heat exchanger performance and louvered fins generally provide the best results.     

整体翅片式微通道换热器的数值模拟

采用CFD数值模拟的方法,对整体翅片式微通道换热器空气侧的流动与传热特性进行研究,用场协同的原理分析翅片参数(长度、高度、间距)对换热器性能的影响,并与传统微通道换热器空气侧性能进行对比。结果表明:在考查的翅片尺寸范围内,随着翅片长度的缩短、高度的增加、间距的减小,温度场和速度场的协同性随之改善,有利于换热效果的提高;与传统的微通道换热器相比,整体翅片式微通道换热器空气侧换热性能变差,但是阻力大大降低,总体性能有所改善。     

Assessment of boiling heat transfer correlations in the modelling of fin and tube heat exchangers

A new way to assess the performance of refrigeration system models is presented in this paper, based on the estimation of cycle parameters, such as the evaporation temperature which will determine the validity of the method. This paper is the first of a series which will also study the influence of the heat transfer coefficient models on the estimation of the refrigeration cycle parameters. It focuses on fin and tube evaporators and includes the dehumidification process of humid air. The flow through the heat exchanger is considered to be steady and the refrigerant flow inside the tubes is considered one-dimensional. The evaporator model is discretised in cells where 1D mass, momentum and energy conservation equations are solved by using an iterative procedure called SEWTLE. This procedure is based on decoupling the calculation of the fluid flows from each other assuming that the tube temperature field is known at each fluid iteration. Special attention is paid to the correlations utilised for the evaluation of heat transfer coefficients as well as the friction factor on the air and on the refrigerant side. A comparison between calculated values and measured results is made on the basis of the evaporation temperature. The experimental results used in this work correspond to an air-to-water heat pump and have been obtained by using R-22 and R-290 as refrigerants.     

Effects of fin shape on condensation in herringbone microfin tubes

Effects of fin height and helix angle on condensation inside a herringbone microfin tube have been experimentally investigated with five types of herringbone microfin tubes. Heat transfer coefficients are about 2–4 times higher than that of the helical microfin tube under high mass velocity conditions. In the low mass velocity, they are equal to that of the helical microfin tube. The heat transfer enhancement increases with fin height up to 0.18 mm; higher fin heights show enhancement values similar to the 0.18 mm results. Pressure drop increases with the fin height. Larger helix angle yields higher heat transfer and higher pressure drop. For the lowest fin and/or smallest helix angle, the pressure drop is comparable with that of the helical microfin tube, while the heat transfer enhancement is higher. The enhancement mechanism is discussed from flow pattern observations. Effect of mass transfer resistance for R410A is estimated and negligible effects have been proved.     

Efficiency and optimization of an annular fin with combined heat and mass transfer – An analytical solution

An analysis was carried out to study the efficiency of annular fin when subjected to simultaneous heat and mass transfer mechanisms. The temperature and humidity ratio differences are the driving forces for the heat and mass transfer, respectively. Analytical solutions are obtained for the temperature distribution over the fin surface when the fin is fully wet. The effect of the atmospheric pressure on the fin efficiency was also studied, in addition to fin optimum dimensions. It is demonstrated that the closed-form solutions for a dry-fin case presented in many text books are special cases for the solutions presented in this paper.     

Characterisation of the hydraulic maldistribution in a heat exchanger by local measurement of convective heat transfer coefficients using infrared thermography

A methodology was developed to characterise the heat exchangers' performance decrease due to two-phase flow maldistribution. It consists in measuring the spatial distribution of the local heat transfer coefficients with a rapid, non-invasive and fluid independent method. The method is based on the infrared (IR) thermography measurement of the temperature response to an oscillating heat flux. The amplitude of the measured temperatures is compared to the solution of an analytical model. The problem is solved iteratively to obtain the heat transfer coefficients. This method has been applied to evaluate the uneven phase distribution of an air–water mixture in a compact heat exchanger. The exchanger is composed of seven multiport flat tubes, a vertical downward header and horizontal channels. Experiments were performed for mass flux from 29 kg m⁻² s⁻¹ to 116 kg m⁻² s⁻¹ and for quality from 0.10 to 0.70.     

Theoretical analysis of the thermal performance of a plate heat exchanger at various chevron angles using lithium bromide solution with nanofluid

Nanofluids technology has been rapidly developing over the last two decades. In this paper, the performance of a lithium bromide (LiBr) solution with and without nanoparticles in plate heat exchanger (PHE) for various chevron angles and mass flow rates was investigated. As a result, the heat transfer rate and the overall heat transfer coefficient in 60°/60° PHE is over 100% higher than that of 30°/30° PHE, and the effectiveness of the PHE in 60°/60° PHE is about 70% higher than that of 30°/30° PHE. By using nanoparticle in the working fluid, the heat transfer performance can increase significantly. The heat transfer rate of 3 vol.% nanofluids increased about 3–8% compare to that of LiBr solution for all chevron PHEs. Besides, the 60°/60° PHE using 3 vol.% nanofluids produced the largest heat transfer rate and heat exchange effectiveness under given operating conditions.     

平翅片传热与流动特性的数值模拟

利用计算流体力学软件Fluent研究翅片管换热器的换热和压降特性。使用流固耦合准确确定翅片及管内外侧的传热边界条件，并利用场协同理论进行分析。将计算出的结果与试验结果进行比较。结果表明，数值模拟与实验结果吻合较好，数值计算可为换热器的结构优化和产品的研究开发提供依据。     

Ammonia evaporation in a mixed configuration chevron plate heat exchanger with and without miscible oil

An experimental study was conducted to determine the effects of miscible lubricant oil on evaporation of ammonia in a vertical chevron plate heat exchanger. The heat exchanger was configured in a U-type counter flow arrangement with mixed (30°/60°) chevron plate configuration. Experiments were carried out for four saturation temperatures ranging from −25 °C to −2 °C for a fixed ammonia mass flux rate of 6.5 kg m⁻² s⁻¹ and over a range of heat flux levels resulting in a vapor quality at the heat exchanger exit ranging between 0.5 and 0.9. For a given saturation temperature, experiments were performed for 0%, 3%, 6% and 9% oil concentrations, by volume in ammonia. The oil concentration, exit vapor quality, heat flux and saturation temperature were found to have significant effects on the heat transfer coefficient and pressure drop of ammonia. Based on the experimental data, correlations to estimate two phase Nusselt number and friction factor, generalized for the whole range of oil concentration have been presented.     

Performance of finned-tube heat exchangers under frosting conditions: I. Simulation model

This paper describes an analytical model that was developed to predict the performance of finned-tube heat exchangers under frosting conditions. The method models the frost growth mechanism and heat exchanger performance in a comprehensive manner. The results include frost accumulation and its effect on energy transfer in relation to varying humidities, fin densities and ambient conditions.     

A study on airside performance of geometry combination fins using large scale model

This study investigated the pressure drop and heat transfer characteristics of heat exchanger according to the arrangement of fins as well as fin configuration by the similitude experiments with the finned-tube geometry scaled as large as four times. Colburn j factor, Fanning friction factor, f, and goodness factors are compared to each other to estimate performance of each case for four different kinds of fins, which are louver, double side slit, single side slit and plain fin. Results show that heat transfer would be altered by fin arrangement and that friction loss is more affected by fin configurations than by the fin arrangements. In particular, heat transfer depends more on the shape of front row than that of rear row. The heat transfer rate of combined fin arrangement increases a lot more under the same pressure drop than that of conventional fin arrangement. This indicates that the heat exchanger of higher efficiency would be designed by the proper combination of fins of different shapes.     

单一金属高螺纹翅片管换热器的探索与实践

通过对管壳式换热器几种常见翅片管的结构特点、换热性能和制造经济性进行分析比较和研究,研制出传热效率更高、结构更紧凑、制造成本更低廉的新型换热元件——单一金属轧制的高螺纹翅片管;提出了传热学意义上最佳翅片截面轮廓线的通用解析式。通过生产实践与不断改进,形成了一组高螺纹翅片管系列产品,并成功地应用于为透平压缩机配套的大中型管壳式换热器。     

Evaluation of thermal contact conductance using a new experimental-numerical method in fin-tube heat exchangers

In a fin-tube heat exchanger the contact between fin collar and tube surface is obtained through mechanical expansion of tubes. Since the interfaces between the tubes and fins consist partially of metal-to-metal contact and partially of air, the features of heat transfer through the contact interfaces have not been fully investigated. The present study aims at the development of a new tool including an experiment and a numerical calculation for the estimation of the thermal contact conductance between the fin collar and tube surface, and pursues the evaluation of the factors affecting the thermal contact conductance in a fin-tube heat exchanger. Heat exchangers fabricated for the current study have been put to the test for heat balance in a vacuum chamber with water as an internal fluid. And a finite difference numerical scheme has been used for the data reduction of the experimental data to evaluate the thermal contact conductance. Fin-tube heat exchangers employed in the current research are of tube diameter of 7 mm with different tube expansion ratios, fin spacings, and fin types. The results of the present study imply that these parameters as well as hydrophilic fin coating have a significant effect on the thermal contact conductance. It has been discovered that the portion of the thermal contact resistance is not negligible compared with the total thermal resistance in a fin-tube heat exchanger, and this means that in order to reduce the thermal contact resistance thoughtful care should be taken in fabricating heat exchangers.     

铜翅片换热器开发应用的分析Ⅱ--铜翅片换热器对空调器性能影响分析

在Part Ⅰ中对铜翅片效率、铜换热单元传热系数以及整个铜换热器换热特性分析的基础上，理论分析了空调换热器采用铜翅片后，换热器单体性能以及空调系统性能的变化。结果显示：采用铜片铜管换热器后(不论是蒸发器，还是冷凝器)，制冷系统的制冷系数较之铝翅片换热器均有提高。蒸发器换热性能的变化比冷凝器换热性能的变化对系统性能的影响要大。当同时采用铜片蒸发器和铜片冷凝器时，传热强化系数按照通常所能达到的1．1计算时，COP随着工况变化而变化的范围可以达到3．0％-10．0％；如果单独设计换热器使换热强化系数达到1．3时，则COP的提高可达4．0％～25．0％。以上对铜翅片换热器开发应用的分析，为空调换热器的高性能化、小型化提供一条可行的发展方向。