Experimental study and modelling of heat transfer during condensation of pure fluid and binary mixture on a bundle of horizontal finned tubes

An experimental investigation was conducted to measure the local heat transfer coefficient for each row in a trapezoidal finned horizontal tube bundle during condensation of both pure fluid (HFC 134a) and several compositions of the non-azeotropic binary mixture HFC 23/HFC 134a. The test section is a 13×3 (rows × columns) tube bundle and the heat transfer coefficient is measured using the modified Wilson plot method. The inlet vapour temperature is fixed at 40 °C and the water flow rate in each active row ranges from 170 to 600 l/h. The test series cover five different finned tubes all commercially available, K11 (11 fins/inch), K19 (19 fins/inch), K26 (26 fins/inch), K32 (32 fins/inch), K40 (40 fins/inch) and their performances were compared. The experimental results were checked against available models predicting the heat transfer coefficient during condensation of pure fluids on banks of finned tubes. Modelling of heat exchange during condensation of binary mixtures on bundles of finned tubes based on the curve condensation model is presented.     

Performance characteristics correlation for round tube and plate finned heat exchangers: Equations relatives aux performances d'échangeurs de chaleur constitués de tubes ronds et de plaques à ailettes

A number of correlation equations describing the performance characteristics of round tube and plate fin have been published in the open literature. However, many of these correlations are restricted to flat finned heat exchangers and a limited number of geometrical configurations. In this study, 28 heat exchanger samples were tested in an open circuit thermal wind tunnel over a velocity range of 1 to 20 m/s for a number of geometries. The geometrical variations include the number of tube rows, fin thickness and the spacing between fins, rows and tubes. Both flat and corrugated fins were tested and the results were correlated in terms of j and f factors as a function of Reynolds number and the geometrical parameters of the heat exchangers. An important feature of this correlation is the novel way in which the geometric parameters are expressed in the correlation. Ratios of these parameters are derived from consideration of the physics of the flow and heat transfer in the heat exchangers. This results in a more accurate and physically meaningful correlation which can be applied to a broader range of geometries. The correlation was validated against test data in the literature for round tube and plate fin with good agreement. It was found that the fin type affects the heat transfer and friction factor, and that the number of tube rows has a negligible effect on the friction factor. The number of tube rows effect was found to be influenced by the fin and tube geometries as well as the Reynolds number.Un certain nombre d'équations pour des caractéristiques du rendement des échangeurs de chaleur à tubes ronds plaques à ailettes ont été publiés dans le littérature. Cependant, dans bien des cas, ces corrélations se limitent aux échangeurs à ailette plate dans un nombre limité de configurations géométriques. Dans cette étude, 28 échangeurs de chaleur ont été testés utilisant une soufflerie à circuit ouvert avec une vitesse d'air de 1 à 20 m/s pour plusieurs formes géométriques. Les variations géométriques portaient sur le nombre de rangées de tubes, l'épaisseur des ailettes et la distance séparant des ailettes, des rangées et des tubes. Les ailettes plates et ondulées ont été testées et les corrélations en termes de facteurs j et f en fonction du nombre de Reynolds et les paramètres géométriques des échangeurs de chaleur. Un aspect important de cette corrélation est le façon originale d'exprimer des paramètres géométriques. Les rapports de ces paramètres sont obtenus à partir des flux et transferts de chaleur dans des échangeurs de chaleur. Ce procedé permet d'obtenir une corrélation plus précise et utilé qui s'applique à une gamme de formes géomátriques plus large. La corrélation a été validée en fonction des données concernant des échangeurs à tube et à plaque à ailettes dans la littérature: les données expérimentales et théoriques concordent bien. On a montré que le type d'ailette exerce une influence sur le transfert de chaleur et le facteur de frottement. Cependant, le nombre de rangées de tubes a un effet negligeable sur le coéfficient de frottement. On a démontré que l'effet nombre de rangées de tube est influencé par les géométries des ailettes et des tubes ainsi que par le nombre de Reynolds.     

Effects of thermal conduction in microchannel gas coolers for carbon dioxide

A numerical code was developed for an accurate fully three-dimensional simulation of crossflow compact heat exchangers using finned flat tubes with internal microchannels; such components are often employed as gas coolers in transcritical refrigerating machines CO₂ operated. The equations describing the system were discretised by means of a finite-volume and finite-element hybrid technique for strictly adhering to the real heat transfer process regarding the finned surfaces. The numerical code uses recent correlations by different authors for predicting the heat transfer coefficients both refrigerant side and air side. The results of simulations are verified against experimental data reported in the open literature.The aim of this work is to investigate the effects of thermal conduction inside metal on the overall performance of the considered gas coolers; high-resolution meshes for the discretisation of separating wall and fins makes it possible to avoid much of the approximations typical of the traditional approaches. In particular the efficiency of finned surfaces, the real distribution of thermal fluxes between the two fin roots and the effects of thermal conduction along the walls of microchannel flat tubes are extensively discussed.The numerical simulations confirm that the traditional approach for describing fins, which assumes them as adiabatic at the middle section in order to decouple the equations accounting for the effect of different temperatures at the fin roots, can be considered acceptable in a wide range of applications. In a similar way, the conduction on fins along the direction of the air velocity and the longitudinal conduction on tubes produce a negligible effect on the performance of the considered class of heat exchangers.     

Prediction of heat transfer coefficient during condensation of water and R-134a on single horizontal integral-fin tubes

This paper presents a few salient features of an investigation carried out to study the heat transfer augmentation during condensation of water and R-134a vapor on horizontal integral-fin tubes. The experimental investigation was performed on two different experimental set-ups for water and R-134a. The test-sections were manufactured by machining fins over plain copper tubes of 24.4 ± 0.6 mm outside diameter. The performance of two types of finned tubes viz. circular integral-fin tubes (CIFTs) and spine integral-fin tubes (SIFTs) was studied for the condensation of water and R-134a. These tubes were positioned one by one inside the test-condenser to perform the experiments. All together the experiments were conducted for the condensation on 10 different test-section tubes. With the help of the experimental results, authors have developed an empirical equation. This equation predicts the condensing heat transfer coefficient from their own experimental data for the condensation over CIFTs and SIFTs within a range of ± 15% and experimental data of other thirteen investigators in a range of ± 35% for condensation of water and different refrigerants.     

双金属复合翅片管振动特性的研究

双金属复合翅片管是一种高效传热元件,容易发生流体诱导振动破坏,对其进行振动模态理论分析具有重要的工程指导意义。针对双金属复合翅片管的结构特征,将其简化为串、并联刚度系统,采用组合截面等效弯曲刚度、等效扭转刚度和等效抗拉压刚度,并结合等效质量和等效转动惯量的方法,对其弯曲、扭转和轴向振动模态进行理论解析。为了验证理论分析方法的准确性,对双金属复合翅片管的振动模态进行了实验测试和有限元分析。研究了翅片几何参数对双金属复合翅片管振动频率的影响规律。结果表明,对于矩形翅片形式的钢铝双金属翅片管,其弯曲、扭转和轴向振动频率均随翅片高度和翅片厚度的增大而减小,随翅片间距的增大而增大。     

CYRANO: a computational model for the detailed design of plate-fin-and-tube heat exchangers using pure and mixed refrigerants

A computational model for the detailed design of finned coils has been developed. This programme discretises heat exchangers into tube elements for which the governing equations are solved using local values of temperature, pressure, physical properties and heat transfer coefficients. Single-phase, condenser and evaporator cases can be automatically treated using water, R22, R134a, and refrigerant mixtures based on R32, R125, and R134a. The software can handle non-conventional coil circuits with different numbers of inlets and outlets, non-uniform air distribution at the coil inlet face, using smooth, wavy and louvered fins, and smooth and internally finned tubes. The programme has been validated on seven finned coils using pure fluids, with and without moisture condensation on the fins. Comparisons with tests show errors of less than 5% on the coil duty and of the order of 30% on the refrigerant pressure drop. A performance simulation of a coil using R22 and a ternary mixture is presented to validate the programme algorithms developed for mixtures.     

Influence of the angle of inclination of round-finned tubes in a staggered tube bundle on the free convective heat exchange between it and an unbounded air space

Results of experimental investigations of the average heat transfer from five-row staggered bundles of tubes with knurled spiral fins, operating in the free air-convection regime, in the case where the angel of inclination of the longitudinal axis of the tubes changed from 0 to 60° are presented. The investigations were carried out by the method of complete heat simulation. The average heat transfer was measured in the direction of a free air flow in each row of a tube bundle. The experimental data were generalized by similarity equations for calculating the average heat transfer from the tube bundles and their individual rows in the range of change in the Rayleigh number characteristic of the operating conditions of industrial heat exchangers assembled from finned tubes of the type of the above-indicated tubes.     

Measurement of spray boiling refrigerant coefficients in an integral-fin tube bundle segment simulating a full bundle

Outside (refrigerant) boiling coefficients for a combination of spray and drip boiling for a low pressure refrigerant have been obtained from overall heat transfer coefficients in a 1024 fins per meter tube bundle segment. The tubes were heated by water on the inside; liquid refrigerant was sprayed and/or dripped on the outside. Also, refrigerant vapor was supplied at the bottom of the bundle segment. This configuration simulates an actual flooded evaporator under spray boiling conditions. The dripping corresponds to liquid film falling from upper rows while the inlet vapor is equivalent to the vaporized refrigerant rising from lower tubes; the refrigerant vapor can influence heat transfer performance by the combined effects of gas convection and liquid shear on the tubes. For a nominal heat flux of 23,975 W/m², a bundle average outside heat transfer coefficient of 8522 W/m² °C, based on nominal tube outer diameter, was found at an average bundle vapor mass flux equal to 12.4 kg/s m². The distributor plate below the bundle enhanced the heat transfer, especially at lower vapor mass fluxes, by providing a level of liquid hold-up just below the bottom tube row.     

Nucleate boiling heat transfer coefficients of flammable refrigerants on various enhanced tubes

In this study, nucleate boiling heat transfer coefficients (HTCs) of five flammable refrigerants of propylene (R1270), propane (R290), isobutane (R600a), butane (R600), and dimethylether (RE170) were measured at the liquid temperature of 7 °C on a low fin tube of 1023 fins per meter, Turbo-B, and Thermoexcel-E tubes. All data were taken from 80 to 10 kW m⁻² with an interval of 10 kW m⁻² in the decreasing order of heat flux. Flammable refrigerants' data showed a typical trend that nucleate boiling HTCs obtained on enhanced tubes also increase with the vapor pressure. Fluids with lower reduced pressure such as DME, isobutene, and butane took more advantage of the heat transfer enhancement mechanism of enhanced tubes than those with higher reduced pressure such as propylene and propane. Finally, Thermoexcel-E showed the highest heat transfer enhancement ratios of 2.3–9.4 among the tubes tested due to its sub-channels and re-entrant cavities.     

Effect of strip location on the air-side pressure drop and heat transfer in strip fin-and-tube heat exchangerEffet de la localisation des gaufrures d’ailettes sur la chute de pression de l''air et sur le transfert de chaleur dans un échangeur à tubes à ailettes

This work discusses about the pressure drop and heat transfer characteristics of a plain fin, a strip fin and combinations thereof in the fin-and-tube heat exchanger. Experimental apparatus and test algorithm using scale-up model fin geometry are discussed. The present work uses the electric heaters inside of the tube as the heating source to simulate the hot water loop in the wind tunnel test. The test results of the plain and strip fins are compared with the existing correlation and experimental data. It was found that a hybrid fin, plain fin at front row and strip fin at rear row, was more effective to enhance the heat transfer than that of the whole strip fin at the same fan power.

Résumé

Dans l’article, on traite des caractéristiques de la chute de pression et du transfert de chaleur d’une ailette lisse, d’une ailette gaufrée et de combinaisons de ces deux types d’ailettes dans un échangeur de chaleur à tubes ailetés. Le dispositif expérimental et l’algorithme utilisant une géométrie des ailettes échelonnées sont présentés. On utilise des équipements de chauffage électriques à l’intérieur du tube, comme source de chaleur, pour simuler la boucle d’eau chaude dans le tunnel expérimental. Les résultats expérimentaux concernant les ailettes lisses et les ailettes gaufrées sont comparés avec la corrélation existante et les données expérimentales. On a observé que des ailettes hybrides, c’est-à-dire lisses à la rangée frontale et gaufrées à la rangée arrière, étaient plus efficaces pour intensifier le transfert de chaleur que des ailettes entièrement gaufrées, à la même puissance du ventilateur.     

Effect of circuit arrangement on the performance of air-cooled condensersEffet de la disposition du circuit sur la performance des condenseurs refroidis par l'air

An experimental study was carried out to investigate the effect of circuitry on the performance of wavy finned condensers. A total of eight arrangements were made and tested. The arrangements included six 1-circuit and two 2-circuit arrangements. For the one-circuit arrangement, the test results indicate that counter-cross flow would give better performance than other arrangements. However, heat conduction along the fins may offset the benefits of the counter-cross arrangement. This study has proposed two modifications to the counter-cross flow arrangement. For the two-circuit arrangement, a unique characteristic of “pressure gain” was observed when one circuit is completely condensed and the other is still in two-phase region.

Résumé

On a effectué une étude expérimentale afin d'étudier l'effet de la disposition du circuit sur la performance des condenseurs à ailettes ondulées. Huit dispositions ont été fabriquées et étudiées: six de ces dispositions comportaient un circuit et les deux autres dispositions avaient deux circuits. Les résultats obtenus avec les dispositions à un circuit indiquent que le flux à contre-courant sembler avoir une performance accrue par rapport aux autres dispositions. Cependant, la conduction de chaleur le long des ailettes pourrait réduire l'avantage de la disposition à contre-courant. Les auteurs propose deux modifications de la disposition à contre-courant. Une augmentation de pression a été observée avec la disposition à un circuit lorsqu'une condensation totale était obtenue dans l'un des deux circuits pendant que l'autre circuit restait en régime diphasique.     

Numerical analysis of evaporation performance in a finned-tube heat exchanger

This study discusses the effects of the heat exchanger type, refrigerant, inner tube configuration, and fin geometry on evaporator performance by adopting updated correlations of EVSIM, a numerical analysis model based on the tube-by-tube method developed by Domanski. The heat exchanger types considered are the cross-counter flow type and cross-parallel flow type. The refrigerants considered for the numerical test as a working fluid are R-134a, R-410A and R-22. For inner tube configuration, enhanced tube and smooth tube cases are considered. For the air side evaporation performance, heat exchangers using plate fins, wavy fins and slit fins are analyzed. Results show that the heat transfer rate of the cross-counter flow type heat exchanger is 3% higher than that of the cross-parallel flow type with R-22. The total heat transfer rate of the evaporator using R-410A is higher than those using R-22 and R-134a, while the total pressure drop of R-410A is lower than those of R-22 and R-134a. The heat transfer rate of the evaporator using enhanced tubes is two times higher than that using smooth tubes, but the pressure drop of the enhanced tube is 45–50% higher than that of the smooth tubes. The evaporation performance of slit fins is superior to that of plate fins by 54%.     

水平管外二氧化碳膜状凝结传热分析

综述了水平低翅片管外凝结传热的基本模型,阐述了二氧化碳制冷剂的物性特点,讨论了表面张力与凝液滞留角及二氧化碳管外凝结换热系数的关系,分析了翅片密度、环形翅片管尺寸对翅片效率、滞留角、凝结换热系数以及传热增强比的影响,优化了外翅片管的齿高与齿距,并求得相应的强化传热增强比.结果表明,对于根径为20mm的低翅片管,最佳翅片密度为每米435个翅片,最佳齿高为5.1mm,最佳齿距2.3mm.     

The Tube to Tube Through Transmission Technique for Inspecting Finned Heat Exchanger Tubes

Finned tubes are used in chemical processing and air conditioner heat exchangers. Conventional electromagnetic methods cannot be used to inspect the finned tubes for flaws because of the interference caused by the fins. We describe here tests of a recently developed method, the Tube to Tube Through Transmission (T4) technique which uses coplanar exciter and detector coils in neighboring tubes. As the method is new there is, as yet, little theoretical or experimental explanation for it. We report first, therefore, studies of the magnetic fields and the induced eddy currents. Since the coupling path between exciter and detector coils is primarily diffusive, skin effect equations can be applied to give useful approximations for signal analysis. Signals from machined full circumferential grooves in finned heat exchanger tubes were analyzed using these equations, and it was found that the depth of the grooves or general thinning could be estimated to within ±10%.     

Free-Convective Heat Transfer from Cylindrical Surfaces with Transverse Sectional Finning

A mathematical model has been developed and numerical calculations of heat transfer have been performed for a horizontal heated cylindrical base with transverse sectional finning under the conditions of free-convective flow of an external heat-transfer agent. The effect of concurrent flows from lower fins on the heat transfer of upper fins has been studied. Local thermal characteristics of individual sectional fins on different portions of the cylindrical base and total heat fluxes removed by a finned surface have been determined. Results of the calculation have been compared to experimental data and satisfactory agreement of them has been obtained.     

Flow boiling of ammonia in a plain and a low finned horizontal tubeEbullition en éncoulement d'ammoniac dans un tube lisse ou un tube ailetté

Experimental data of the local heat transfer coeffcient of flow boiling ammonia in dependence of vapor fraction, mass flux and local heat flux is presented. Two horizontal test sections of 450 mm length and an inner diameter of 10 mm have been used, one being a plain tube, one being a spirally low finned tube. A constant wall temperature boundary has been aimed for the test section by heating with a fluid condensing on the tube outside. Local heat transfer coeffcients and pressure drops have been measured in the range −40 < T_sat < 4°C, 0 < x< 0.9, 50 < < 150 kg/m² s and 2 < ΔT_w < 15 K with resulting heat fluxes of 17 < < 75 kW/m². The vapor quality is denoted as x, is the mass flux and ΔT_w the wall superheat. The measured data is carefully evaluated using a finite element model of the tube with regard to the circumferential heat flow distribution. The smooth tube results are compared with recently published data and the correlation from Zürcher (Zürcher, O., Thome, J.R., Favrat, D. Evaporation of ammonia in a smooth horizontal tube: heat transfer measurements and predictions. Journal of Heat Transfer, 1999;121:89–101), and with the correlations of Steiner (Steiner D. Strömungssieden gesättigter Flüssigkeiten. VDI-Wärmeatlas, vol. 8. VDI-Verlag, 1997) and Kattan (Kattan N, Thome JR, Favrat D. Flow boiling in horizontal tubes: part 3 — development of a new heat transfer model based on flow pattern. Transactions of the ASME, 1998;120). The results of the low finned tube are not matched by any known correlation.     

Heat transfer and pressure drop in a plate-type evaporator

A plate-type evaporator, working with natural refrigerant circulation, has been investigated both experimentally and theoretically. Motivated by the phase-out of ozone-depleting substances, HCFC22 was compared to HFC134a and two zeotropic refrigerant mixtures. The effect of different separator liquid levels, i.e. refrigerant flows, and its influence on heat transfer was also studied. The investigated plate-type evaporator consists of thirteen vertical flow channels and its size is 3.0 m × 0.5 m. The heat source for the evaporator is a falling water film on the outside of the plate. Experimental studies have been carried out using a test facility that enabled detailed measurements of heat transfer and pressure drop. Experiments were compared to results from a calculation method that simultaneously calculates heat transfer and pressure drop in a variable number of steps along the evaporator. The calculation method is based on a pressure drop correlation proposed by the VDI-Wärmeatlas and a heat transfer correlation for vertical tubes proposed by Steiner and Taborek. For different evaporator duties, heat transfer was over predicted by 12% for pure fluids by 15% for mixtures. Calculated pressure drops were well within ±5% of the measured values. Changes in heat transfer due to different flows were closely predicted by the proposed calculation method.     

椭圆管换热器对空调室内机声场影响的数值模拟

椭圆管换热器由于良好的流动和换热特性,在换热设备中有广泛的应用。本文利用计算流体力学方法,对椭圆管翅片换热器应用于空调室内机的流场和声场进行数值模拟,并与圆管作对比。计算结果表明,换热管形状对贯流风机内部偏心涡的形成位置和大小没有影响;与采用相同截面积的圆管翅片换热器的室内机相比,相同条件下,采用椭圆管换热器对于室内机增加风量,改善制冷性能,降低噪声尤其是低频噪声方面有良好的效果。其中,长短轴之比为2的椭圆管可以降低室内机噪声4 d B。     

Rheology, flow behaviour and heat transfer of ice slurries: a review of the state of the art

This paper reviews recent studies on rheology, flow behaviour and heat transfer of two-phase aqueous secondary refrigerants (ice slurries). The difficulties in measuring their rheological properties for which standard rheometers prove to be poorly adapted are analysed. Special attention is paid to “vane-in-cup” rheometers, which can make it possible to determine the yield stress. Pressure losses in cylindrical tubes have been measured by many authors. Even if the results generally agree with two-phase flow theory, divergences in sensitivity to the solid fraction can be observed. The stratification observed by some authors for small-Reynolds-number flows and its effects on the pressure drop are addressed. In a last part, information concerning numerical values of the heat transfer coefficient of ice slurries are summarized. Heat transfer coefficients depend on many parameters, but are largely influenced by the flow regime. A geometry of heat exchanger is then proposed, which has already been effective for single-phase flows, and which may enhance heat transfer in ice slurry flows.

Résumé

Cet article passe en revue des études récentes concernant la rhéologie, le comportement des écoulements et le transfert thermique des frigoporteurs diphasiques aqueux (coulis de glace). Les difficultés de mesure des propriétés rhéologiques, pour lesquelles des rhéomètres courants s'avèrent mal adaptés, sont analysées. Une attention particulière est portée sur les rhéomètres du type « vane-in-cup » qui permettent de déterminer la contrainte seuil d'un matériau. Les pertes de charge linéaires dans des canalisations cylindriques ont été mesurées par de nombreux auteurs. Même si les résultats sont genéralement conformes à la théorie des écoulements diphasiques, on peut observer des divergences quant à la sensibilité à la concentration en glace. La stratification, observée par quelques auteurs pour des écoulements à faible nombre de Reynolds, et ses effets sur la chute de pression sont étudiés. Dans une dernière partie, les valeurs numériques du coefficient de transfert thermique des coulis de glace sont récapitulées. Les coefficients de transfert thermique dépendent de nombreux paramètres, mais sont fortement influencés par le régime d'écoulement. Nous proposons enfin un type d'échangeur de chaleur, dont l'efficacité a déjà été remarquée pour des écoulements monophasiques, et il pourrait s'avérer tout à fait adapté à des écoulements de coulis de glace.     

Condensing heat transfer for R114/R12 mixtures on horizontal finned tubes

Two titanium tubes with external fins were tested in the horizontal orientation to determine heat transfer performance with R114, R12, and selected non-azeotropic mixtures of the two condensing on the outside surface. For the single-component situation, data were in excellent agreement with predictions from a modified Katz-Keller method, and little performance distinction was found between the tubes or between the pure refrigerants. All mixtures depressed performance below single-component levels, with even low second-component concentrations causing substantial degradation (up to 55% performance reduction for 5% R12). Gas chromatograph composition analyses of vapour from the condenser shell showed elevated concentrations of the more volatile component (R12), evidence that an added transport resistance contributed to the observed mixture performance reductions. If previously suggested benefits of mixtures in heat pump applications are to be realized, the associated condensers should be in a configuration so as to mitigate these performance penalties.