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.     

Oligochaete worms as biological describers of the interactions between surface and groundwaters: A first synthesis

The paper focuses on the definition of the vulnerability of phreatic waters to surface water pollution. The amplification of water exchanges between surface and underground waters, because of human activities, is considered as a risk of contamination of aquifers. The aim of this paper is to present a first synthesis of oligochaetes as biological describers of these exchanges. Field investigations have shown that several rare and/or stygophilous taxa are found in surface (coarse) sediments of rivers upstream and/or downstream of dams or gravel operations. These observations relate to species such as Propappus volki, Pristina spp., Chaetogaster parvus, Rhyacodrilus carsticus, R. falciformis, Phallodrilus spp. The data suggest that these species invade surface waters from phreatic or interstitial waters when water flows are amplified between the two. Flow amplification is considered as a consequence of dams' and gravel operations' hydraulic impact. But the lack of hydraulic validation of the presumed active exchanges describer's (AED) ecological status given to those species remains a problem, giving the opportunity to examine the link between hydraulics and biology. Preliminary results on two gravel pits of La Wantzenau (Rhine alluvial valley, France) have shown relations between the distribution of oligochaetes, especially Lumbriculidae, and soil filtration properties. Two other studies were planned along an industrially polluted tract of the River Rhǒne (Grand Gravier Island). Horizontal and vertical zonation of the river banks were performed by studying simultaneously hydraulics and oligochaetes. Preliminary results have shown that the absolute and relative abundances of taxa such as Phallodrilus sp. and S. velutinus increase in relation to the permeability of superficial coarse substrate in the horizontal plane, and to the current velocity in underground porous media in the vertical plane. The increasing percentages of P. barbatus and the decreasing percentages of Tubificidae reveal active exchange zones where superficial waters are renewed by groundwaters. Some other species such as P. vejdovskyi seem to be potential describers of water exchanges.     

Influence of the heat flux in mixture boiling: experiments and correlations

Heat transfer at nucleate pool boiling of the binary and ternary refrigerant mixtures R404A, R407C and R507 at the outside of a horizontal tube with emery ground surface has been investigated in a wide range of pressures and heat fluxes. Together with experimental data of Bednar and Bier for wide boiling ethane/n-butane mixtures, the results of these comparatively narrow boiling mixtures are used to investigate the influence of heat flux q on the heat transfer coefficient as predicted by various correlations for nucleate boiling of mixtures. At comparatively high saturation pressures with experimental -values markedly smaller than the molar average of the pure components, the ,q-relationships predicted differ significantly from the experimental, particularly for wide boiling mixtures.     

Beyond CFCs: Extending the search for new refrigerants

Previous work in developing environmentally acceptable alternatives to fully halogenated chlorofluorocarbons (CFCs) has concentrated almost exclusively on methane and ethane based compounds. A review of toxicity and boiling point data for a large variety of fluorine compounds reveals additional classes of compounds which may be suitable as refrigerants. Fluorinated derivatives of dimethyl ether and cyclopropane appear to have both low toxicity and suitable boiling points. They also have a relatively simple structure which means that they should have a reasonably good cycle efficiency. Propane based CFCs may also be useful if simpler compounds prove to be unacceptable. Specific compounds that warrant further investigation include bis-difluoromethyl ether (for R114), difluoromethyl dichlofluoromethyl ether (for R113), difluoromethyl fluoromethyl ether (for R11) and hexafluorocyclopropane (for R12). In addition, the compound trifluoroiodomethane may be a useful alternative to R13B1 in fire extinguishers. A cooperative programme of synthesizing and evaluating fluorinated derivatives of dimethyl ether and cyclopropane is recommended.     

Refrigeration with ammonia

Ammonia is widely used as a refrigerant in industrial systems for food refrigeration, distribution warehousing and process cooling. It has more recently been proposed for use in applications such as water chilling for air-conditioning systems but has not yet received widespread acceptance in this field. This review paper assesses the reasons why ammonia is so popular in industrial systems, the reasons why it is deemed less suitable for other applications and the possible benefits at local, national and international levels that might be gained by more general acceptance of ammonia as a refrigerant. The paper also considers other possible applications which might benefit from the use of ammonia as refrigerant.     

Condensation heat transfer of binary refrigerant mixtures of R22 and R114 inside a horizontal tube with internal spiral grooves

An experimental study of the condensation of pure and mixed refrigerants of R22 and R114 inside a spirally grooved horizontal copper tube has been carried out. A double-tube counterflow condenser in the pressure range 3–21 bar and at a mass flow-rate 26–70 kg h⁻¹ was used. The axial distributions of refrigerant, tube wall and cooling water temperatures, wall heat flux density and vapour quality are shown graphically. The variation of tube wall temperature around the circumference of the tube is also shown. The local Nusselt number depends on the molar fraction, whereas the average Nusselt number can be correlated by an equation which is modified from a previously established equation for pure refrigerants inside a horizontal smooth tube. The frictional pressure drop evaluated is correlated well by the Lockhart-Martinelli parameters and is independent of the concentration of the mixture.     

Evaluation of supplementary/retrofit refrigerants for automobile air-conditioners charged with CFC12Evaluation des frigorigènes supplémentaires / de remplacement pour des systèmes de conditionnement d'air automobiles utilisant du CFC12

In this study, thermodynamic performance of supplementary/retrofit refrigerant mixtures for CFC12 used in existing automobile air-conditioners was examined. A thermodynamic computer analysis of an automobile air-conditioner was carried out for the initial screening of possible mixture candidates, and refrigerant mixtures composed of HCFC22, HFC134a, HCFC142b, RE170 (dimethylether), HC290 (propane), and HC600a (iso-butane) were proposed to supplement CFC12. Also a breadboard type refrigeration test facility was manufactured to verify the performance of the alternative refrigerant mixtures proposed through the computer analysis. Test results showed that HFC134a/RE170 mixture with zero ozone depletion potential is the best long term candidate to supplement CFC12. On the other hand, HCFC22/HFC134a/RE170 and HCFC22/HFC134a/HCFC142b mixtures are good only as short term supplementary/retrofit alternatives since they contain HCFC22. A hydrocarbon mixture of HC290/HC600a showed a good performance but its use in existing automobile air-conditioners should be carefully considered due to its flammability.     

用于小样本模式识别的最小平方支持向量机

为了进一步提高支持向量机分类器的推广性能,采用最小二乘原理,形成最小平方支持向量机,编制了相应的MATLAB程序,并将其应用于小样本模式识别中.仿真实验结果表明,最小平方支持向量机分类器在小样本模式识别中,有着优良的推广性能.     

遗传算法和BP算法相结合进行图像匹配

将神经网络理论应用于图像匹配 ,提出采用遗传学习算法进行全局寻优、利用BP算法进行精确训练、优化BP(backpropagation)神经网络权重学习和训练的神经网络图像匹配算法 .实验表明该算法的收敛性能及学习速度优于传统的BP神经网络图像匹配算法及其他同类改进算法 ,具有寻优的全局性和精确性 .     

基于数据融合和挖掘技术的航海避碰决策系统

介绍了数据融合和数据挖掘技术，并把该技术应用于航海避碰决策系统中．建立了航海避碰决策系统的数据融合和数据挖掘综合处理的模型，并将基于多知识库的数据挖掘与信息融合技术与模糊神经网络数据挖掘技术相结合，利用模糊推理、神经网络技术和D—S证据理论实现目标识别，从而得到对于各目标类型的最终认识．