紧凑传热管束受限空间内沸腾强化换热特性

海水淡化装置以及太阳能或余热吸收式制冷机中的蒸发换热器,采用管排外降膜式蒸发方式,它具有很多优点,但管间距离较大,以致尺寸较大,供液方式较复杂。将传热管束紧凑排列置于饱和状态液体中,将其变为满液式蒸发换热器,利用传热管束间受限空间内早期沸腾强化机理,将中小热负荷条件下的自然对流换热转化为核沸腾换热,在间隙尺寸适宜时,其换热性能可能优于降膜式蒸发换热器。对紧凑传热管束在受限空间内沸腾强化换热进行实验研究,确认了满液式蒸发换热器具有良好的换热性能,在中小热负荷条件下甚至超过降膜式蒸发换热器。     

Falling Films on Arrays of Horizontal Tubes with R-134a,Part I: Boiling Heat Transfer Results for Four Types of Tubes

A new falling film heat transfer test facility has been built for the measurement of local heat transfer coefficients on a vertical array of horizontal tubes, including flow visualization capabilities, for use with refrigerants. Presently, the facility has been used for evaporation tests on four types of tubes at three tube pitches and three nominal heat flux levels for R-134a at 5°C. A new method for determining local heat transfer coefficients using hot water heating has been applied, and test results for a wide range of liquid film Reynolds numbers have been measured for arrays made of plain, Turbo-BII HP, Gewa-B, and High-Flux tubes. The results show that there is a transition to partial dryout as the film Reynolds number is reduced, marked by a sharp falloff in heat transfer. Above this transition, the heat transfer coefficients are nearly insensitive to the film Reynolds number, apparently because vigorous nucleate boiling is always seen in the liquid film. The corresponding nucleate pool boiling data for the four types of tubes were also measured for direct comparison purposes. Overall, about 15,000 local heat transfer data points were obtained in this study as a function of heat flux, film Reynolds number, tube spacing, and type.     

管内插入物单相流体强化传热的实验研究

采用管内插入物增强锅炉烟管的放热系数是行之有效的手段,它不影响换热器的机械强度,便于拆装、检修和更换。在设计烟管锅炉,特别是改造烟管锅炉中,它是最有希望的强化传热方式之一。本文介绍了内插物强化传热实验装置的设计调试,及其一种内插件——螺旋线圈强化传热的实验研究和有关理论探讨。     

Numerical simulation of a closed wet cooling tower with novel design

A closed wet cooling tower with novel design was proposed and numerically investigated. The studied cooling tower consists of two main parts: one heat and mass transfer unit (HMTU) and one heat transfer unit (HTU). In the HMTU, copper tubes are arranged as heat transfer tubes while plastic tubes are collocated to enlarge the mass transfer area between the spray water and the airflow. In the HTU, only copper tubes are adopted as heat transfer tubes. Heat and mass transfer process takes place among the process water, airflow and spray water in the HMTU, while in the HTU only heat transfer between the process water and the spray water is observed. A transient one dimensional distributed-parameter model was adopted to evaluate the cooling tower performance under different operating conditions. Determination of heat and mass transfer coefficients, as well as the influence of Lewis number on the cooling tower performance, was presented.     

A Large Eddy Simulation of Plate-Fin and Tube Heat Exchangers with Small Diameter Tubes

Plate-fin and tube heat exchangers are extensively studied both experimentally and numerically. However, data on the fluid flow and heat transfer in the exchanger passage with small diameter tubes have not been accumulated enough. With a large eddy simulation technique (LES), this study performs a detailed investigation of the fluid flow and heat transfer in a plate-and-tube channel with tubes of diameters as small as 5.2 mm. The conservation equations for mass, heat, and momentum were solved by the proposed LES model. It was found that the LES model is appropriate to predict the fluid flow and heat transfer. Compared to heat exchangers of larges tubes, the heat exchangers exhibit much higher heat transfer coefficients with small tubes. The fin efficiencies are improved with small tubes.     

Effects of hydrophilic surface treatment on evaporation heat transfer at the outside wall of horizontal tubes

An experimental study has been conducted to investigate the effects of hydrophilic surface treatment on evaporation heat transfer at the outside wall of various kinds of copper tubes. Plain, spiral, corrugated, and low-finned tubes were selected as test tubes. In this work, to increase the wettability of distilled water on copper tubes, a novel hydrophilic surface treatment method using plasma was employed. The experiments show that every kind of hydrophilic surface treated tube tested in the work exhibits superior evaporation heat transfer performance as compared with that of the same kind of untreated tube. It is found out that during the evaporation process, the high wettability of the surface obtained through hydrophilic treatment induces film flow on the tubes while sessile drops are formed on untreated tubes. The film has a smaller thickness as well as a greater heat transfer area than the sessile drops, and this yields higher heat transfer rate for hydrophilic surface treated tubes than that for untreated tubes.     

Finite element analysis of the flow and heat transfer of solid particles in moving beds

A numerical analysis for the flow and heat transfer of solid particles in moving beds of heat exchangers is presented. The solid particles pass through a bundle of heat source tubes as the result of the gravitational force. Heat energy is transferred through direct contact of particles with the heat source tubes. A viscous-plastic fluid model and a convective heat transfer model are employed in the analysis. The flow field dominantly determines the total heat transfer in the heat exchanger. As the velocities of solid particles around the heat source tubes increase, the heat transfer from the tubes also increases. Examples are presented to show the performance of the numerical model. The effect of flow on heat transfer has also been studied. © 1998 John Wiley & Sons, Ltd.     

Study on the design procedure for a multi-cool/heat tube system

The objective of this study is to contribute to widespread use of earth-to-air heat exchangers by proposing a design procedure. In this paper, it is discussed the design method when an earth-to-air heat exchanger system consists of multiple pipes with a close arrangement.A numerical model for this multi-cool/heat tube system was developed and it was verified by field measurements. With taking into account the thermal interference between tubes, the heat transfer performance was evaluated under various design conditions such as number of tubes, arrangement interval, air velocity and length, and soil properties. Based on these results, an estimation method for the heat transfer rate for the multi-cool/heat tube system is proposed.     

管外冷凝强化换热管的结构及发展趋势

介绍了各种类型的管外冷凝强化换热管,分析了其强化机理及结构特点,并总结得出:管外冷凝强化管的换热系数与管型有关,且各管型的结构参数对强化传热具有重要的作用.对国内外管外冷凝强化技术研究工作进行分析,结果表明,目前管外冷凝强化换热管的研究主要集中于翅片形状、翅片密度、翅片高度等结构参数对换热性能的影响.强化换热管的冷凝传热性能不仅与翅片结构参数有关,而且也与管材的表面特性和导热系数有关.管外冷凝强化换热管的研究重点是开发新型三维结构翅片的双侧强化管并研究其传热关联式,以及研究不锈钢等低成本材料制造的强化管换热管的传热性能和强化结构的优化.     

An experimental heat transfer study for helically flowing outside petal-shaped finned tubes with different geometrical parameters

The performances of shell-side heat transfer and pressure drop were experimentally studied in a helically baffled single tube heat exchanger, where water was used as a working medium. The tested tubes included one smooth tube and five petal-shaped fin tubes (PF tubes) with different geometrical parameters for improving the heat transfer of the shell side. It was shown that, compared with the smooth tube, five PF tubes significantly increased the values of Nusselt numbers. The Nusselt numbers increased with the fin height and decreased with the fin pitch. In the range of the present experiments, it was found that the Nusselt numbers for the PF tubes were increased by up to 233%, while the pressure drop was increased by less than 111%, as compared with that for the smooth tube. It is a promising route to use a PF tube instead of smooth tube for improving the performance of a helically baffled heat exchanger.     

传热强化管管内熵产分析与热力性能评价

根据热力学第二定律,对传热强化管管内热力过程进行熵产分析,建立了基于流动与传热过程熵增原理的管内传热熵产方程,导出恒热流和恒壁温2种常见工况下的无因次熵产数表达式.在不同雷诺数和进口温差条件下,对2种螺旋槽管和光管进行恒壁温工况的熵产分析和热力性能评价,分析了传热和流动摩阻引起的熵产变化规律及2种不可逆损失占总熵产的份额.结果表明,熵产分析可用于评价传热强化管的综合热力性能,确定合理的运行工况、结构参数及强化换热形式,为强化管的应用评估及优化设计提供参考.     

The effective use of heat transfer additives for steam condensation

Reported herein are the results of the experimental investigations on the effect of small amounts of heat transfer additives (C_add<1000 ppm) on steam condensation in a bundle of horizontal tubes that are internally cooled. The condenser was designed with 12 tubes in an array of three horizontals and four verticals with a 2-in. horizontal and 1.5-in. vertical in-line pitch. By using effective additives, the condensation heat transfer rate can be augmented as much as 30% as compared with heat transfer that operated without additives under the same operating condition. The steam condensation, which occurred in our experiments while using effective additives, was mostly dropwise like. When heat transfer additives function effectively, the condensate-droplets become more dispersed and have a smaller shape than those produced without additives. These droplets, unlike traditional turbulence, start at the top portion of the condenser tubes and cover most of the tubes. Such a flow behavior can be explained by the Marangoni effect (in terms of thermodynamic equilibrium) in connection with obtained surface tension data. Furthermore, we extend our discussion to the dynamic effect of surface tension in the steam condensation process and propose a general criterion for selecting effective additives. In our experiments, we noted that the use of heat transfer additives such as 2-ethyl-1-hexanol for steam condensation was highly effective.     

Techno-economic analysis of H2 production processes using fluidized bed heat exchangers with steam reforming – Part 1: Oxygen carrier aided combustion

This work presents the techno-economic assessment for a new process where a fluidized bed heat exchanger (FBHE) is used as heat source for steam reforming in a hydrogen production plant. This suggested process configuration is compared with a reference case representing a conventional steam methane reforming (SMR) large-scale hydrogen production plant. The use of a FBHE as a heat source for the endothermic reforming is an advantage because of the high heat transfer coefficient to the reformer tubes. The suggested process configuration utilizes oxygen carrier particles as bed material and a bubbling fluidized bed reactor with immersed reformer tubes to ensure sufficient heat production for the reforming and improved heat transfer to the reformer tubes compared a conventional plant. The results include a comparison of hydrogen production efficiency and levelized production costs (LCOH) of the two plants where the production efficiency is more than 11% higher and the LCOH is more than 7% lower for the suggested process configuration.     

“Zero Fouling” Self-Cleaning Heat Exchanger

Conventional shell and tube heat exchangers sometimes have to use two severely fouling process streams, one in the tubes and one in the shell. This paper presents the design of a self-cleaning heat exchanger that applies the self-cleaning mechanism in the tubes of two parallel bundles handling the fouling process streams. For the transfer of heat between both bundles, a small circulating flow of conditioned water is used as an intermediate fluid, a fraction of which evaporates on the outside of the tubes of the high-temperature bundle and condenses on the outside of the tubes of the low-temperature bundle. This novel design, which consists of two parallel bundles in one shell, experiences very high film coefficients at the outside surface of both tube bundles and does not suffer from any fouling. Therefore, it is referred to as a “zero fouling” self-cleaning heat exchanger. In this paper, a conventional severely fouling crude oil preheater will be compared with a zero fouling self-cleaning heat exchanger for the same service.     

Heat and mass transfer studies on plate fin-and-elliptical tube type metal hydride reactors

In the present work a two-dimensional transient model to study the heat and mass transfer characteristics of plate fin-and-elliptical tube type metal hydride reactors is presented. The relevant governing equations are solved numerically. The heat transfer coefficient and pressure drop on external fin side are estimated using available correlations. Effects of external fluid flow rate and temperature on the fin-and-tube metal hydride reactor are studied. Results show that the use of elliptical metal hydride tubes in place of the standard circular tubes give rise to compact systems in addition to considerably lower fan power consumption, with very little change in the average heat and hydrogen transfer rates. Even though the performance of the reactor depends very much on the fin-and-tube arrangement, for all the arrangements considered here, the reactors with elliptical tubes were found to perform better in terms of compactness, weight and fan power consumption. Considering the aspects of mean hydride bed thickness, tube wall thickness, tube mass, compactness, heat and hydrogen transfer rates and fan power consumption, elliptical tubes of 0.6 eccentricity appear to offer the best solution for the given application.     

Heat transfer characteristics of falling film evaporation on horizontal tube arrays

A falling film heat transfer test facility has been built for the measurement of falling film evaporation in a vacuum of about 1000 Pa. At this condition, only convective evaporation occurred in the liquid film. The Reynolds numbers of falling film over a range from 21.6 to 108.1 were tested on six-tube arrays made of enhanced or smooth tubes. Results show that the tubes with both enhanced outer and inner surfaces give high heat flux. Besides, as the Reynolds number increases, the heat transfer enhancement ratio of falling film evaporation decreases. A semi-analytical correlation is established to predict the heat transfer coefficients of falling film evaporation on smooth tube arrays, considering the contributions of partially dryout and fully wet regimes, respectively. For enhanced tubes, the heat transfer enhancement ratios to the smooth tubes were also correlated.     

带有小螺旋角的内外螺旋翅片管高压加热器的工业试验

工业现场试验的结果表明,带有小螺旋角的内外螺旋翅片管（简称内外螺旋翅片管或ＩＯＳＦ管）用于电站高压加热器有着显著的传热强化效果,其实测总传热系数是光滑管加热器的１．４３倍,可相应节省换热面积３０％。在等面积下使用,则可收到明显的节能效果。     

Falling film evaporation heat transfer of water/salt mixtures from roll-worked enhanced tubes and tube bundle

An experimental study is carried out for enhancement of falling film evaporation heat transfer of pure water and water/salt mixtures on horizontal smooth tube and two kinds of structured tube bundles under atmospheric pressure. The experimental results show that the low-cost roll-worked tube can greatly enhance the evaporation heat transfer performance of the falling film, and make it comparable to that of expensive commercial enhanced tubes such as GEWA-T tubes, TE tubes and HF tubes, even at low and moderate heat flux levels. The average evaporation heat transfer coefficients for the roll-worked tube bundle are basically independent from the parameters tested such as flow and heating conditions, salt-concentrations, as well as geometries of the tube bundles. The present experimental data result in a constant heat transfer coefficient; α≈20 kW/m² K, in the convective heat transfer range of the heat fluxes <10⁵ W/m².     

氧化膜的生长对管壁温度和氧化膜温度的影响

管壁温度是评估电站锅炉受热面的热效率和锅炉安全运行的重要参数,根据传热学理论并利用L-M经验公式对管道的传热过程进行了研究,建立了具有氧化膜的受热面管道传热的数值模型,并对某再热器管道的传热过程进行计算,得到了管道各个界面温度与氧化膜厚度随着管道运行时间的变化趋势,定量地解释了管道各个界面温度随时间增长的原因,提出了氧化膜增长的敏感系数.     

Heat transfer analysis of plain and dimpled tubes with different spacings

Heat transfer enhancement by modifying the surface of tubes is commonly practiced throughout the world. Grooves, dimples, flutes or corrugations are placed inside and outside the surface of tubes and channels for enhancement. In this article, a novel method for heat transfer enhancement by varying the spacing between the tubes is reported. A comparison is made between the heat transfer performance of plain tubes and dimpled tubes at different spacings. For analysis, an experimental setup is fabricated and assembled. The flow is externally forced laminar flow of air over a hot tube maintained at constant temperature. Four different velocities of air 0.4, 0.6, 0.8, and 1.0 m/s are considered in this study. Tube surface temperature, heat transfer coefficient, heat transfer rate and Nusselt number are the parameters studied to analyze the thermal behavior of tubes at different spacings of 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 cm. From the experimental investigation it is found that, apart from heat transfer enhancement by providing dimples on the tube surfaces, there is an optimal spacing between the tubes after which no further improvement is obtained. In this study, 3.0 cm is found to be the optimal spacing for both plain and dimpled tubes. However, the percentage value of heat transfer enhancement is greater with optimal spacing and for all velocities of air in dimpled tubes.