滴状冷凝传热机理的研究 (Ⅰ)传热表面的制备及传热性能实验

采用离子注入技术对紫铜表面进行处理,在几种表面上都获得了稳定的滴状冷凝.对各种表面进行了冷凝传热性能实验,并对实验结果进行了分析讨论.     

大气压附近蒸汽温度对滴状冷凝传热的影响

<正>滴状冷凝作为一种高效传热方式,自从被人们发现以来,许多研究者在这一领域作了大量研究工作.早期的研究主要集中在实现滴状冷凝的方法上.随着对滴状冷凝现象认识的加深,人们逐渐加强了对滴状冷凝传热机理的研究,提出了许多观点来解释滴状冷凝高效传热的原因,其中主要有两种假说,一种是液膜破裂假说,另一种是固定成核中心假说.     

滴膜共存表面强化冷凝传热

<正>滴状冷凝具有很高的传热速率,其冷凝传热系数是膜状冷凝的几倍至几十倍,但是,由于长期维持滴状冷凝表面制备技术的限制,该技术距工业化应用还有一定的距离,在滴状冷凝的研究过程中,经常发现滴膜共存冷凝现象,而且传热系数较普通的膜状冷凝有较大的提高,在以往的实验研究中,往往不能肯定地排除蒸汽中有机杂质的影响,滴膜共存表面的实验研究也间接地证实了蒸汽中是不含有机杂质的,滴膜共存表面也是强化冷凝传热很实用的技术,是滴状冷凝传热过程全面工业化的过渡阶段.所以,研究滴膜共存表面强化冷凝传热的特性具有重要的理论意义和实用价值.Kumagai等报道了在竖平面上垂直分割的滴膜共存表面上冷凝传热的实验.结果表明,当滴状冷凝区和膜状冷凝区的面积比为1:1时,平均热负荷要高于在通常表面上滴状冷凝和膜状冷凝热负荷的算术平均值,而且平均热负荷与表面分割方式和分割数目有关.他们在一种特殊分割形式的表面上得到的最大热负荷高于当全部表面为滴状冷凝时的最大热负荷.     

实现滴状冷凝新途径的研究——(Ⅰ)基本概念及实验结果

本文从表面科学的角度出发,对目前实现滴状冷凝的各种方法进行了评述.提出了实现滴状冷凝的新途径,并通过实验研究确认其可行性.在不锈钢和紫铜两种圆管外表面上,采用了不同的表面处理技术进行处理.对每种表面都做了垂直管外的水蒸汽冷凝实验.在几种表面上,获得了稳定的滴状冷凝,并整理出各种表面的传热实验结果及滴状冷凝照片.对成滴表面的特性,进行了分析与探讨.     

表面分割方式对滴膜共存冷凝传热特性的实验研究

实验研究了滴膜面积比为1：1且滴状区域采用较厚的有机涂层时，垂直管外滴膜共存表面的滴膜区域分割方式对冷凝传热特性的影响。结果表明，在表面分割方式一定时，滴膜共存表面的冷凝传热性能随着表面分割数的增加而增加，但总仍是介于全部表面为滴状冷凝和全表面为膜状冷凝型态之间的传热特性。分析表面分割数分别为2和4的滴膜共存表面的传热结果，发现凝液环的缓冲作用对于滴膜共存表面的冷凝强化传热具有重要作用。滴膜共存表面上滴状冷凝区面积对其下游膜状冷凝传热的影响不仅与滴膜区域的面积比有关，而且实验操作条件也有一定的控制作用，且两者之间存在最佳匹配值，即此时的强化传热效果最好。     

Filmwise Condensation Heat Transfer Enhancement with Dropwise and Filmwise Coexisting Condensation Surfaces

Six surfaces were prepared with defferent surface division patterns for the experimental investigationof steam condensation heat transfer characteristics for dropwise and filmwise coexisting(DFC)condensationsurfaces under atmospheric pressure Dropwise condensation(DWC)was promoted with an ultrathin polytetrafluoroethylene(PTFE)film,which was prepared by the dynamic ion-beam mixed implantation(DIMI)method.The results showed that the condensation phenomena at the intersection between the dropwise andfilmwise condensation regios were quite different for different relative positions of the dropwise and filmwisecondensation regions.The experimental results revealed that the condensation heat transfer characteristics werehighly influenced by the surface division number and the relative area ratio of the dropwise and filmwise conden-sation regions.The impact of thesc findings on heat transfer enhancement mechanism for condensation heattransfer is discussed in detail.     

An experimental study of the effects of surface characteristics and material thermal properties on dropwise condensation

In this investigation, a study of the dropwise mode of condensation of water vapour at near atmospheric pressure was undertaken. Three vertically oriented condenser surfaces of different materials, copper, brass and stainless steel, were carefully prepared and heat transfer data were obtained over a long period of time as the surfaces continued to deteriorate. The condensation heat transfer coefficient was found to decrease with increasing surface deterioration. It is also evident that further work is needed before a general model can be developed to predict heat transfer in dropwise condensation. However the effect of surface characteristics and material thermal properties on strong dropwise condensation at small vapour to surface temperature differences is evident from the present results.     

离子注入制备滴状冷凝表面材料的研究

<正>滴状冷凝是一种高效传热方式.从本世纪三十年代发现这种现象以来,许多研究者一直致力于滴状冷凝工业应用的研究,但这个愿望迄今仍未实现.要在金属表面上实现滴状冷凝必须降低其表面能.以往实现滴状冷凝的方法一般都是采用在金属表面外加一层低表面能材料,靠物理或化学吸附与表面结合来降低金属表面能.由于这些材料的本身特点和与金属表面的结合方式带来了对冷凝液的污染、腐蚀金属表面、使用寿命短、成本昂贵和附加热阻等问题     

实现滴状冷凝新途径的研究——(Ⅲ)滴状冷凝传热系数的测定和寿命实验

采用直接测温的方法,测定了新表面材料的滴状冷凝传热特性,并同国外滴状冷凝传热研究的结果进行比较.分析讨论了Cu-Cr表面和机械抛光表面传热性能的差异.对Cu-Cr和机械抛光两种表面进行了寿命实验,滴状冷凝现已超过8500小时.     

Dropwise condensation on L-B film surface

A method of achieving dropwise condensation using Langmuir-Blodgett (L-B) built-up film technology is presented. One and three layers of barium stearate monomolecular film were prepared on the copper plates, and excellent dropwise condensation of steam was formed on the surfaces, respectively. The heat transfer coefficients of dropwise condensation on these treated surfaces are more than 30 times higher than that of filmwise condensation on a bare surface.     

A NEW METHOD FOR ACHIEVING DROPWISE CONDENSATION (Ⅱ) NEW SURFACE MATERIALS FOR DROPWISE CONDENSATION

Technology of ion-plating and ion-beam mixing was employed to prepare surface alloys on copper tubes. Elements chosen for the purpose are Cr, Fe, Al, N, Bi, Sb, Sn, Se and In. Excellant dropwise condensation of steam was then obtained on some external surfaces of these copper tubes and the mechanism for the dropwise condensation was analysed.The surface characteristics of the Cu-Cr and Cu-Fe alloy layers were analysed by using XPS, AES and AES-PRO of PHI-550 multiple function electron spectroscopy.     

限制热阻对珠状凝结换热的影响

应用作者提出的数值模拟模型 ,对不同材料表面珠状凝结换热进行了数值模拟 .计算结果表明 :珠状凝结换热系数随冷凝壁材料导热系数的降低而降低     

聚四氟乙烯薄膜表面滴状冷凝传热实验研究

为了探索实现滴状冷凝传热的新表面涂层 ,采用离子束动态混合注入技术制备了紫铜基聚四氟乙烯(PTFE)薄膜 ,显著地提高了水蒸气冷凝传热性能。实验表明 ,该表面涂层能够实现常压水蒸气稳定的滴状冷凝形态 ,在表面过冷度 9.8— 1 4 .2K范围内 ,滴状冷凝传热通量和传热系数比膜状冷凝结果提高 30—47倍。同时发现 ,在大气压附近 ,热通量和冷凝传热系数都随着水蒸气蒸汽温度的增加而增加 ,但表面过冷度随蒸气温度增加呈弱增加趋势     

实现滴状冷凝新途径的研究——(Ⅱ)滴状冷凝传热与新表面材料

提出采用离子镀技术加离子束混合的工艺,在紫铜圆管外表层,制备Cr、Fe、Al、N、Bi、Sb、Sn、Se、In的铜基合金材料.对每种表面进行了垂直管外水蒸汽的冷凝实验.对Cu-Gr和Cu-Fe表面,用PHI-550型多功能电子能谱仪的XPS、AES和AES-PRO三种功能,进行化学成分及相对含量,主要元素的价态及其在表面法线方向的浓度分布以及合金层厚度等内容的分析.对能够形成滴状冷凝的表面的机理进行了分析.     

聚合物表面性能对强化冷凝传热的影响

对离子束动态混合注入(DIMI)技术制备的黄铜、紫铜、不锈钢和碳钢管基聚四氟乙烯(PTFE)表面的冷凝传热实验发现,用不同加工条件制备的表面具有不同的化学组成、不均匀的表面状态以及不同的物理化学性质,从而导致不同的冷凝成滴面积和传热性能,而且表面加工条件对滴状冷凝传热的寿命有至关重要的作用,不同基体材料应有不同的最佳制备工艺条件.不同工艺条件下制备黄铜基PTFE表面水蒸气竖直管外冷凝传热通量比相应的膜状冷凝提高0.3-4.6倍,冷凝传热系数提高1.6-28.6倍.实验结果也表明冷凝表面基体材料对冷凝传热性能有一定的影响.     

金属基底超疏水表面制备技术发展及滴状冷凝强化传热应用展望

蒸汽冷凝过程在石油化工领域有着广泛的应用,滴状冷凝传热模式相比于传统的膜状冷凝方式其传热系数与传热效率大大提高,可作为节约能源和高效利用资源的重要手段。随着金属基底超疏水表面制备技术的发展,蒸汽滴状冷凝研究的不断深入,为滴状冷凝强化传热在工业上应用提供了理论依据和制备方法。介绍了近年来金属基底超疏水表面制备技术发展,评述各方法的特点,提出滴状冷凝强化传热工业化应用所面临的问题。     

Effects of surface topography of material on nucleation site density of dropwise condensation

Heterogeneous nucleation is very important for the initial droplet formation of dropwise condensation, and it is directly related with the surface properties of materials. In this paper, the relationship of nucleation site density with surface topography in nanometer scale for dropwise condensation was experimentally studied on surfaces of magnesium in view of that magnesium can react with hot water (condensate) and make the chemical compositions of the surface changed, and thus leave the nucleation sites in nanometer on the surface. The differential box-counting was used to calculate the fractal dimension of these magnesium surfaces. And condensation experiments were carried out on the magnesium surfaces with different topography under same subcoolings and condensation time. The results showed that surface topography has a great influence on nucleation site density. The larger the fractal dimensions the more the nucleation sites. A new formula to calculate the nucleation site density was proposed based on the modification of Rose's formula with the experimental data. And this new formula can reflect the effect of surface fractal dimension on the nucleation sites.     

微纳结构超疏水表面参数影响含不凝气蒸汽冷凝传热的理论分析

超疏水表面结构参数对滴状冷凝传热性能、液滴生长和分布均会产生影响。首先利用竖直壁面液滴的受力平衡确定了不凝气条件下蒸汽冷凝过程中液滴的脱落半径,随后建立了含不凝气蒸汽滴状冷凝的传热模型,研究了不同不凝气浓度和过冷度下超疏水表面微柱的柱间距对滴状冷凝传热性能的影响,得到了使超疏水表面冷凝传热性能达到最佳的柱间距值,并对其随不凝气浓度和过冷度的变化规律进行了分析。结果表明在所研究的过冷度范围内,当不凝气浓度较低时（<20%）,最佳柱间距随不凝气浓度升高而增大;而当不凝气浓度高于20%时,最佳柱间距随不凝气浓度升高而减小。为驱动含不凝气蒸汽冷凝传热性能的超疏水微纳结构参数的优化提供了必要的基础数据。     

A NEW METHOD FOR ACHIEVING DROPWISE CONDENSATION (Ⅰ) BASIC CONCEPT AND EXPERIMENTAL RESULTS

Various methods for achieving dropwise condensation (DWC) are reviewed from the point of view of surface science. The objective of the present work is stated. In the experimental study, the surfaces of stainless steel and copper tubes employed were treated with various surface techniques, steam condensation under atmospheric pressure was carried out and steady DWC observed on these surfaces, and the mechanism of DWC formation on the treated surfaces was analysed. Experimental results and photographs of DWC are presented.     

初始冷凝液滴尺寸分布的分子团聚模型

Initial dropwise condensation of moist air on a hydrophobic self-assembled monolayer（SAM）surface was investigated with a high speed camera and a microscope.The transient initial droplet size distribution and the growth of micro-droplets by direct condensation on the surface without coalescence were analyzed from the concept of molecular clustering.Based on the general characteristics of nucleation of dropwise condensation,a molecular clustering physical model was proposed to describe the state of steam molecules in the bulk steam phase before condensing on the cooled surface.