【作者】 王军；

【导师】 黄树红； 陈良才；

【作者基本信息】 华中科技大学 ， 热能工程， 2009， 博士

【摘要】 蒸发式空冷器广泛用于炼油、化工、冶金等流程工业。本文提出的一种新型蒸发式空冷器——增膜型蒸发式空冷器,在管束的每上下两管之间装上亲水的平板,上管落下的热水在增膜板表面铺展成膜,与空气传热传质的面积增大,高温水膜在板上被冷却成低温水膜,与没有增膜板相比,低温水膜流到下管上,扩大了管内外温差,强化了下管的换热能力。研究建立了换热管和增模板的传热传质模型。以水膜和空气之间的温差和含湿量差作为传热传质驱动力,建立了可以显式差分求解的数学模型,模型形式简洁,易于求解。传统模型一般假设热流体与水膜并流,水膜与空气逆流。在吸收传统的换热管一维传热传质模型优点的基础上,研究建立了用于增膜型空冷器的二维传热传质模型,考虑了热流体与水膜错流、水膜与空气逆流和空气绕流管外水膜换热系数随管外环向角变化的实际情况,所以模型中5个独立的未知变量(热流体温度、空气温度、空气含湿量、水膜温度、水膜流量)都是在水平管轴向和铅垂方向二维分布的,与传统的铅垂方向一维模型有明显区别。准确获取水膜厚度是计算蒸发式空冷器管外换热系数的关键之一。研究提出了一种水膜厚度测量方法——电阻法。利用金属换热管电阻为百欧级、水膜电阻为兆欧级、空气电阻为无穷大的特点,在千分尺上和金属换热管上接上两电极,即可测出水膜厚度,测量精度达μm。测量了换热管外壁环向上水膜厚度随角度变化的数据,测量了增膜板上水膜厚度随高度变化的关系。建立了换热管和增膜板上水膜厚度和水膜热阻的模型。针对二维传热传质模型数值求解时,二元迭代函数曲线斜率绝对值小于1和迭代值衰减振荡的特点,证明了迭代值衰减振荡二元简单迭代法根的存在性、唯一性和收敛性,推论了二元弦割法迭代求解的收敛性。模型求解使用的显式迭代算法,比隐式迭代算法节省计算机内存。用Matlab软件编制了多管程传热传质计算软件。此软件的编制提高了对蒸发式空冷器传热传质性能的研究效率。分析了增膜板对多管程蒸发式空冷器中各管程和各排换热管强化传热程度的变化。增膜板对第一管程第一排管传热的全管强化度较小,到第二排时全管强化度就迅速增加,到第三排管时全管强化度达最大值,随后各管程各排管的全管强化度缓慢减小,但都有强化传热作用。从水膜在换热管外的流动方向看,经增膜板冷却的水膜落到管子顶部时,对传热的沿角强化度最高,随着水沿管壁流动,水温逐渐升高,沿角强化度逐渐减小。增膜板对管子传热沿角强化度的变化规律在管长方向基本相同,故热流体温度变化对增膜板的沿角强化度γ几乎无影响。通过实验,测量和研究了增模板的强化传热传质能力。通过正交实验探索了影响蒸发空冷器换热的主要因素,通过多因素的单变量实验探索了光管管束、增膜板管束、迎面风速、单位面积喷淋量对蒸发空冷器换热性能的影响。实验结果表明,增膜板管束换热能力的提高量与工况有关;在某些工况条件下,与光管管束相比,增膜板管束换热能力的提高量可达25%。更多还原

【Abstract】 Evaporative air coolers are found in a wide range of applications, such as petrochemical plants and steel plants. A new type of evaporative air cooler also called film-increasing type evaporative air cooler (FITEAC) is developed in this paper. In the FITEAC, pieces of hydrophilic plates are placed in the spaces between a layer of tubes and another lower layer of tubes to change hot water falling from the upper layer of tubes into pieces of cooler water film. Placement of these hydrophilic plates enhances heat and mass transfer for the lower layer of tubes because temperature difference between the cooler water film and hot fluid inside the tubes is larger than that between hot water and the hot fluid.improved models for calculation of heat and mass transfer on the surfaces of plates and tubes were built in the paper. Temperature difference and moisture content difference are chosen as the driving forces correspondingly of heat and mass transfer and a system of specific difference equations (SSDE) is developed to solve easily the mathematical model for heat and mass transfer on plates and tubes. Based on traditional one-dimensional heat and mass transfer model for tubes, a two-dimensional model was built in the paper. In traditonal models, it is assumed that hot fluid parallelflows with water film out of tubes and the water film also counterflows with moist air, however, it is assumed that hot fluid crossflows with the water film in the model in the paper and heat transfer coefficient of air and water out of tubes varies with angles. So five variables varying independently which are temperature of air, moisture content of air, temperature of water film, mass flux of water film, temperature of hot fluid are distributed two-dimensionally in the plane made of horizontal axes of tube and vertical line and that is different with the traditional one-dimensional model in vertical direction sharply.Thickness of water film is a key factor for calculating heat transfer coefficient of water film out of tubes in an evaporative air cooler. A new method used to measure thickness of water film, called resistance method, was proposed in the test. Beacause resistance values of metal heat tubes, water film and air are very different, such as, that of metal heat tubes is about 10~6Ω, that of water film is about 100Ω, that of air is nearly infinity, thickness of water film can be measured by two electrodes which are set at a micrometer and a tube, and the precision of the mothod is 1μm. Thickness of water film at some angles on the surface of tube and on the surface of plate is measured by this method. Based on thickness of water film, a heat transfer coefficient model of water film was built.Capability of film-increasing plates for enhancing heat and mass transfer is validated in the tests. Main factors influencing heat and mass transfer in an evaporative air cooler was investigated by orthogonal tests. Effect of velocity of air, mass flow rate of water film, temperature of hot fluid on evaperative heat transfer was also explored in tests. Comparing heat transfer capacity of traditional tubes with that of the film-increasing tubes, it was find that the capacity of film-increasing tubes is 25% higher than that of traditional tubes for some cases.Existence, exclusive and astringency of two-valuabled silmple iterative equation and two-valuabled secant method was discussed for the case that two-valuabled iteratived equation curve is smaller than 1 and the iterative value is attenuation. Based on two-dimensional heat and mass transfer model built in the paper, a software is writed in MATLAB software to calculate heat transfer of an evaporative air cooler with multipass tubes. A specific iterative arithmetic is proposed and used in the software, which ties up fower EMS memory. The software is a efficient tool used to study heat and mass transfer performance of an evaporative air cooler.The present work is supported by the National Natural Science Foundation of China(No. 2007CB206901).更多还原