【作者】 于春健；

【导师】 马学虎；

【作者基本信息】 大连理工大学 ， 化学工程， 2012， 博士

【摘要】 沸腾是一种高效的传热方式,被广泛地应用于诸多工业领域。为了提高沸腾传热效率,研究者们提出了不同的强化方法。其中,限制工作液体所在的空间尺度被认为是一种有效的强化方法。其缺陷是会降低高热通量条件下的沸腾传热效率,原因是此时大量气泡挤在受限空间中无法脱离,这些气泡相互合并后致使加热表面被气膜覆盖进而导致传热效率降低。本文提出了新的强化结构表面,基本解决了气泡难以释放的问题,提高了核沸腾传热性能,并通过高速摄像及红外热像法研究了结构表面的强化机理,在实验的基础上建立了物理及数学模型。设计出一种特殊构型表面,将加热表面划分成具有不同功能的区域,部分区域的空间受限,用来促进气泡形成、生长和脱离,而其它区域上方为自由空间,其作用是存储液体和释放气泡。在受限空间中生成的气泡很容易排放到自由空间中,引起的尾流又能够将自由空间中的液体抽吸到受限空间中,从而完成一个传热循环。通过传热性能测试,得到了不同工作液体在光滑和不同规格的强化表面上的沸腾曲线。实验结果表明,水和乙醇在结构强化表面上的沸腾传热特性得到了明显强化,水在两种强化表面上的平均传热系数分别为光滑表面的2.3和2.5倍,乙醇则分别为3.5和2.3倍。利用高速摄像研究了水和乙醇在光滑及强化表面上的气泡生成、生长及脱离等动力学行为。通过图像处理及分析,得到了气泡直径、核化点密度以及气泡脱离频率的数据,分析了在强化表面上沸腾传热的强化机理。实验结果表明,两种空间受限表面上的气泡均始终产生于空间受限区域,自由空间区域则没有气泡生成。水在两种强化表面上的平均核化点数量分别为光滑表面的2.2和3.3倍,在一种强化表面上气泡平均脱离频率为光滑表面的2.1倍,是沸腾得以强化的关键因素。以乙醇为工质时,两种空间受限强化表面上的平均气泡直径分别为光滑表面上的3.2和2.0倍,平均核化点数量分别为光滑表面的3.3和5倍,平均气泡脱离频率均为光滑表面的1.3倍,是沸腾得以强化的关键因素。在乙醇的沸腾过程中发现了一种新的‘喷射态’气泡形态。运用强化元件确定了金属加热表面上沸腾时产生气泡的核化点位置并运用红外测温手段对加热表面过热度波动幅度及波动频率进行了测定,以研究沸腾时金属加热表面与气泡间是否存在液膜的科学问题。实验结果表明,在实验热通量范围内,加热表面的过热度波动幅度始终未超过1K,而理论计算和文献研究的结果都表明没有液膜存在的加热表面的过热度波动幅度至少为5K。强化表面与光滑表面的平均过热度波动频率的比值在实验热通量范围内平均值为1.1,强化表面与光滑表面的气泡脱离频率的比值在实验热通量范围内平均值为1,这表明二者具有一致的趋势。气泡脱离频率要明显高于表面温度波动频率,强化表面E0.15上气泡脱离频率与表面过热度波动频率的比值介于1-2.4之间,平均值为1.5,而光滑表面上的该项数值介于1-2之间,平均值为1.4。这说明气泡脱离频率和表面温度波动频率在数值上无法对应。低过热度波动幅度,过热度波动频率与气泡脱离频率在趋势上的一致性以及在数值上的差异性共同表明,加热壁面在沸腾过程中没有发生干壁现象,即加热壁面与气泡之间始终有液膜存在。在光滑加热表面的某些区域涂覆超亲水或超疏水涂层,使加热表面呈现不同的润湿特性,制备出局部改性表面。通过传热实验研究了改性表面的沸腾传热特性,并利用红外热像技术对表面上的温度分布及变化状况进行了研究,对传热机理进行了探讨。结果表明,局部改性表面的传热特性得到了强化,并且其传热性能也超越了完全改性表面。表面温度分布研究表明,核化点影响范围的扩大是局部改性表面传热特性提高的主要原因,而气泡脱离频率的影响较小通过理论分析,构建了受限空间表面在大气压下发生池沸腾时的物理及数学模型。基于实验数据,运用指数回归的方法建立了用于计算受限空间沸腾时气泡直径,核化点密度,气泡脱离频率及传热通量的经验关联式。该模型能够分别独立计算气泡直径,脱离频率和核化点密度这三个沸腾过程中的关键参数,再通过这三个参数计算整个过程中的热通量。与以往模型不同的是,在计算这三个参数的时候都引入了受限空间尺度参数,使计算更为准确。对水在两种强化表面上的沸腾传热计算结果表明,本实验所涉及的热通量范围内,本模型能够预测液体在受限空间沸腾时的气泡直径,脱离频率及核化点密度,比文献报导的计算方法更具准确性,能够准确地预测传热通量。更多还原

【Abstract】 Boiling heat transfer is an efficient way which is widely used in various industrial fields. In order to improve the efficiency of boiling heat transfer, many enhanced methods have been proposed within the past several decades. Confined space is one of most efficient methods for boiling heat transfer enhancement. However, its heat transfer efficiency is significantly reduced by the flourish bubbles which can’t be released from the confined space. To solve this problem, this paper presents an enhanced surface to improve the nucleate boiling; subsequently the heat transfer enhancement mechanism was investigated by high-speed CCD and Infrared methods.A new and special surface is designed to enhance boiling heat transfer by dividing a heat surface into different functional areas. Boiling spaces of part of the areas are confined to promote bubble formation, growth and detachment, while the other spaces are kept open to store the liquid and release bubbles. Boiling curves are acquired for water and ethanol on plain and enhanced structure surfaces to test the heat transfer characteristics. Experimental results show that boiling heat transfer characteristics are well improved on the enhanced surfaces, average boiling heat transfer coefficient of water on the two enhanced surfaces are improved to2.3and2.5times of that on plain surface, respectively. For ethanol, responding values are3.5and2.3, respectively.Bubble generation, growth, detachment dynamics are studied by high-speed camera visualization for water and ethanol on different surfaces. Data of the diameter of the bubble, the nucleation site density and the bubble departure frequency are acquired by the image processing. The boiling heat transfer enhancement mechanisms on the enhanced surface are analyzed based on the image processing results. The analysis indicates that all the bubbles on the enhanced surface are generated in the confined space while no bubble emerges from the free space. For water, average nucleation site density on the two enhanced surface are2.2and3.3times of that on plain surface, respectively, average bubble departure frequency on one enhanced surface is2.1times of that on plain surface, are key factors to the boiling enhancement. For ethanol, bubble diameter, nucleation site density and bubble departure frequency on the two enhanced surfaces are3.2and2.3.3and5.1.3and1.3times of those on plain surfaces, respectively, are key factors to boiling heat transfer. Specially, a new form of bubbles is found in the boiling process of ethanol, as ’jet-state’ bubble. The enhanced structure surface is used to determine the exact positions of the nucleation site on the thin copper heating surface and infrared camera is used to test the superheat fluctuation range and frequency to investigate the scientific topic-whether a liquid film exists between heating surface and bubbles. The results show that the fluctuations of the heating surface superheat did not exceed1K at all the measurement points. It conflicts with theoretic calculation and literature results that superheat fluctuation should be at least5K if no liquid film exists between heating surface and bubbles.The results show that the fluctuations of the heating surface superheat did not exceed1K at all the measurement points, the superheat fluctuation frequency curves have the same trend with the bubble departure frequency curves but lower in magnitude. The low superheat fluctuations on the metal heating surface indicate that no dry-out phenomenon occurs during the boiling process, which identifies that a liquid film always exists between bubbles and heating surface. The relationship between the superheat fluctuations and bubble departure frequency also indicates that bubbles are not directly generated from heating surface but are generated within a thin layer of liquid closed to the surface. These results also approve that the liquid film does exist between the heating surface and bubbles. The ratio of superheat fluctuation frequency on enhanced surface to that on plain surface is1.1. while the ratio of bubble departure frequency is1. Bubble departure frequency is obviously higher than superheat fluctuation frequency, the ratio values on enhanced surface are between1and2.4,1.5for average, while the ratio values on plain surface are between1and2.1.4for average. The low superheat fluctuations, the same trend and the difference in value between the bubble departure frequency and superheat fluctuation frequency together indicate that on the metal heating surface no dry-out phenomenon occurs during the boiling process, which identifies that a liquid film always exists between bubbles and heating surface.The heating surface is modified to different wetting properties. Experimental study of boiling heat transfer is performed to investigate the heat transfer characteristics of these surfaces. An Infrared thermography is used to investigate the temperature distribution on the boiling surface, and thus to analyze the boiling mechanisms. Heat transfer experimental results show that the heat transfer characteristics of the specially designed surface are superior to the traditional wetting-property-modified surfaces and also the plain surface. The surface temperature distribution obtained by Infrared thermography shows that the extension of the impact scope of nucleation points is the main reason for the improvement of heat transfer of the surface, while the effect of bubble departure frequency is small.Based on theoretical analysis, physical and mathematical models are established for the confined space boiling under atmospheric pressure. Experimental data are used to verify its accuracy and precision of the model predictions, and compared with other models in literature. Comparison shows that the model can better predict the boiling heat transfer characteristics for liquid boiling in the confined space.更多还原