降膜蒸发液膜形成实验和数值模拟研究

发布时间：2018-09-11 07:33

【摘要】：海水和苦咸水淡化为解决淡水资源短缺问题提供了行之有效的方法,由于水平管降膜蒸发具有小温差换热、较低的蒸发温度、较高的传热系数等诸多优点,从而得到了广泛的应用。研究水平管降膜蒸发技术,可为设计生产更加高效的水平管降膜蒸发器提供重要的技术依据和理论基础。本文利用高速摄像的实验方法研究了四种布液器孔间距在不同喷淋密度和管形下的管间流型和柱状流的降膜波长;采用激光诱导荧光技术研究了开孔参数为单孔和三孔时液膜在管表面的铺展分布。结果表明:同一个管形的孔间距小时,流型转变Re数小;光管的滴-滴柱、滴柱-柱流型转变Re数小于螺旋翅片管的转变Re数,两管形的柱-柱帘流型转变Re数相似。同一管形的液柱波长随Re数的增大而降低;波长随孔间距的增大而增大;Re数一定时螺旋翅片管的波长小于光管的波长。在一定的Re数范围内,液膜轴向铺展存在极限。单孔下随Re数增大,液膜在管表面顶端和底部的轴向铺展长度差值减小。三孔下两液柱中心距随时间呈周期性变动,Re=302时两液柱中心距较Re=172时大。最厚液膜在两液柱中心位置略微左右移动,出现在中心位置的几率为30%。利用VOF方法模拟了水平管外液体的降膜流动,研究了单孔和三孔时液膜在管壁的铺展规律。随着接触角从10°增大到60°,水在管表面各周向角的轴向铺展长度减小,达到铺展长度最短时对应的周向角越小,液膜铺展长度稳定的周向区域增大,相应周向角的液膜厚度增大,即液膜厚度与铺展长度呈相反的分布。随着入口速度的增大,同一周向角的铺展长度呈增大的趋势,且增大的幅度较小。管径32mm的液膜厚度大于管径19mm和25mm的液膜厚度,管径32mm的液膜铺展长度较另两管径的铺展长度短。三孔下周向角90°时XZ平面的液膜厚度分布,出现两个波峰和三个波谷,波峰位于两孔之间,波谷位于液体入口处,在液膜铺展的两侧边缘处液膜厚度突起。当液体入口速度小,液膜轴向铺展长度较短,液膜厚度波动较小,波峰和波谷的差值小。
[Abstract]:Desalination of seawater and brackish water provides an effective method to solve the problem of shortage of fresh water resources. Due to its advantages such as small temperature difference heat transfer, low evaporation temperature, high heat transfer coefficient and so on, horizontal tube falling film evaporation has many advantages, such as small temperature difference heat transfer, low evaporation temperature, high heat transfer coefficient, etc. Thus it has been widely used. The study of horizontal tube falling film evaporation technology can provide an important technical and theoretical basis for the design and production of a more efficient horizontal tube falling film evaporator. In this paper, the flow patterns between tubes and the falling film wavelengths of columnar flows with different spray densities and tube shapes are studied by means of high speed camera. Laser induced fluorescence technique was used to study the spreading distribution of the liquid film on the surface of the tube when the opening parameters were single hole and three hole. The results show that the Re number of flow pattern transition is small when the hole spacing of the same tube is small, the Re number of drip-drop column and drip-column flow pattern transition is smaller than that of spiral finned tube, and the Re number of flow pattern transition from column to column curtain is similar between the two tubes. The wavelength of the same tube decreases with the increase of the Re number, and the wavelength of the helical fin tube is smaller than that of the light tube when the number of re increases with the increase of the hole spacing. In a certain range of Re numbers, there is a limit for the axial spreading of liquid film. The axial spreading length difference of the liquid film at the top and bottom of the tube surface decreases with the increase of the Re number under the single hole. The center distance of the two liquid columns under the three holes varies periodically with time: re = 302.The center distance of the two liquid columns is larger than that of Re= 172. The thickest liquid film moves slightly around the center of the two columns, and the probability of appearing in the center is 30. The liquid falling film flow outside the horizontal tube was simulated by VOF method, and the spreading law of the liquid film in the tube wall was studied with one hole and three holes. With the increase of the contact angle from 10 掳to 60 掳, the axial spreading length of water at each circumferential angle on the surface of the pipe decreases, and the corresponding circumferential angle becomes smaller when the spreading length is the shortest, and the circumferential region of the stable spreading length of the liquid film increases, and the thickness of the liquid film at the corresponding circumferential angle increases. That is, the thickness of liquid film is opposite to the spreading length. With the increase of inlet velocity, the spreading length of the same circumferential angle tends to increase, and the amplitude of increase is smaller. The liquid film thickness of diameter 32mm is larger than that of 19mm and 25mm, and the liquid film spreading length of 32mm is shorter than that of the other two diameter. Two peaks and three troughs appear in the liquid film thickness distribution of the XZ plane at 90 掳angle of the three holes next week. The wave peaks are located between the two holes and the trough is located at the inlet of the liquid, and the thickness of the liquid film protrudes at the edge of the liquid film spreading. When the inlet velocity of the liquid is small, the axial spreading length of the liquid film is short, the thickness of the liquid film fluctuates less, and the difference between the peak and the trough is small.
【学位授予单位】：内蒙古工业大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P747;TK172

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