【作者】 刘洋；

【导师】 牟兴森；

【作者基本信息】 大连理工大学 ， 热能工程， 2024， 硕士

【摘要】 水平管降膜蒸发器是低温多效蒸馏海水淡化设备的核心,具有换热效率高、传热温差小等优势。管外液膜的流动特性对降膜蒸发器的传热表现有着重要影响。现有的研究中,学者们大多通过测量液膜厚度来间接计算液膜速度。在液滴撞击液膜的研究中,液滴撞击水平或倾斜表面液膜的动力学特性受到广泛关注,而对其撞击水平管表面液膜的研究则较少。本文搭建了降膜流动实验台,通过向水中加入示踪粒子,拍摄其运行轨迹来获得液膜流速的分布规律;通过记录液滴撞击液膜后的铺展进程与形态变化,研究液滴撞击圆管表面液膜的铺展特性与速度特性。主要研究内容及结论如下:在管外液膜流动特性的实验研究中,随着喷淋密度的增加,管间依次出现滴状流、滴柱状流、柱状流、柱帘状流和帘状流。管间距增大时,各流态的临界转换雷诺数均增大。柱状流态划分为顺排柱状流与错排柱状流两种,流体粘性越大,越容易形成顺排柱状流。在轴向距离L~*=0-0.3截面处,液膜切向速度表现出随周向角先减小后增大再减小的趋势,其中第一个极小值点出现在周向角30°-60°附近,极大值点出现在90°-105°附近。位于L~*=0.4-0.5截面时,切向速度随周向角度先增大后减小,极大值点出现在90°-105°附近。同一周向角度处,液膜的切向速度随管间距、喷淋密度的增大而增大。在液滴扩展特性的实验研究中,通过拍摄液滴撞击液膜后的铺展过程,观察到液滴撞击液膜时外表面会出现多条小波纹,随着液滴铺展,液滴顶部出现“弹坑”。液滴表面重复出现波纹扩展、“弹坑”震荡变形的过程,最终形成稳定铺展液膜。韦伯数为11、22时,由于液滴速度较小,撞击后在液膜表面形成规则的波状水花与圆弧形边缘线。随着韦伯数增加至32、50,液滴的速度随之增大,撞击后形成带有突起尖端的水花与不规则波浪边缘线。当曲率比改变时,由于液滴携带的能量不变,因此形成波形水花。但扩展边缘线在曲率比为5.2时呈现不规则波浪形,随着曲率比的增大,扩展边缘线逐渐变为规则的圆弧形。液滴扩展过程中,周向速度随周向角度的增大,呈现先保持稳定后减小再保持稳定最终减小的分布规律,存在两个滞速区间。轴向上,速度随着轴向距离的增加单调减小。在相同的周向角度和轴向距离,周向与轴向速度均随着韦伯数的增大而增大,同时周向速度曲线的滞速区间对应的圆周角范围增大。当曲率比减小时,在相同的周向角和轴向距离,周向与轴向速度均随之增大,且周向速度曲线的两个滞速区间的间隔也增大。此外,本文还分析了液滴扩展边缘的形状变化规律,随着液滴的扩展,边缘线越来越陡峭。同时,韦伯数的增大和曲率比的减小都会使扩展边缘线更加陡峭。更多还原

【Abstract】 Horizontal tube falling film evaporator is the key of low temperature multi-effect evaporation equipment,and it has many advantages such as high heat transfer efficiency and small heat transfer temperature difference.The flow characteristics of the liquid film have important influence on the heat transfer of the falling film evaporator.There is almost no precise measurement and research on the velocity of the liquid film outside the tube in existing research,and most of them indirectly calculate the liquid film velocity by measuring the thickness of the film.In the study of droplet impact on liquid film,scholars mostly study the dynamic characteristics of droplet impact on horizontal or inclined surface liquid films,while there is relatively little research on the impact on horizontal tube liquid films.In this article,by building a falling film flow experimental platform,adding tracer particles to the water,and capturing the trajectory of the tracer particles obtain the distribution law of liquid film flow velocity.In addition,the morphology of the droplet after hitting the liquid film is captured to study the spreading characteristics and velocity characteristics.The main research contents and conclusions are as follows.In the experimental study of flow characteristics outside the tube,as the spray density increases,five flow patterns appear sequentially between the tubes:droplet flow,droplet column flow,column flow,column sheet flow,and sheet flow.As the spacing between tubes increases,the critical Reynolds number for each flow pattern increase,and the fluid viscosity is greater,so it is easier to form parallel columnar flow.The tangential velocity of the liquid film decreases first,then increases,and then decreases again with the circumferential angle on the L~*=0-0.3 section.The first minimum point appears around the circumferential angle of30°-60°,and the maximum point appears around 90°-105°.When located at the L~*=0.4-0.5 section,the tangential velocity first increases and then decreases with the circumferential angle,and the maximum point is around 90°-105°.At the same circumferential angle,the tangential velocity of the liquid film increases with the increase of tube spacing and spray density.In the experiment of droplet spreading characteristics,small ripples appear on the droplet surface after hitting the liquid film by capturing images.As the droplets spread,"pit"appears at the top.The process of ripple spreading and"pit"oscillation deformation repeated at the top of the droplet,ultimately forming a stable spreading liquid film on the surface.When the Weber number is 11 and 22,due to the lower droplet velocity,regular wave spray and circular arc edge are formed on the surface of the liquid film.As the Weber number is 32 and 50,the velocity of droplet increases,resulting in the formation of spray with protruding tips and irregular wave edges.When the curvature ratio changes,due to the constant Weber number,the energy carried by the droplet remains unchanged,resulting in wave spray.When the curvature ratio is 5.2,the edge line becomes irregular wave shape.As the curvature ratio increases,the edge line gradually becomes regular circular arc shape.In the circumferential direction,the velocity first stabilizes and then decreases,next stabilizes and finally decreases with the circumferential angle,and there are two stable intervals.In the axial direction,the velocity decreases with the axial distance.At the same circumferential angle and axial distance,the velocity increases with the Weber number,the range of circumferential angle corresponding to the stable interval increases.When the curvature ratio decreases,the velocity increases and the space between the two stable intervals in the circumferential direction also increases.In addition,the shape change law of the spreading edge of the droplet is also analyzed.The edge line becomes steeper as the droplets spread out.At the same time,the increase of the number of Webers and the decrease of the curvature ratio also make the edge lines steeper.更多还原