红外热像法研究降膜流动

采用高精度红外热成像技术研究固体平壁上加热和冷却液膜流动性能,比较了不同加热强度下液膜表面的温度场特征以及加热温差对固壁上液体分布的影响。传递过程中,界面上浓度和温度变化引起的表面张力梯度,驱使低界面张力处的液体流向高界面张力处（Marangoni效应）,从而造成了液体特殊的流动现象与分布特征。在受热/冷却降膜过程中,由于流速和传热的差异,液膜表面存在一定的温度梯度。横向的温度梯度远大于流动方向的温度梯度,引起的Marangoni效应会造成受热液膜收缩和冷却液膜扩展,从而明显地影响液膜的相界面积及其传递性能。     

Marangoni效应对降膜加热流动的影响

采用红外热像仪和智能化薄膜厚度测试仪研究了垂直板上受热降膜的流动特性, 得到液膜表面温度分布、液膜宽度、液膜厚度、局部液膜厚度随时间变化关系等参数 通过对这些参数的分析,可知降膜加热流动时,液膜表面出现轴向和径向的温度梯度,由此产生的表面张力梯度引起Marangoni效应对降膜流动有着很大的影响降膜加热流动过程中,Marangoni 效应使液膜产生较大的收缩,膜厚增加,表面波动加强.     

电解质溶液的受热降膜传热与Marangoni效应

采用红外热成像技术研究了电解质Na2SO4-H2O溶液受热降膜传热过程中的Marangoni效应。实验结果表明,降膜表面的流向和径向上存在分别由温度和浓度分布不均引起的热和溶质Marangoni效应。降膜径向热Marangoni效应引起了液膜的收缩,使液膜厚度增加,增大了液膜热阻;流向热Marangoni效应加剧了液膜的湍动,减小了液膜滞流层厚度,降低了液膜热阻;径向溶质Marangoni效应则减弱了热Marangoni效应对液膜的收缩作用,促进了液膜的扩展。分析发现在本实验条件下,表征Marangoni效应大小的Marangoni数Ma不能准确地反映不同雷诺数Re下Marangoni效应对降膜流动的影响,为此引入判断Marangoni效应对降膜流动影响程度的Marangoni效应影响因子ε,ε与实验结果具有较好的一致性。     

γ-氨基丁酸溶液降膜传热过程流动特性研究

本课题组借助高分辨率红外热像仪,研究了γ-氨基丁酸（GABA）水溶液在不同温度和流速条件下的降膜流动过程中,Marangoni效应对其流动和传热的影响。实验发现,受热降膜流动过程中液膜存在明显的收缩现象,横向热Marangoni效应显著,在相同的进料温度下,加热温度越高,液膜收缩越明显。同时由于降膜表面更新加快,GABA溶液降膜流动时的溶剂蒸发速度较常规加热方式大大提高。以上结果对于GABA的提浓、富集具有重要意义。     

受热降膜Marangoni效应临界状态表征

采用红外热成像技术,研究了水和Na2SO4水溶液受热降膜的Marangoni效应及对降膜流动的影响。发现在降膜的流向和径向都存在热Marangoni效应,但Marangoni数却不能较好地表征Marangoni效应对液膜流动的影响程度。因而引入Marangoni效应影响因子ε。结果表明,ε能较好地表征Marangoni效应对液膜流动的影响程度,且不同溶液受热降膜存在一个近似相同的临界值(εLc)。当εLεLc,降膜流向热Marangoni效应将引起降膜明显的扰动。根据εLc值,可获得临界Marangoni数MaTc,L。     

降膜传质过程Marangoni效应稳定性分析

通过建立降膜流动传质模型，并考虑液相流动及非线性浓度分布的影响，利用线性微扰理论得到了降膜流动传质过程Marangoni效应发生的理论临界条件.用所建立的降膜流动传质实验装置得出实测传质系数的变化，由此获得Marangoni效应发生的实测临界条件，实测值与理论值基本符合，从而为工业上常见的降膜流动传质过程中Marangoni现象的预测和利用提供了理论依据.     

皂膜解吸Marangoni对流观察及分析

利用竖直流动皂膜装置,使用纹影光学方法观察了由于丙酮从皂液中解吸,在微米级厚度皂膜上出现的滚筒状Marangoni对流结构;通过建立皂膜传质数学模型及求解,分析了丙酮从皂膜解吸过程中浓度及表面张力的变化。结果表明:由于微米级的皂膜厚度,在皂膜平面法向方向皂膜内皂液丙酮浓度变化很小,但浓度梯度较大,对应的表面张力梯度较大,此较大表面张力梯度是丙酮从皂膜解吸过程中出现Marangoni对流的主要成因。     

激光诱导荧光技术观测填料表面非等温降膜流动形态变化

非等温降膜流动在精馏过程中较为常见,其流动特性较为特殊,对填料表面的质量和热量传递有重要影响。本文搭建了可视化的实验装置,在保证液相室温进料的前提下升高填料温度,在规整填料表面构造液相被加热的非等温降膜流动,运用激光诱导荧光技术对液膜形态变化进行观测分析,对液体润湿面积、液膜厚度等参数进行比较。结果表明,随着填料温度的升高,液体润湿面积显著减小,波谷处液膜厚度显著增加,液膜逐渐收缩为沟流,填料表面的不规则液滴数目增多,干板现象越发严重。液体的聚并和分散轨迹消失,液相再分布及表面更新能力降低。因此,填料表面的非等温降膜流动液膜收缩现象十分明显,降低传热传质能力,需在工业生产中引起足够重视。     

不同管径毛细管的毛细蒸发特性实验

以无水乙醇为工质,采用3种不同管径的圆形毛细管进行毛细蒸发实验。研究结果表明：毛细管毛细蒸发过程中,蒸发界面移动后会延展形成一层液膜,这层液膜可以大幅提高液体蒸发速率。这是因为液膜蒸发引起的温度变化会通过Marangoni效应转化为表面张力差,使液膜在蒸发的同时得到不断补充,形成高效的蒸发-补充机制。随管径增大,液膜厚度增加,蒸发稳定性变好,速率变化幅度减小。工质整体蒸发速率随管径变大呈递减趋势。     

倾斜波纹板上液膜流动的CFD研究

利用VOF法建立了液膜在倾斜波纹板上的气-液两相流CFD模型,并根据液膜流动特点提出了表面张力动量源项和气液界面作用力动量源项.模拟结果与文献实验值吻合较好,表明本文提出的CFD液膜流动模型具有一定的可靠性.通过模拟不同性质的液体在不同表面结构波纹板上的流动过程发现,波纹板表面微观结构以及液体性质尤其是液体的表面张力对连续液膜的形成有重要作用,表明通过改变波纹板面微观结构以及降低液体的表面张力可以促进连续液膜的形成,对提高气液之间的传质效率有重要意义.     

振动平板上降膜流动换热特性

为强化平板降膜换热效率,运用VOF模型和动网格方法的耦合技术,编写UDF函数,建立了振动平板上降膜流动换热计算模型。考虑了表面张力和壁面接触角对液膜流动的影响,研究了不同参数对降膜换热的影响,计算结果表明：振动使得平板上的液膜剧烈波动,强化了液膜传热传质效果;随着振幅和频率的增大,液膜换热效果提高,当频率和振幅同时增大时,换热效果明显提高,在研究的参数范围内,当频率f=50 Hz,振幅A=2.0 mm时,相比于无振动平板降膜,换热效果提高了27%。     

功能表面材料与流体界面相互作用对垂直降液膜流动特性的影响

通过调节水温度、添加表面活性剂以及铝合金壁面表面改性处理来改变降膜流体与固体表面之间的表面自由能差值,运用JDC-2000型精密测微仪测定垂直降液膜的厚度,研究固体表面和液体间相互作用对流体垂直降膜流动特性的影响;考察了液膜雷诺数、流体温度、添加表面活性剂、固体表面材料物理化学性质等因素对垂直壁面降液膜流动特性的影响规律。实验结果表明:改变固体表面与降液膜流体的物理化学特性,即改变固液界面的相互作用能够改变流体的降膜流动特性。降液膜平均厚度随液固表面自由能差的增大而减小。     

竖直流动皂膜中解吸导致的Marangoni对流观察

In gas-liquid mass transfer processes,Marangoni convection may occur due to the surface tension gradient produced by mass transfer near the interface.With a falling soap film tunnel and the Schlieren optical method,the Marangoni convection patterns along the film surface were observed directly in the desorption process of acetone from the falling soap film.The Schlieren images showed the regular roll convection in the thin falling soap film during the acetone desorption.The hydraulic characteristics were determined experimentally by measuring the variation of acetone concentration in the film and the surface tension of the soap liquid.The results show that the acetone concentration gradient vertical to the falling direction is very small because the thickness of the soap film is in the order of 10^-6 m.The variation of acetone concentration along the falling film is large,so there is a significant surface tension gradient,resulting in the Marangoni roll convection.The experimental results and a qualitative analysis may be helpful to understand the mechanism of Marangoni convection near the interface in the mass transfer.     

Experimental investigation of maldistribution in vertical plate falling film tower

Falling film towers are popular in process industries due to moderate air side pressure drop, less energy ingesting liquid distribution and easy cleanup features. Two major limitations of falling film towers are the uneven distribution of liquid across solid surfaces (maldistribution) and contraction of falling liquid film on the solid surface (poor wetting). Maldistribution of liquid is primarily dependent on the type of liquid distributor and scope of liquid flow (slit opening) across solid surfaces. In the current communication; the influence of liquid distributor on maldistribution problem for vertical plate falling film tower has been experimentally investigated. Four types of liquid distributors: plain tube, spray nozzle, perforated plate, and branch tube distributor have been tested. Effect of parameters: slit opening, liquid flow rate, and the number of plates on maldistribution are also explored. It is found that branch tube distributor is best suited for vertical plate falling film tower application.     

CFD Simulation of Liquid Film Flow on Inclined Plates

A two‐phase flow CFD model using the volume of fluid (VOF) method is presented for predicting the hydrodynamics of falling film flow on inclined plates, corresponding to the surface texture of structured packing. Using the proposed CFD model the influence of the solid surface microstructure, liquid properties and gas flow rate on the flow behavior was investigated. From the simulated results it was shown that under the condition of no gas flow the liquid flow patterns are dependent on the microstructure of the plates, and proper microstructuring of the solid surface will improve the formation of a continuous liquid film. It was also found that liquid properties, especially surface tension, play an important role in determining the thin‐film pattern. However, there are very different liquid film patterns under the action of gas flow. Thinner liquid films break easily, but thicker liquid films can remain continuous even at higher gas flow rates, which demonstrates that all factors affecting the liquid film thickness will affect the liquid film patterns under conditions of counter‐current two‐phase flow.