固液界面接触角对膜状冷凝传热强化的初步分析

引　言冷凝有两种形态 :膜状冷凝和滴状冷凝 .当液固表面自由能差Δσ≥ 33.3mJ·m- 2 [1] ,即液体完全不润湿固体表面时 ,蒸气在表面上将呈现滴状冷凝 ,其传热系数是膜状冷凝的几倍到几十倍 .这已是公认的事实 .但是 ,当表面自由能差在 0～ 33.3mJ·m- 2 范围内时 ,表面强化冷凝传热的效果将取决于表面自由能差值的大小 ,差值越大强化效果越明显 .为了更清楚地表示冷凝传热特性随冷凝液与表面自由能差的变化关系 ,把 0和 33.3mJ·m- 2 分别作为第 1临界值和第 2临界值 .当固液表面自由能差小于第 1临界值时 ,冷凝形态为传统的膜状冷凝 .…     

孔板波纹填料的结构参数对其性能的影响

在内径Φ30 0mm的填料塔中研究了孔板波纹填料的结构参数对液相轴向混合特性、流体力学和传质性能的影响 .研究结果表明 ,轴向返混参数随开孔率、倾角的增加及盘高的减小而增加 ,填料层压降随开孔率、盘高的增加及倾角的减小而降低 ,传质单元高度随倾角的增加、盘高的减小而降低 ,开孔率对传质单元高度的影响有一个适宜值 .通过对实验数据的回归分析得出了轴向返混参数、填料层压降及传质单元高度的关联式 .该研究结果对孔板波纹填料的开发与设计具有参考价值     

气-液-液萃取的流体力学研究

在液-液萃取过程中,提高分散相的表面更新速率可有效提高萃取的传质效率。研究发现,在萃取过程中使用气体搅拌可以增加液液之间的接触面积,促进液相内的湍动和循环。本文研究了气-液-液三相下油滴的流动形态,并对不同填料的流体力学性能进行了测定。实验结果表明,气相速度的增加可导致气含率、液含率的增加,从而提高分散相在填料萃取塔中的停留时间,在一定的速度范围内明显降低萃取的表观传质单元高度,极大地强化传质效果。通过与散装填料对比,发现规整填料更利于强化萃取效果,其液泛速度平均增加25%。     

新型垂直板规整填料流体力学及传质性能

采用氧解吸实验,在直径190 mm的有机玻璃塔内,液相喷淋密度10～38 m³·m^-2·h^-1,F因子0.2～3.2 m·s^-1·（kg·m^-3）^0.5的实验条件下测定了一种新型垂直板规整填料的流体力学及传质性能。实验结果表明：垂直板填料的操作压降及传质性能均显著优于商业波纹填料。通过与几种经改进的250型波纹填料相比发现,两者泛点F因子整体上相当;在较高液体喷淋密度下,垂直板填料传质性能及压降均高于改进250型波纹填料;在低喷淋密度下,垂直板填料可实现压降低于改进250型波纹填料,而两者传质性能相当。此外,对填料结构改进对其性能的影响进行了单因素考察。     

Computational Fluid Dynamics Simulation of 13CO Distillation in Structured Packing

Physical 3D models were established for corrugated packing used in the enrichment of the isotope ¹³C. Computational fluid dynamics (CFD) simulation results indicated that common corrugated packing was not well wetted when used for isotope distillation. It is concluded that liquid misdistribution in the packed tower results from the structure of the packing rather than from the height of the packing beds. The existence of entrainment was also demonstrated by CFD simulation. It is proved that mass transfer equations based on the Nusselt theory are not suitable for distillation calculation in such a corrugated packing system. By comparison, the recently developed structured packing model with a corrugation geometry based on the right‐angled triangle, known as Zigzag‐pak, describes vapor‐liquid distribution properties well and has significant advantages over common corrugated packing due to its better liquid distribution character.     

Mass Transfer In Trickle‐Bed Reactors With Structured Packing

An experimental investigation was carried out to examine the fluid dynamic and mass transfer behavior of structured packing, with the liquid and gas phase flowing co‐currently downwards in the column. Liquid to packing mass transfer coefficients for three positions within the pack were measured by an electrochemical method, varying both the liquid and gas loads as well as the physical properties of the liquid phase. Due to the high void fraction of structured packing, much higher liquid flow rates can be used than in traditional particle trickle‐beds. It was found that in the range studied, the gas superficial velocity has no effect on the mass transfer rate and thus, a general mass transfer correlation in terms of liquid Reynolds number only, was developed. Wetted areas and the radial distribution of liquid through the packing element were determined by a colorimetric method. Within the range of liquid flow rates investigated, complete wetting is not achieved, even with the low viscosity solutions. The measured ratios of hydraulic to geometric area, agree reasonably well with values predicted by existing relationships.     

Prediction of effective area and liquid hold-up in structured packings by CFD

Interfacial effective area and liquid hold-up in structured packing geometries are investigated using the volume of fluid method. Three-dimensional numerical simulations of gas–liquid flow on inclined plane plate and in a structured packing are performed. The VOF method is used to capture the gas–liquid interface motion. After a first validation case on the wetting phenomena prediction on an inclined plane plate, the effective interfacial area, the liquid hold-up and the degree of wetting of packing are studied as function of liquid flow rate and wall surface characteristic (adherence contact angle). Results show that the liquid flow-rate and the contact angle play a significant role. It is found that the interfacial effective area and the degree of wetting of packing increase as the liquid flow rate increases and as the contact angle decreases. Moreover, under the influence of the contact angle, different liquid film shapes are observed. The simulations results are compared to experimental data available in literature. This work shows that the CFD is a powerful tool to investigate performance characteristics of structured packings. Moreover, this work shows how CFD can be used as an effective tool to provide information on fluid flow behavior and determination of interfacial area, liquid hold-up and minimum flow-rate to ensure complete wetting. These parameters could be further used in process simulation at larger scale for the development and the design of efficient packings.     

波纹板填料表面液体降膜流动的特性分析

基于降膜流动实验台,结合计算流体力学方法(Computational Fluid Dynamics, CFD)研究了波纹板表面液体的平均液膜厚度和有效润湿面积等定量信息,并通过三维模拟进一步分析了喷淋密度和波纹倾斜角度β对降膜流动特性的影响。结果表明,液体在波纹板表面的流动并非均匀,分为沟流和溪流两种形式;当喷淋密度较小时,液体在波谷内形成沟流,当喷淋密度达到400 m3/(m2?h),液体跨越相邻波纹进行溪流流动;两种形式波纹板整体的润湿性能均较差,且液膜厚度分布不均;波纹倾斜角度对降膜流动特性影响较大,90°时更有利于提高有效润湿面积。     

Numerical simulation and experimental study of the characteristics of packing feature size on liquid flow in a rotating packed bed

Rotating packed bed has high efficiency of gas–liquid mass transfer. So it is significant to investigate fluid motion in rotating packed bed. Numerical simulations of the effects of packing feature size on liquid flow characteristics in a rotating packed bed are reported in this paper. The particle image velocimetry is compared with the numerical simulations to validate the turbulent model. Results show that the liquid exists in the packing zone in the form of droplet and liquid line, and the cavity is droplet. When the radial thickness of the packing is less than 0.101 m, liquid line and droplets appear in the cavity. When rotational speed and radial thickness of the packing increase, the average diameter of the droplets becomes smaller, and the droplet size distribution becomes uniform. As the initial velocity of the liquid increases, the average droplet diameter increases and the uniformity of particle size distribution become worse. The droplet velocity increases with the radial thickness of the packing increasing, and gradually decreases when it reaches the cavity region. The effect of packing thickness is most substantial through linear fitting. The predicted and simulated values are within ±15%. The cumulative volume distribution curves of the experimental and simulated droplets are consistent with the R-R distribution.     

Mass transfer characteristics of liquid films flowing down a vertical wire in a counter current gas flow

The wetted-wire packing, mainly consisting of a bundle of vertical parallel wires, is a promising concept for the use in separation columns. To investigate the multiphase flow inside the packing in detail and to estimate the performance of the packing, experiments on liquid films on a single vertical wire in a counter current gas flow were carried out. To get information about the interfacial area, an optical measurement of the film thickness was carried out with a digital high speed camera and image recognition tools. By measuring the evaporation of water and aqueous polyvinylpyrrolidone solutions into air, the gas-side mass transfer was determined. The liquid-side mass transfer was examined by measuring the desorption of CO₂ from water into air. The results show that the mass transfer coefficients are comparable to those appearing in common structured packings. When assuming a sufficiently high wire packing density, a specific interfacial area similar to corrugated sheet structured packings can be reached. Previous studies predicted a low pressure drop per packing height and extended capacity limits compared to common packings. In consideration of these results, the wetted wire packing therefore is shown to be suitable especially for absorption processes where a low pressure drop is favourable.     

The effects of nanostructure and composition on the hydrophobic properties of solid surfaces

The effects of nanoroughness and chemical composition on the contact and sliding angles on hydrophobic surfaces were studied theoretically and experimentally. A theoretical model based on forces developed at the contact area between a liquid drop and hydrophobic smooth or nanoroughened surface was developed and compared with the existing models, which are based on forces developed at the periphery between the drop and the solid surface. The contact area based model gives rise to an interfacial adhesion strength parameter that better describes the drop-sliding phenomenon. Consequently, relationships were derived describing the dependence between the interfacial adhesion strength of the liquid drop to the surface of a given composition, the mass of the drop, the measured contact angles and the sliding angle. For a given surface chemistry, the sliding angle on a nanometric roughened surface can be predicted based on measurements of contact angles and the sliding angle on the respective smooth surface. Various hydrophobic coatings having different surface nanoroughnesses were prepared and, subsequently, contact angles and sliding angles on them as a function of drop volume were measured. The validity of the proposed model was investigated and compared with the existing models and the proposed model demonstrated good agreement with experimental results.     

聚丙烯孔板波纹填料表面改性研究

采用液相化学法对聚丙烯孔板波纹填料进行表面改性,并对改性后的填料作性能测试。结果表明,改性后的聚丙烯填料表面粗糙度增大并引入含氧极性基团,其表面水接触角从８８°降低至４５°,临界表面张力由２８×－３Ｎ／ｍ提高到５５×１０－３Ｎ／ｍ,气相总传质单元数平均增加２４％,传质性能大为改善。     

立装规整填料的性能研究

以空气-水为工质研究了立装规整填料的流体力学性能;利用CO_2-空气-水体系研究其传质性能,并与相同试验条件下的平装规整填料进行对比。结果表明,在相同条件下,2者的压降没有明显变化,但立装填料比平装填料的壁流量和液相总传质单元高度分别降低了10.7%和17.4%,其流体力学和传质性能均优于平装填料。     

毛细管中泰勒流液侧传质特性及其强化机理的CFD模拟

采用计算流体力学（CFD）的方法,研究了圆管中泰勒流的液侧传质特性,分析了泰勒气泡上局部传质特性,并研究了气泡上升速度、液膜长度和液栓长度对液膜处和气泡半球帽处平均传质系数的影响。结果表明,泰勒气泡表面局部传质系数存在3个峰值,液膜处的平均传质系数随气泡上升速度增大显著增大,随液膜长度增大而减小,而半球帽处的平均传质系数随气泡速度和液膜长度的增大变化较小,即膜接触时间增加时,液膜处的传质系数降低,而半球帽处传质系数变化较小。另外,引入场协同原则对单元胞内速度场和浓度场进行分析,解释了局部传质特性及强化机理。最后,给出了分别预测短和长膜接触时间下泰勒流液侧体积传质系数的关联式,该式在较宽的管径尺度范围（0.25～3 mm）内的预测误差在±20%以内。     

改性塑料规整填料的流体力学和传质性能研究

主要研究了不同比表面积和开孔率塑料规整填料表面处理前后的流体力学和传质性能。结果表明：经过表面改性后,传质性能可提高约20%～35%,开孔填料经表面改性后阻力不会增加,并且在低喷淋密度时传质效率提高显著。     

液膜性质的小尺度研究

Structured packing is a good candidate for CO₂ capture process because of its higher mass transfer efficiency and lower pressure drop.Now,the challenging problem of CO₂ capture and storage demands more and more efficiency equipment.The aim of the present study is to investigate the liquid film characteristics under counter current gas phase and throw some insight into the enhancing mechanism of mass transfer performance in structured packing.A high speed digital camera,non-intrusive measurement technique,was used.Water and air were working fluids.Experiments were carried out for different gas/liquid flow rates and different inclination angles.The time-average and instantaneous film widths for each set of flow parameters were calculated.It is shown that the effects of gas phase could be neglected for lower flow rate,and then,become more pronounced at higher flow rate.According to instantaneous film width,three different stages can be distinguished.One is the constant width of liquid film.The second is the slight decrease of film width and the smooth surface.This kind of character will lead to less interfacial area and deteriorate the packing mass transfer performance.For the third stage,the variation of film width shows clearly chaotic behavior.The prediction model was also developed in present work.The predicted and experimental results are in good agreement.     

引入液固界面效应的滴状冷凝传热模型

针对液固界面相互作用对滴状冷凝传热的影响,以Rose滴状冷凝传热模型为基础,考虑接触角、脱落直径对冷凝传热的影响,对滴状冷凝过程中液滴空间序列上的构象,作时间序列上的重构,建立了包含液固界面效应的滴状冷凝传热模型.模型计算结果表明液固表面自由能差越大、接触角滞后越小则越有利于冷凝传热.为滴状冷凝文献数据间存在差异的原因提供了一个新的解释,即液固界面效应的影响.模型可计算得到在不同界面条件下的不同传热结果,模型计算结果与Rose实验值以及本文滴状冷凝传热实验较为吻合.     

共轭环翅填料的流体力学与传质性能

以正犁削金属表面翅成形技术加工出共轭环翅填料,用富氧水解吸方法测定了38 mm铝材共轭环翅填料的流体力学和传质性能,并在相同喷淋密度和气速条件下,与同尺寸光滑表面的共轭环填料进行比较。结果表示,共轭环翅填料的单位填料高度压降比普通共轭环填料平均增加15.2%,而传质单元高度则平均降低18.3%;在中等喷淋密度60 m3/(m2.h)以下,共轭环翅填料具有较好的综合性能。     

液相流动方式对波纹规整填料性能的影响

根据液相在波纹规整填料片上呈现渗流、膜状流等不同的流动方式,选择5种不同的波纹规整填料对其流体力学和传质性能进行研究,以探究液相在波纹片上的流动方式对波纹规整填料性能的影响.研究结果表明,液相呈渗流流动的泡沫碳化硅波纹规整填料(SCFP型)有利于液体横向扩散和液膜均匀分布,当液相喷淋密度和气相F因子均较小时,其压降最低,传质效率最高;液相主要呈渗流流动、兼有膜状流动的双层错孔丝网填料(DMⅢ型)有利于波纹片两侧液体交换,强化液体在流动过程中的扰动,其压降及传质性能略逊于SCFP型填料;液相主要呈膜状流动的BX型、DMⅠ型及DMⅡ型填料波纹片表面液膜较厚,横向扩散能力差,其传质效率低于SCFP型和DMⅢ型填料.研究揭示了依靠渗流作用的波纹规整填料具有较好的应用性能,为波纹规整填料的进一步发展开拓了新思路.