微波强化液桥式螺旋降膜蒸发器数值模拟

微波加热薄膜蒸发技术在促进极性/非极性混合物分离领域潜力巨大,但仍面临着能源利用效率低和加热不均的挑战,而电场分布不均是其根本原因,但影响电场分布的因素十分复杂且不可控,因此,从蒸发器结构及流体流动形式视角出发可为解决微波能高效利用的瓶颈提供新思路。为此本文提出了液桥式螺旋降膜蒸发器,通过COMSOL建立三维模型并模拟计算了微波能强化蒸发器上的螺旋降膜流动与蒸发过程,以蒸发率和温度变异系数作为评价指标,探究液膜厚度、螺距、蒸发器直径、流量以及时间对微波能利用效率的影响规律,研究结果表明该种结构在一定微波入射功率下,液膜蒸发率可达29.26%,温度变异系数降至0.0867,为微波能强化蒸发分离装置的设计提供了依据。     

基于液膜流型的双入口管柱式气液分离器性能研究

管柱式气液分离器(gas-liquid cylindrical cyclone,GLCC)是一种耦合离心力与重力作用的分离设备,常用于深海油气分离。气相带液(liquid carry-over,LCO)是影响GLCC分离性能的关键问题,且LCO率与GLCC上部筒体内的液膜流型有密切关系。因此,通过调控液膜流型来控制LCO率是一种可行方法。提出了一种向上分支的双入口管柱式气液分离器,并在其分支管增设一个阀门以控制两入口间流量比,达到调控液膜流型的目的。利用高速摄像机,通过改变入口气、液流量和阀门开度,系统研究了液膜的分布特征;并利用数值模拟对GLCC液膜流型、内部流线及速度特性做了研究。支路流通面积比从100%改变至0时,流经倾斜管主路的流量增多,液膜占据的筒体壁面高度沿轴向逐渐降低,且液膜集中在倾斜管主路入口附近并形成对分离性能有利的旋环流流型;模拟结果显示,在前述过程中,旋流场涡核中心逐渐稳定,有利于抑制LCO的发生。调节入口阀门开度以调控液膜流型是改善GLCC分离性能的可行手段。     

Analysis of two-phase contacting in a rotating spiral channel

The separation of one component from a multicomponent fluid solution is commonly achieved by bringing the solution into contact with a second immiscible phase, as in liquid–liquid or gas–liquid contactors. Counter-current flow of the phases allows high purity separations and existing approaches typically force one of the phases to disperse into the other, producing the close contact necessary for the solute species to be distributed by diffusion throughout the volume of each phase. This phase mixing leaves the contacting strongly dependent on fluid and interface mechanical properties. Thus an approach that suits one phase and solute system will not necessarily suit others. In addition, it is then necessary to separate the dispersed phase which can be a major drawback of the operation sometimes posing substantial difficulties in easily emulsified liquid–liquid systems or in easily foamed gas–liquid systems. A new approach, which has recently been demonstrated experimentally for microchannels, uses a rotating spiral channel to allow controlled contacting giving a very high ratio of interfacial surface area to fluid volume but avoids phase mixing. Its application to larger channels, up to millimetres in size, is considered here. The two phases are forced to flow side by side in parallel layers along the narrow spiral channel. Selection of spiral parameters, rotation rate and pressure gradient along the channel controls the flow rate ratio of the phases and the relative thickness of the phase layers. This allows adjustment to reach the optimum mass transfer (limited strongly neither by one phase nor the other) and adaptation to phase and solute systems having widely differing fluid viscosities, densities, solute diffusivities and interface equilibria. With appropriate control over these parameters, a single device is, in principle, capable of application to a wide range of separation requirements. This rotating spiral contacting is, however, a new technology and remains to be investigated and tested in detail. The present work develops a model yielding both quantitative prediction of flow and mass transfer in the contacting channel and a framework for determining suitable designs, operating conditions and mass transfer performance for liquid–liquid and gas–liquid operations.     

Pattern formation in a confined polymer film induced by a temperature gradient

We have studied a morphological instability of a double layer comprising the polymer film and air gap confined between the two plates set to different temperatures. The temperature gradient across the double layer causes the breakup of the polymer film into well-defined columnar, striped or spiral structures spanning the two plates. The pattern formation mechanisms have been discussed. The formed patterns can be transferred to produce PDMS stamp, a key element of soft lithography for future microfabrication.     

Spiral column configuration for protein separation by high-speed countercurrent chromatography

Retention of the stationary phase of aqueous–aqueous polymer phase systems is improved by a spiral column configuration which utilizes the radially acting centrifugal force along the spiral pitch to retain the heavier phase in the outer portion and the lighter phase in the inner portion of the spiral channel. For the separation of proteins which has low mass transfer rates, the system needs further modification of the separation channel to interrupt the laminar flow and enhance mixing of the two phases.     

重力对垂直相分离冷凝管内流型调控的影响

随着重力的降低,冷凝换热性能急剧恶化,特别在微重力条件下,流型与传热极不协同,所需的冷凝长度要比地面常规重力情况大一个数量级。基于非能动相分离概念的适用于不同重力条件的新型相分离冷凝管,促进了流型与传热的协同,极大强化了冷凝换热。在地面常规重力、小重力和微重力情况下,数值研究了新型冷凝管内垂直上升的空气-水两相流型的调控过程。通过研究得出:在三种不同重力条件下,均呈现“气在壁面,液在中心”的全新分布模式,特别是在微重力情况下,环隙区域内完全被气体占据,液体完全在核心区域内流动;重力越小,调控后液膜厚度减小幅度越大,特别在微重力条件下,减薄到1/32;重力越小,调控后薄液膜主导的冷凝换热量提高幅度越大,特别在微重力条件下,调控后提高到57.4倍,极大地强化了冷凝换热。总体来说,重力越小,流型调控过程越有利于强化冷凝换热。     

Morphology development and characterization of the phase-separated structure resulting from the thermal-induced phase separation phenomenon in polymer solutions under a temperature gradient

This paper studied the morphological development during the fabrication of anisotropic polymeric materials using the thermal-induced phase separation phenomenon (spinodal decomposition) in a model binary polymer solution under a linear spatial temperature gradient using mathematical modeling and computer simulation. The model incorporated the non-linear Cahn-Hilliard theory for spinodal decomposition and the Flory-Huggins theory for polymer solution thermodynamics. Moreover, the slow mode theory and Rouse law were used to account for polymer diffusion. The two-dimensional numerical results showed that an anisotropic morphology was developed when a temperature gradient was imposed along the polymer solution sample. The droplet size and droplet density decrease as temperature increases during the intermediate stage of spinodal decomposition. The spatial temperature gradient, however, had insignificant effect on the droplet shape.     

An atomic force microscopy study of ozone etching of a polystyrene/polyisoprene block copolymer

The ozone etching of a commercial poly(styrene)/poly(isoprene) (PS/PI) block copolymer (Kraton D1117) was studied by atomic force microscopy. The copolymer contains 17% PS and forms a cylindrical phase in the melt. The copolymer dewetted when spin coated onto a silicon wafer but the film was stable on a grown silicon oxide layer. The structure of the stripe pattern formed was examined on substrates with different oxide layer thicknesses (surface energies). Finally etching by ozone was investigated. For low ozone doses, no degradation of polymer was observable. Extended ozone treatment resulted in more obvious degradation, but the etching was non-selective.     

固体平壁上受热降膜分布模型

在降膜受热流动过程中,由于液膜边缘与膜中央之间存在速度差异,造成液膜横向温度分布不均,由此在液膜横向产生了表面张力梯度, 即引起了自液膜边缘向液膜中央的Marangoni流动,从而使得液膜收缩变形. Marangoni流动引起的收缩效应与液体润湿性及流动压力的扩展效应相互作用, 在液膜边缘形成了凸起区. 根据液膜边缘凸起区内的受力平衡和物料平衡关系,同时考虑温度引起的表面张力梯度对液膜流动的影响,建立了受热降膜收缩模型. 此模型显示, 较低的壁温、较小的固液接触角以及较大的液体流量有利于液膜在加热固体壁面上的扩展. 通过与实验数据的对比显示,该模型较准确地预测了受热液膜下落初始过程中的液膜分布, 能够为传质传热过程及其设备的优化设计提供理论依据.     

新型外分液结构调控水平管间歇流流型

相变传热广泛存在于换热领域,而两相流中的厚液膜是恶化相变传热的根本原因。提出在换热管壁面设计新型网状外分液结构,利用金属网进行间歇流气液非能动相分离,从而调控流型、强化传热。通过搭建空气-水水平管冷态实验台,证实当间歇流经过外分液结构时,气体在表面张力作用下被金属网孔拦截,液桥和厚液膜则被分离出换热管,使管内液相减少,气相汇聚,有效增大气相与管壁的接触概率;多级分液后气液两相长度比呈振荡上升,甚至将间歇流调控为分层流。减小液速或增大气速,分液效果越好,间歇流最终转变为分层流所需经历的外分液级数越少,预期此结构可显著提高相变传热效果。     

Phase separation induced ordered patterns in thin polymer blend films

Strategies for the utilization of phase separation to generate ordered pattern in polymer thin film are reviewed. First, the fundamental theory and factors influencing phase separation in polymer thin film are discussed. That is followed by a discussion of the formation of ordered patterns induced by phase separation in polymer thin films under the influence of a chemical heterogeneous substrate, convection or breath figures. The mechanisms and the conditions for well-ordered structures generated by phase separation are then discussed to show that multi-scaled/multi-component patterns, stimuli-responsive patterns may be developed by controlling the preparation conditions or exposing the sample to different environments more complex structures. Finally, applications of fabricated patterns in pattern generation and reproduction, antireflecting coating, catalysis, bio-chips and optoelectronics are also discussed.     

相分离概念调控水平管分层流流型

冷凝传热广泛存在于各种冷凝器中,传统冷凝管内厚液膜将蒸汽与冷壁面隔开,是恶化冷凝传热的根本原因。提出采用非能动相分离概念,在冷凝管内设置柱状金属丝网,管壁与丝网之间形成环形间隙。液体在表面张力作用下被捕获到丝网内,气相在环隙内流动,使气液相分布与传热协同。为验证这一新颖学术思想,开展空气-水两相流实验,获得相分离概念调控水平管内分层流实验结果。发现当水平管内具有一定液位高度时,全部液体被捕获到网内流动,管壁完全被气相覆盖,实现“气托液”模式。水平管内液位较低时,部分液体被捕获至丝网内,气相与管壁接触面积增大。按以上相分离方法对分层流的调控,在发生冷凝传热时,预期可实现高强度冷凝传热。     

卧螺离心机用于盐泥固液分离时转鼓转速对流场特性的影响

对矿井采卤制盐工艺产生的盐泥使用卧螺离心机进行固液分离,应用计算流体力学软件Fluent,基于多相流Eulerian模型、RNG k-ε湍流模型及多重参考系（MRF）方法,对卧螺离心机流体域进行三维数值模拟,采用仿真与实验相结合的方式,研究了转鼓转速与分离特性的关系,结果表明：卧螺离心机在考虑螺旋叶片情况下比忽略螺旋叶片时切向速度滞后系数有明显提升;提出修正离心液压的概念,通过该理论数值证明分离液静压值与理论值的误差源于切向速度的滞后;当转鼓转速达3000r/min以上继续提高时,沉渣固相质量分数不会随之继续增大;分离液中固相微粒的沉降速度随转速增大会小幅均匀增大;实验表明卧螺离心机用于高浓度卤水高速离心时,将在径向产生一定程度的NaCl浓度梯度,溶质浓度在分离液外层较高,内层较低。     

Mechanism of micro-particle motion across falling liquid cylinder for PM2.5 separation (Ⅰ)Trajectory of particle motion towards surface and separation radius

提出了横掠液柱流的速度场和温度场推动PM_2.5微粒附面运动机理,建立了微粒附面运动微分方程和数值积分反演方法,计算粒子运动轨迹并预测可吸收的微粒运动最大分离半径。定义最大分离半径与液柱表面之间附面层的厚度为分离厚度,以气溶胶流体通过该区域的体积流量与横掠单液柱的总体积流量之比代表单液柱吸收效率;热泳推动力是强化吸收效率的主要因素。基于单液柱吸收效率,按串联模型导出规则排列的液柱群整体分离效率计算公式,依据液柱交叉流几何结构、流体流动和气液两相传热传质参数即可确定整体分离效率。对交叉流Reynolds数为170的实例计算显示,直径4 mm的单液柱吸收效率为1.18%,由195排液柱群组成的长度为1170 mm的分离通道整体分离效率达到90%。     

Orientational phase-separated domains in a polyolefin blend under a temperature gradient field

We investigate the effect of a temperature gradient on the orientation of phase-separated structures in a polyolefin blend system. Phase contrast optical microscopy (PCOM) has been used to measure the morphology of phase separation via spinodal decomposition as a function of phase separation time and temperature gradient. The bicontinuous and interconnected tubelike structure, the characteristic morphology of the spinodal decomposition process, exhibits a preferential alignment along the direction of temperature gradient after phase separation. The orientation of the bicontinuous and interconnected tubelike structures gradually increases with phase separation time and temperature gradients. Also the orientation of phase-separated domains can respond really quickly to the change in the direction of external temperature gradient field. The results suggest that “thermal force” induced by the temperature inhomogeneity might play an important role in aligning phase-separated domains preferentially along the temperature gradient direction.     

分相式多孔壁微通道流动冷凝传热强化与减阻

微通道换热器应用广泛,强化传热和减阻是新型换热器设计的重要目标。为了同时实现这两相目标,本文提出了一种分相式多孔壁微通道冷凝器,利用微针肋阵列组成的多孔壁在冷凝传热过程中实现了汽液两相分离。采用实验研究方法对比了分相式多孔壁微通道与普通实心壁微通道的流动和传热特性,结果证明分相式微通道在冷凝传热中同时具备强化传热和减阻的作用。深入研究了通道内两相流动摩擦耗散原理并提出了相分离减阻理论,指出汽液两相流内部摩擦耗散的减小是分相流减阻的关键。另一方面,分相过程使针肋换热面侧壁直接与高温蒸汽接触,极大消减了蒸汽与换热壁面之间的传热液膜厚度。沿流动方向不断扩展的液通道截面与不断减缩的汽通道截面积适应了流动冷凝过程延工质流动方向"水渐多,汽渐少"的规律,保证沿程传热效果不会恶化。     

Pulsation cyclic liquid-liquid chromatography

This article considers new methods of liquid-liquid chromatography with one phase retained in a free state in a spiral column during the second phase pumping through it by imparting liquids with alternate motion. At that, compared with the known methods, the instrumentation of extraction-chromatographic separation of the components mixture becomes much easier and less expensive. The retention of the stationary phase in a spiral pulsation-chromatography column has been experimentally investigated. The results of experiments and calculations prove the prospectiveness of pulsation-cyclic liquid chromatography.     

垂直上升气液两相弹状流模型

基于等效弹单元思想,改进了预测垂直上升管中充分发展气液弹状流流动特性的模型。 模型中考虑了界面切应力对液膜运动的影响;并在液弹空隙度预测中引入临界气体夹带速度的概念,以此来描述弹状流中大气泡尾部的混合特性。本文提出的模型还考虑了管径对液弹空隙度的影响。弹状流模型的计算结果得到本文及其他作者实验数据的验证。     

Phase-separation behavior in a binary mixture fluid layer subjected to a vertical temperature gradient

The phase separation behavior in a layer of an isobutyric acid (IBA)-water binary mixture was observed from the bottom and side of the layer, where the layer was subjected to a vertical temperature gradient spanning the critical temperature. As a result, the following conclusions were obtained. (1) Patterns characterized by a polygonal cell including one droplet, similar to those observed by Assenheimer et al. (Physica A 208 (1994) 373), were formed in a relatively thick liquid layer, e.g., thick, in a limited range of the temperature gradient, although the patterns ultimately disappeared. (2) The growth and extinction rates of the phase-separated droplets increased with ΔT/d, and increased particularly rapidly at high ΔT/d. (3) The formation of the pattern of polygonal cells each with a droplet is considered to be due to thermocapillary motion along the droplet. (4) The concentration-driven buoyancy convection affects the growth rate of the phase-separated droplets. (5) The extinction rate of the phase-separated droplets at the late stage is affected by the thermocapillary motion along the droplet surface.     

双列螺旋槽液膜密封相变现象及性能

为了探究相变现象对密封性能的影响规律,通过联立N-S方程与质量输运方程,建立了液膜密封相变模型,使用有限体积法对控制方程进行离散,对双列螺旋槽液膜密封相变现象进行了仿真模拟,获得了液膜流线及相态分布并分析了结构参数对相变区域与密封性能的影响。结果表明：液膜发生相变后物性参数发生变化,密封间隙内流场与端面压力分布发生明显改变。内侧螺旋槽可以提供稳定的开启力并保证密封端面处于较好的润滑状态,但同时导致密封泄漏增加。通过减小外侧螺旋槽槽面宽比、槽台宽比、螺旋角、槽深或增大外侧螺旋槽槽数均可降低密封泄漏量,提升密封性能。