盐水液滴降压蒸发析盐过程数值模拟

盐水溶液的降压蒸发广泛应用于海水淡化和工业制盐等领域,因此研究盐水在降压过程中的蒸发特性具有重要意义,有助于解决我国水资源缺乏问题。本文通过数值模拟的方法研究了降压环境下盐水液滴蒸发析盐过程,获得了盐析质量和液滴温度随时间的变化。采用的工质为饱和盐水,液滴的初始温度分别为20℃、15℃、10℃;环境压力从0.1MPa降至2000～10000Pa。通过与实验数据相对比,验证了本文模型的有效性。通过该数学模型,分析了影响析盐率和液滴温度变化的主要因素。结果表明:液滴直径越大,在蒸发过程中其析盐率越高,但温度变化越慢;压降速率越快,液滴蒸发速率越快,析盐率越大,温度变化也越快;液滴初始温度越高,蒸发速率越快,液滴表面析盐率越高,但不同初始温度的盐水液滴,在蒸发过程中其最终温度趋于一致。     

盐水液滴降压环境下蒸发过程

盐水溶液在降压环境下的蒸发过程的研究,主要集中在海水淡化领域的降膜蒸发过程应用方面和工业制盐方面的平坦表面的蒸发过程的研究。而本文主要集中研究各种实验因素对于液滴温度变化的影响。对于多组分液滴降压环境下相变过程的研究,实验采用浓度为15%和6%的盐水溶液作为一组比较工质,在初始环境压力为94.5～97 kPa,最终环境压力范围为50～3000 Pa,液滴的初始温度范围为7～30℃,初始直径范围为1～3 mm的条件下进行实验。通过实验数据分析可知:盐水液滴在降压蒸发过程中的中心温度变化有析盐和不析盐两种现象,随着水分不断地蒸发,当液滴浓度达到22.4%而且温度足够低时就会析出盐分,否则,不会出现析盐现象;同时分析不同浓度、不同最终环境压力、不同初始温度和不同初始直径对液滴相变过程和温度变化的影响,并且观察环境压力降低和液滴温度变化之间的关系。     

乙醇溶液液滴降压蒸发过程传热传质特性

针对单个乙醇溶液液滴在降压环境下蒸发的传热传质过程建立了数学模型。模型基于液相的能量守恒和 传质扩散理论,利用经典拓展模型计算液滴的质量蒸发率,并引入活度系数考虑液滴表面的蒸气分压。采用液 滴悬挂法进行实验,分别记录了乙醇溶液液滴和乙酸溶液液滴在降压蒸发过程中的液滴内温度变化。将实验数 据与计算结果对比,验证了模型的有效性。通过模型计算获得了液滴内部温度分布以及浓度分布随时间的变化。 结果表明:快速降压阶段空气流动较快,加之乙醇工质易挥发,液滴表面温度下降迅速,液滴内部温差和乙醇 浓度梯度较大;压力稳定后,空气流速为零,液滴内部温差和乙醇浓度梯度逐渐减小。由于液滴内部的热扩散 速率大于传质扩散系数,内部温度随时间的变化比浓度随时间的变化更快。     

低压环境下海水液滴蒸发模型研究

针对低压环境下的多种盐分组成的海水液滴(NaCl、MgCl_2、CaCl_2、KCl的质量分数分别为20.07%、1.29%、0.412%、0.399%)的蒸发过程进行了模型研究,模型将能量守恒方程和传质扩散方程进行耦合。通过模型计算,分析了液滴内组分含量和温度分布随时间的变化,获得了环境压力和环境温度对液滴温度变化的影响。结果表明,在蒸发过程中,液滴内部的各种盐含量升高过程不是成单一的线性增加的,在液滴的蒸发和内部盐含量扩散的共同作用下,盐含量表现出在先快速上升之后,逐渐下降继而又有所回升的趋势。液滴温度下降至最低温度后,也逐渐回升。并且环境压力越低,液滴温度下降越快,最低温度越低,温度回升越快。环境温度越高,液滴最低温度越高,温度回升越快。     

更正启事

<正>《化工学报》2015年第66卷第7期《盐水液滴降压蒸发析盐过程传热传质特性》一文中基金项目更正为:国家自然科学基金项目(51406055);河北省自然科学基金项目(E2012502069);中央高校基本科研业务费专项资金资助项目(2014MS106)。     

基板温度对超疏水表面液滴蒸发影响

基于椭球模型,模拟了不同基板温度条件下超疏水表面上固着纯水液滴的蒸发过程,探究了蒸发过程中液滴蒸发时间、接触角演变行为、液滴表面温度和蒸发速率等参量。结果表明:基板温度越高,液滴蒸发寿命越短,且蒸发时间随基板温度呈线性关系。液滴蒸发过程中,接触角随蒸发时间成非线性变化。因较大的高径比和较小的热扩散率,基板温度对液滴蒸发初始阶段影响较小。近接触线区域液滴表面温度梯度较大,其他区域温度梯度较小;且基板温度越高,除近接触线区域液滴表面温差越小,这说明蒸发冷却效应受到了抑制。同一体积条件下,基板温度越高,液滴蒸发速率越大,且蒸发速率随基板温度呈指数变化。     

超疏水表面液滴蒸发内部流体流动与传热分析

基于椭球模型,模拟了超疏水表面上固着纯水液滴的蒸发过程,探究了蒸发过程中液滴外形、体积、接触角、表面温度和蒸发速率的变化,以及液滴内部温度和流线的分布情况。结果表明:液滴蒸发过程中,液滴顶部局部表面温度最大,接触线处最小,且随蒸发的进行表面温度逐渐减小。液滴体积演变行为区别于亲水表面,这表明基板特性影响了液滴蒸发机理。液滴蒸发初始时刻,全局蒸发速率最大,随蒸发的进行,全局蒸发速率逐渐减小。液滴表面温度分布不均匀使得液滴表面产生了张力梯度,张力梯度引起了液滴内部流体流动;近气液界面处流体由液滴顶部向下流动,液滴内部流体由下向上流动,形成了Marangoni流。流体流动伴随着热量的传递,导致接触线附近温度较低,液滴内部中心处温度较高。     

水平及竖直基底上微小固着液滴的蒸发特性分析

为揭示非水平表面上微小蒸发液滴的传热传质特性,本文在准稳态模型的假设下构造三维液滴模型,综合考虑了蒸气扩散、蒸发冷却以及气相域中的自然对流这3种传输机理,对水平以及竖直基底上液滴的蒸发过程进行数值研究。通过分析气液界面上温度分布、蒸发通量分布及总蒸发率的变化,重点探究了基底过热度以及重力的改变对液滴蒸发特性的影响。结果表明：与水平基底上温度的对称分布不同,竖直基底上气液界面温度分布表现出明显的非对称性,且非对称性随基底过热度的升高而增强,最低温度点不再位于液滴顶点,而向一侧偏移。此外,水平基底上气液界面局部蒸发通量呈对称分布,各截面分布相似,而竖直基底上局部蒸发通量分布则呈现出显著的非对称性以及各截面异性,非对称性随着基底过热度的升高而增强,这是重力改变后气相域自然对流发生改变的结果。与水平基底相比,竖直基底上蒸发率更高,总蒸发时间更少。最后,基底由水平变为竖直时,液滴内部流场由对称双涡转变为非对称单涡,单涡流速显著大于双涡流速,液滴内流速随基底过热度的上升而增大,单涡环流造成了气液界面温度分布的改变以及最低温度点的偏移。     

湿工况下泡沫金属内换热和压降的数值模拟和实验验证

泡沫金属应用到换热器空气侧有望提高析湿工况下的换热性能。为了了解湿空气在泡沫金属内的热质传递和压降特性,建立了泡沫金属内液滴形成、生长和运动特性的数值模型。基于液滴成核数目和成核临界半径得出液滴形成过程的传质率模型;通过建立液滴与湿空气相界面附近湿空气中水蒸气的组分守恒方程,得出液滴生长过程的传质率模型;通过对不同孔棱柱表面液滴的受力分析,建立在重力和风力的共同作用下的液滴接触角模型。将液滴形成及生长的传质率模型和接触角模型分别作为质量源项和表面张力源项,加入连续性方程、动量方程和能量方程组中,实现对泡沫金属内液滴生长、形成和运动过程模拟。模型的实验验证结果表明,换热量预测值与实验结果的最大偏差为11.9%,压降预测值与实验结果的最大偏差为17.7%。     

Al_2O_3纳米流体液滴蒸发特性的数值模拟研究

纳米流体液滴蒸发现象在电子设备冷却、喷墨打印以及医学检测等领域都有广泛应用。为了研究水基Al_2O_3纳米流体液滴的蒸发特性,建立了纳米流体液滴蒸发的二维瞬态模型,考虑了纳米颗粒输运行为以及液滴内部流动的影响,并采用任意拉格朗日-欧拉法(ALE)捕捉气液运动界面。基于所建立的模型,分析了水基Al_2O_3纳米流体液滴内部Marangoni流、纳米颗粒初始浓度以及基板温度对纳米流体液滴蒸发特性的影响规律。结果表明,液滴内部Marangoni流会影响气液界面温度分布和蒸发速率。由于液滴内部纳米颗粒浓度分布和气液界面温度发生变化,纳米流体液滴的蒸发速率随着纳米颗粒初始浓度和基板温度升高而增加。     

Mathematical Modeling of Drying of Liquid/Solid Slurries in Steady State One-Dimensional Flow

A mathematical model of simultaneous mass, heat and momentum transfer for two-phase flow of a gas and a solid/liquid slurry was developed. The model was applied to calculation of the drying process of coal-water slurry droplets in a gas medium in a steady one-dimensional flow. The model was based on the well-known two-stage drying process for slurry droplets. After the first period of drying, in which the evaporation rate is controlled by the gas phase resistance, the evaporating liquid diffuses through the porous shell (crust) and then, by convection, into the gas medium. Inside the dry external crust of the drop, a wet central core forms, which shrinks as evaporation proceeds. The temperature of the slurry droplet rises. The process ends when the temperature of the dry outer crust reaches the coal ignition temperature in the case of combustion or when the moisture of the particle reaches the final required moisture. The developed model was based on one-dimensional balance equations of mass, energy and momentum for the liquid/solid and gas phases. The system of governing equations was represented by first-order differential equations and solved simultaneously. The numerical solution of the governing equations was obtained using Gear's method. The model permitted calculation     

Mathematical Modeling of Drying of Liquid/Solid Slurries in Steady State One-Dimensional Flow

ABSTRACT

A mathematical model of simultaneous mass, heat and momentum transfer for two-phase flow of a gas and a solid/liquid slurry was developed. The model was applied to calculation of the drying process of coal-water slurry droplets in a gas medium in a steady one-dimensional flow. The model was based on the well-known two-stage drying process for slurry droplets. After the first period of drying, in which the evaporation rate is controlled by the gas phase resistance, the evaporating liquid diffuses through the porous shell (crust) and then, by convection, into the gas medium. Inside the dry external crust of the drop, a wet central core forms, which shrinks as evaporation proceeds. The temperature of the slurry droplet rises. The process ends when the temperature of the dry outer crust reaches the coal ignition temperature in the case of combustion or when the moisture of the particle reaches the final required moisture. The developed model was based on one-dimensional balance equations of mass, energy and momentum for the liquid/solid and gas phases. The system of governing equations was represented by first-order differential equations and solved simultaneously. The numerical solution of the governing equations was obtained using Gear's method. The model permitted calculation     

尿素水溶液液滴的蒸发特性

在石英管式炉上通过挂滴法来观察单个尿素水溶液（urea-aqueous-solution,UAS）液滴的具体蒸发过程,比较了不同环境温度以及不同初始直径大小下液滴的蒸发特性。结果表明,尿素溶液液滴在100～1300 ℃的温度范围内呈现出了不同的蒸发行为。在较高的温度下,液滴的蒸发行为较为复杂,如气泡的产生、液滴的变形以及发生微爆的现象;但是,随着环境温度的降低,这些现象就变得非常微弱甚至消失。同时,还定量分析了稳态蒸发常数与温度、液滴初始直径之间的变化关系,发现在初始直径为2.5 mm、温度在100～600 ℃之间变化的情况下,稳态蒸发常数从0.02075 mm2/s增加到了0.23953 mm2/s,增大了10倍左右。此外,还对气流流速为0.25～1.25 m/s范围内的液滴蒸发特性作了实验研究。当液滴周围有强迫气流存在时,液滴与气体间的换热方式由导热转变为对流换热,从而增强了液滴表面的传热传质能力,促进了液滴的蒸发。     

Salt Crystal Growth as Weathering Mechanism of Porous Stone on Historic Masonry

It is well known that in porous stones, NaCl precipitation on and beneath a surface depends on the solution supply and the evaporation rate according to the microclimate and the effective pore structure.In the present work, the study of salt crystal growth mechanisms in large heterogeneous systems like porous stone masonries is attempted. The historic masonries of the Medieval City of Rhodes act as a pilot, due to the intense marine environment and the climatic conditions in favour of NaCl crystallization into the highly porous stones.Weathered monument samples from various depths on masonries exposed to the sea are examined systematically under SEM and EPMA. The obtained results permit the differentiation of crystal growth patterns occurring during distinguished phases of the evaporation process within the porous stone masonry in depth.In the first phase, salt crystals grow favourably in the larger pores, connecting with the empty evaporation channels, and being supplied by solution from the next smaller pores. The isometric crystal habits attaining an equilibrium form correspond to those growing immersed in the solution, when a granular crust is formed.In a second phase, the crystals already exceed the pore size and overlap other smaller pores. As the rate of evaporation exceeds the solution supply, the solution retires and the substrate dries out, the area where the crystal contacts the solution is reduced, and consequently, columnar crystals grow.The pressure exerted by crystallization against the pore walls, when the crystals filling entirely the coarse pores continue growing, leads to disruption.     

HD低温蒸发系统中填料蒸发器传质传热的分析

通过研究系统中填料蒸发器的蒸发传质传热过程以及两相流动特性，采用计算流体力学（computational fluid dynamics，CFD）中离散相与连续相耦合的方法来模拟规整填料内部通道的蒸发传质传热过程，实现了填料蒸发器中两相传质传热的过程以及液滴流动的可视化，为研究气液两相在规整填料内的流动提供了一种模拟方法。通过与实验结果的比较，最终选用RNG k-ε湍流模型来分析规整填料内部气液两相传质传热以及流动情况。数值模拟研究了规整填料板间距对填料内部气液两相传质传热以及液滴运动影响，发现随着板间距的增大，填料内部压力降逐渐降低，出口空气中水蒸气的含量不断减小，液滴蒸发速率降低，液滴进出口质量差减小，气相出口温度逐渐降低，蒸发传质传热效率降低。随着气速的增大，出口空气中水蒸气的含量不断减小，液滴蒸发速率增加，气相出口温度降低，气液两相传质传热效率降低。     

An Improved,Easily Implementable,Porous Media Based Model for Deep-Fat Frying: Part II: Results,Validation and Sensitivity Analysis

The multiphase porous media model, developed in the companion paper, has been applied to frying of a restructured potato slice to obtain temperature, pressure, moisture, oil content, acrylamide content and evaporation rate profiles, providing valuable insight into the frying process. The model is validated by comparing temperature, moisture content and crust thickness profiles from literature experimental results. A novel non-equilibrium formulation, different from the existing food literature, is able to describe well the evaporation process. Post-frying cooling is included through appropriate changes in boundary conditions. It is seen that the oil pickup mostly takes place during post-frying cooling and is due to capillary suction created by the negative pressures from condensation of water–vapour. Acrylamide is formed primarily in the crust region where temperature exceeds 100°C. Sensitivity analyses of the process to surface mass transfer coefficient, evaporation rate constant and oil diffusivity show that they all have significant effects on the process. Development of this mechanistic model that is also more easily implementable than previous models should make computer-aided design and optimization of frying processes closer to reality.