Al2O3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Al₂O₃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Al₂O₃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。     

氧化铝纳米流体液滴瞬态蒸发速率的演化特性分析

以氧化铝纳米流体液滴为研究对象,本文建立了基于任意拉格朗日-欧拉（ALE）法的液滴蒸发瞬态模型,对液滴蒸发过程中蒸汽浓度、纳米颗粒浓度、温度等进行多物理场耦合,并考虑了Marangoni流对液滴蒸发的影响,同时研究还结合蒸发实验可视化结果,分析了氧化铝纳米流体液滴的瞬态蒸发速率随时间的演化规律,讨论了颗粒体积分数和基板温度对蒸发模式的影响。结果表明,在液滴蒸发过程开始时,纳米流体液滴保持定接触半径蒸发模式,气液界面面积逐渐减小,瞬态蒸发速率也呈逐渐减小的趋势;当颗粒体积分数增大至26%时,瞬态蒸发速率曲线达到驻点;蒸发接近完全时,由于Marangoni流影响了内部流场、强化了内部传热,且液滴在已沉积在基板上的颗粒表面形成液膜,瞬态蒸发速率迅速增大。     

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。     

含有纳米颗粒的悬浮纳米液滴蒸发特性的分子动力学模拟

采用非平衡分子动力学模拟方法,研究单个含有固体金属纳米颗粒的悬浮纳米液滴的蒸发特性。模拟结果表明,含有金属纳米颗粒的悬浮球形纳米液滴,在蒸发过程中基本保持球形不变;模拟温度越高,金属纳米颗粒的质量分数越大,纳米流体液滴的球形度越小。当蒸发过程开始时,纳米流体液滴的蒸发速率很大,而且模拟温度越高,蒸发速率越大,随后,蒸发速率急剧下降;随着蒸发过程的进一步进行,蒸发速率缓慢下降。金属纳米颗粒的种类和质量分数,对悬浮纳米流体液滴的蒸发速率影响不大。     

乙二醇基纳米流体黏度的实验研究

实验研究了3种乙二醇基纳米流体（Al₂O₃-EG、ZnO-EG、CuO-EG）在不同质量分数（0.5%/3.0%/5.0%/7.0%）下的相对黏度随温度的变化规律,实验所用乙二醇基纳米流体采用两步法配制获得。结果表明：在30～60℃温度范围内乙二醇基纳米流体的相对黏度同温度之间并无较强的函数关系（单调递增或递减）;但在质量分数较高时,3种乙二醇基纳米流体的相对黏度随温度的变化会出现波动,且以非球形颗粒的ZnO乙二醇基纳米流体的波动最为显著;乙二醇基纳米流体的相对黏度均随纳米颗粒体积分数的增大而增大,其中CuO乙二醇基纳米流体相对黏度的增长速度最快,Al₂O₃乙二醇基纳米流体的增长速度最慢。最后比较分析了文献中相对黏度预测公式与本文实验数据的相符程度。     

二氧化铈/水基纳米流体核沸腾传热特性

对CeO₂纳米流体进行了池沸腾传热特性研究,考察了CeO₂/水基纳米流体的热导率,静态接触角以及沸腾后表面沉积情况对沸腾传热的影响。结果表明,CeO₂纳米流体可提高沸腾传热系数,且纳米流体最佳质量分数为0.05％,其沸腾传热系数较去离子水提高36%。热导率以及接触角随纳米流体质量分数的增加而增加,在本实验范围内,热导率最大增加1%;而纳米流体接触角从50.5°增加到92.9°;表面沉积随纳米流体的质量分数增加越来越明显,去离子水在沉积表面的接触角发生较大变化（51.4°~134.4°）。纳米流体的热导率影响可忽略不计;而接触角和沸腾表面颗粒沉积对纳米流体的强化传热作用影响较大。     

改性SiO2纳米颗粒沸腾沉积层的形成原理及其沸腾换热

实验研究了改性SiO₂纳米流体液滴蒸发后的沉积图案,以及改性SiO₂纳米颗粒沸腾沉积层对沸腾换热的影响。液滴蒸发实验研究表明：改性官能团会影响改性SiO₂纳米颗粒是否吸附在液-气界面,从而推断出在沸腾过程中改性官能团对纳米颗粒沉积方式的影响。沸腾实验研究结果表明：用聚乙二醇基团改性的SiO₂纳米颗粒沸腾沉积层使加热面的平均粗糙度从160 nm大幅增长到977 nm,且能增强纯水的沸腾传热系数;而用磺酸基团改性的SiO₂纳米颗粒沸腾沉积层对加热面的平均粗糙度的改变不明显,只使其增大了60 nm,且恶化了纯水的沸腾传热系数。通过沸腾换热实验结果较好地验证了通过液滴蒸发实验推断出的沸腾过程中改性官能团对纳米颗粒沉积方式的影响。     

纳米流体液滴蒸发的实验研究

采用粒径为纳米级颗粒的液滴作为研究对象,对比纯水液滴的蒸发过程,发现接触线在基板上的钉扎源于颗粒随毛细流向外移动到接触线处、在接触线附近的沉积聚集作用,最后出现了类似于“咖啡环”的环状干燥图案。随着颗粒质量分数的递增,发现蒸发过程中接触角及接触半径也相应发生变化,验证了液滴蒸发的三种蒸发模式。同时发现并解释了粘滑移动现象,以及不同粒径下发生该现象的浓度范围规律。     

DMLS微型换热器内纳米粒子浓度对Al2O3/R141b流动沸腾压降的影响

为探究纳米粒子浓度对纳米流体制冷剂在微细通道中流动沸腾气液两相压降影响,运用超声波振动法制备质量分数为0.05%、0.1%、0.2%、0.3%、0.4%均匀、稳定的Al₂O₃/R141b纳米流体制冷剂,在直接激光烧结（DMLS）微型换热器中,设计系统压力为176 kPa,纳米流体制冷剂入口温度为40℃,在热通量21.2~38.2 kW·m^-2和质量流率183.13~457.83 kg·m^-2·s^-1工况下,研究纳米粒子浓度对Al₂O₃/R141b纳米流体制冷剂流动沸腾气液两相压降影响。研究结果表明：纳米粒子浓度对纳米流体制冷剂在微细通道中流动沸腾气液两相压降有显著影响,气液两相压降随纳米流体制冷剂的纳米粒子浓度增加而减少,在纯制冷剂中R141b加入纳米粒子Al₂O₃,不同质量分数的纳米流体制冷剂流动沸腾气液两相压降降低5.5%~32.6%;通过SEM和表面静态接触角测试方法,发现纳米流体制冷剂沸腾气液两相压降随质量分数增加而减少的原因是纳米颗粒沉积在通道表面,增加了微通道表面的润湿性;对比国际上3种比较经典流动沸腾两相压降模型,并基于Qu-Mudawar关联式和Zhang关联式进行修正,得出两相压降结果的85%数据点位于修正后的关联式模型值的±15%范围之内,同时实验结果与修正后的模型结果偏差MAE值为11.7%,说明修正后关联式能有效预测本工况下实验值。     

三元混合纳米流体稳定性及热性能

采用两步法制备体积分数为0.005%～1%的Al₂O₃-TiO₂-Cu/水三元混合纳米流体,粒子体积比为40∶40∶20。为了提高其稳定性,添加少量的PVP（0.005%）表面活性剂,并用XRD、TEM、紫外分光光度计和沉淀观察法共同表征纳米流体的稳定特性。测量温度为20～60℃的黏度和热导率,并与相对应的单一纳米流体作对比。试验结果表明,三元混合纳米流体由于各种粒子的粒径和表面能不同,小粒径的Al₂O₃颗粒填充在大粒径TiO₂和Cu颗粒形成的间隙里,可形成致密的固液界面层。三元混合纳米流体由于粒子的特殊排布,使其热导率明显高于相同体积分数下对应的单一纳米流体,黏度却无明显增大。当体积分数和温度分别为1%和60℃时,与Al₂O₃/水纳米流体对比,其热导率增大了5.5%,黏度下降了2.6%。热导率随温度和浓度的升高而升高;而黏度随浓度的升高而升高,随温度的升高而下降,这与单一纳米流体的性质一致。最后,基于试验数据,对热导率和黏度分别进行与温度的拟合,R²分别为0.9835和0.9820,能较精确地预测Al₂O₃-TiO₂-Cu/水三元混合纳米流体的热导率和黏度。     

纳米流体液滴在水平加热面上的变形行为特性

采用格子Boltzmann力矩模型模拟了纳米流体液滴在水平加热面上的变形过程,结果表明纳米流体液滴的变形经历了销联、解销阶段,并且由于纳米颗粒的强销联作用,在蒸发的后期可以明显看出环形沉积,这是在考虑布朗运动、粒子间相互作用等作用力后由于纳米颗粒的强销栓作用而表现出的纳米流体的独有性质。模拟结果与文献中的实验结果作了对比,表现出良好的一致性,表明格子Boltzmann方法可以成功模拟纳米流体液滴在水平加热表面蒸发过程中的变形特点。     

Influence of Plate Size on the Evaporation Rate of a Heated Droplet

The purpose of this study is to numerically investigate how the width of a plate influences natural convection around a droplet. Droplets evaporating on hot surfaces have many applications including drying of dishes and paint. Evaporation rate and deposition of particles withheld in the fluid are of great importance in both cases. As a first step to investigate how the drying rate and deposition mechanisms can be controlled, this work aims to investigate how the external flow around a water droplet influences the evaporation rate. Natural convection caused by the hot plate on which the droplet rests is considered and the effect of different widths is examined. Results show that an extension of the plate past the droplet will increase the maximum velocity in the domain due to natural convection while the flow close to the surface is decreased due to the no-slip condition and temperature gradient. A decrease of the evaporation rate is therefore observed when the plate is extended past the droplet as compared to the case when the plate and droplet have the same diameter. Simulations furthermore show that the results from the heat and mass transfer analogy only compare well to the results of Fick's law when the droplet and plate have the same width.     

Transport and deposition patterns in drying sessile droplets

The literature on drying sessile droplets and deposition of suspended material is reviewed including the simple explanation of the “coffee ring” deposit given by Deegan et al.¹ Analytical and numerical solutions for the flow are given, including the effect of Marangoni stresses, pinning or movement of the contact line, and viscous, thermal, gravitational, and other effects. The solution space is explored using dimensionless groups governing mass, momentum, and heat transfer effects in the droplet, external gas, and substrate. The most common types of deposition patterns are summarized, including those produced by pinned contact lines, sticking‐and‐slipping contact lines, and Marangoni effects. The influence of contact‐line deposits is also reviewed, and the effects of colloidal, polymeric, and other depositing materials. Advanced applications from ink‐jet printing to disease diagnosis are discussed as well. The review helps readers take stock of what has been learned and what remains incompletely explained. © 2014 American Institute of Chemical Engineers AIChE J, 60: 1538–1571, 2014     

The separation of two different sized particles in an evaporating droplet

The separation of two different sized particles during evaporation of a dilute droplet is examined both computationally and experimentally. A transport model of the evaporating droplet system was solved using the finite element method to determine the fluid velocity, pressure, vapor concentration surrounding the droplet, temperature, and both particle concentrations. Experimentally, 1 μm and 3 μm polystyrene particles were used during the evaporation of a sessile water droplet. It was determined that to accurately model particle deposition, thermal effects need to be considered. The Marangoni currents in evaporating droplets keep particles suspended in the droplet until the end of the evaporation. Previous models of particle deposition during droplet evaporation have rapid accumulation of particles at the contact line. Our experiments and the experiments of others demonstrate that this is not accurate physically. In addition, to model the separation of two different sized particles the consideration of thermal effects is essential. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3547–3556, 2015     

Distinctive combustion stages of single heavy oil droplet under microgravity

This report presents an investigation on the combustion of single droplets comprised of heavy oil and oil mixtures blending diesel light oil (LO) and a heavy oil residue (HOR). The tests were conducted in a microgravity facility that offered 10 s of free-fall time. Fine wire thermocouples supported the droplets, resulting in a measurement of droplet temperature history. Additional data were the droplet and flame size history. The results identified four distinctive burning stages between ignition and extinction for heavy oil (C class) and HOR-LO blends. They are, in succession, the start-up, inner evaporation, thermal decomposition (pyrolysis) and polymerization stages. The start-up stage denoted an initial transient period, where the LO components burned from the droplet surface and the droplet temperature increased rapidly. The latter three stages featured pronounced droplet swellings and contractions caused by fuel evaporation and decomposition inside the droplet. An evaporation temperature demarcated the start-up stage from the inner evaporation stage, and this temperature corresponded to a plateau in the temperature history of the droplet. Two additional temperatures, termed the decomposition and polymerization temperatures, indicated the ends of the evaporation and decomposition stages. These temperatures were similarly identified by plateaus or inflection points in the time-temperature diagram. The evaporation temperature gradually decreased with increasing the initial LO mass fraction in the droplet, whereas the other two temperatures were almost independent of the oil composition. All three temperatures increased with decreasing initial droplet diameter, but the dependence was very slight. Based on the results, the combustion of heavy oil droplets appears to be dominated by a distillation-like vaporization mechanism, because of the rapid mass transport within the droplets caused by the disruptive burning.     

影响PA6萃取水蒸发效果的因素探讨

讨论了在己内酰胺聚合过程中,三效蒸发工艺对PA6萃取水浓缩液浓度的影响,为蒸发系统节能降耗提供依据。结果表明:温度和真空度相互制约,温度98～118℃,真空度0．040～0．085 MPa,一效蒸发塔底部物料温度与二效蒸发塔的二次蒸汽温度差为6～8℃,回流水量75～105 L/h,调节浓度梯度,控制蒸汽调节阀开度为55%～80%,可得到己内酰胺质量分数约80%的萃取水浓缩液,且一效蒸发塔出料温度稳定,有利于节能。     

氟化铝加压结晶工艺技术研究探析

湿法氟化铝一般采用常压结晶工艺生产,其存在产品结晶水含量高、结晶时间长、产能低等弊端。对此,研究了氟化铝加压结晶工艺技术,考察了结晶压力、结晶时间、晶种添加量、加料温度等因素对氟化铝结晶水含量的影响。优化工艺及条件:将氟硅酸溶液加入反应槽中,加热到70~80 ℃,按照氟硅酸和氢氧化铝物质的量比为1.1:1加入氢氧化铝,在95~105 ℃反应30 min,过滤得到氟化铝溶液和硅胶沉淀;将氟化铝溶液转入高压结晶釜中,添加15%（质量分数）的氟化铝晶种,控制结晶釜内的温度为170 ℃,维持结晶压力为0.7 MPa,结晶时间为3 h;将高压结晶釜内的温度降到80~90 ℃,对结晶后的料浆进行真空抽滤,再经洗涤得到氟化铝软膏;将氟化铝软膏置于120~180 ℃下干燥脱去附着水,再逐步升温至400~600 ℃进行煅烧,除去产品中的结晶水,冷却后得到氟化铝产品。实验结果表明,采用加压结晶工艺制备氟化铝,不仅降低了产品结晶水含量和生产成本,提高了产品质量和产量,而且解决了制约磷化工、氟化工和电解铝行业发展的瓶颈问题。     

考虑结壳现象的液滴干燥过程数值模拟

为更清楚地了解液滴的干燥过程,文中综合考虑溶剂扩散系数与溶液质量分数,溶液质量分数与液滴表面蒸汽压和滴径变化与传热传质之间的耦合关系,建立了包含液滴内部径向热传导方程,液滴内部的传质方程及液滴质量变化方程的液滴蒸发的完整模型。用所建模型对不同操作工况下液滴的挥发过程进行了模拟,描述了干燥过程中液滴质量损失,滴径的变化及液滴表面到中心的溶液组分变化,并对影响液滴干燥速度的重要因素进行了分析。模型模拟结果跟实验结论比较一致。