纳米结构表面液体沸腾传热的分子动力学模拟

为了从纳米尺度了解表面结构和润湿性对池沸腾液体与固体壁面的传热性能,本文采用分子动力学方法研究了超亲水至超疏水不同润湿性的液体氩在光滑表面和含凹、凸半球纳米结构表面的沸腾传热过程,分析了三种表面上液氩在受热过程的形态、温度、热流密度等相关参数的变化情况。结果表明,液氩层沸腾过程大致可分为液氩层吸附于固体表面和液氩层从壁面脱离两个加热阶段,当液氩层吸附于固体表面时,温度升高、热流密度及气态氩原子产生速度均大于液氩层脱离壁面时的情况,在这两个阶段亲水表面上氩原子温度变化有明显的拐点,而疏水表面在两个阶段加热过程相差不大。亲水表面上的微结构能吸附更多液氩原子,促进了气泡产生及加速温度、热流密度的变化,而在疏水及超疏水微结构表面,微纳结构与液氩间的气膜层促进了气泡产生,计算结果为池沸腾传热及微结构选择提供了理论依据。     

滑移壁面蛇形微通道两相流数值模拟

基于Fluent平台,采用CLSVOF方法对滑移壁面蛇形微通道气液两相流动进行了数值计算。计算选用的方法与理论结果具有较好的一致性,同时可以表明疏水壁面会产生滑移现象,且在高度较小的微通道内滑移效果更显著,从而减小通道内流体流动阻力,实现减阻;不同壁面性质通道内流体流动情况的计算结果表明,滑移壁面对截面速度分布趋势几乎没有影响,但上下壁面疏水性不同会影响通道截面最大速度分布。此外接触角及相对粗糙度对滑移特性影响较大,合理设计壁面润湿性及微粗糙元结构可以最大限度发挥滑移现象引起的减阻效果;与无滑移壁面相比,滑移壁面微通道内传热效果更好,且随滑移速度的增大,通道换热增强。     

滑移壁面蛇形微通道两相流数值模拟

基于Fluent平台,采用CLSVOF方法对滑移壁面蛇形微通道气液两相流动进行了数值计算。计算选用的方法与理论结果具有较好的一致性,同时可以表明疏水壁面会产生滑移现象,且在高度较小的微通道内滑移效果更显著,从而减小通道内流体流动阻力,实现减阻;不同壁面性质通道内流体流动情况的计算结果表明,滑移壁面对截面速度分布趋势几乎没有影响,但上下壁面疏水性不同会影响通道截面最大速度分布。此外接触角及相对粗糙度对滑移特性影响较大,合理设计壁面润湿性及微粗糙元结构可以最大限度发挥滑移现象引起的减阻效果;与无滑移壁面相比,滑移壁面微通道内传热效果更好,且随滑移速度的增大,通道换热增强。     

液体通流微小槽道内气泡动力学行为模拟

采用VOF方法,对液体通流微小通道内壁面逸出气泡的形成、生长及脱离运动进行了数值模拟,并讨论了壁面浸润性、液体流速、气体流速对气泡动力学行为的影响.结果表明:气泡生长壁面亲水性增强有利于其从壁面脱离;气泡生长壁面气相覆盖率随壁面接触角的增大而增大;流动阻力因子随壁面接触角的增大而减小.较高的液体流速会导致气泡的脱离时间...     

液滴冲击加热壁面沸腾现象特征分析

采用高速摄像仪对液滴冲击加热壁面过程进行实验观测,分析了不同实验流体的沸腾现象特征,探讨了中间射流及宝塔状气泡的形成机理。观测发现,壁温高于液体对应的Leidenfrost温度时水滴撞击后会出现暴沸现象,由于气泡夹带伴随强烈的核化作用,氯化钠溶液液滴撞击后可以观察到中间射流的产生,醇类液滴则发生完全反弹;壁温低于Leidenfrost温度时液滴在加热壁面会出现泡状沸腾现象,与半球形气泡不同,宝塔状气泡出现在液膜厚度较大的区域。此外定量考察了液滴在加热壁面完全反弹时的最大铺展因子,发现铺展因子仅受Weber数影响,与文献结果比较表明本研究得出的铺展因子经验公式可较好地预测液滴在加热壁面的铺展尺度。     

微纳复合表面HFE-7100/水的BRT现象及沸腾传热特性

HFE-7100/水作为非共沸不互溶工质可以拓宽核状沸腾传热的有效温区,目前关于其在微纳复合表面的沸腾传热特性和气泡运动机理尚不明晰。利用气泡模板电沉积法在铜基表面上制备了具有微纳孔洞的复合结构,测试了HFE-7100/水的沸腾传热特性,并通过可视化探究了沸腾工质转换（BRT）过程中两相工质在表面的润湿状态和气泡运动现象。结果表明,微纳复合表面上HFE-7100/水的BRT过程中,气泡先后经历小气泡聚并、气膜膨胀、轻工质接触壁面核化三个过程。在BRT过程中,HFE-7100与水对热壁面的润湿性存在竞争关系,随着过热度增加,薄的HFE-7100液层难以维持稳定的重工质沸腾,上层水工质可以穿过HFE-7100层对热壁面实现完全润湿,完成BRT过程。与单一工质相比,常压下HFE-7100/水混合工质体系可以在343~423 K下实现高效的核状沸腾传热。该研究揭示了HFE-7100/水在微纳复合表面的沸腾传热特性,为沸腾强化表面设计提供了思路。     

液滴冲击加热壁面沸腾现象特征分析

采用高速摄像仪对液滴冲击加热壁面过程进行实验观测,分析了不同实验流体的沸腾现象特征,探讨了中间射流及宝塔状气泡的形成机理。观测发现,壁温高于液体对应的Leidenfrost温度时水滴撞击后会出现暴沸现象,由于气泡夹带伴随强烈的核化作用,氯化钠溶液液滴撞击后可以观察到中间射流的产生,醇类液滴则发生完全反弹;壁温低于Leidenfrost温度时液滴在加热壁面会出现泡状沸腾现象,与半球形气泡不同,宝塔状气泡出现在液膜厚度较大的区域。此外定量考察了液滴在加热壁面完全反弹时的最大铺展因子,发现铺展因子仅受Weber数影响,与文献结果比较表明本研究得出的铺展因子经验公式可较好地预测液滴在加热壁面的铺展尺度。     

微小有限空间内微气泡控制生长的界面追踪与数值模拟

通过对微机电系统微流体器件中气泡生长实验结果的分析,考虑加热元表面液体微层的作用,将微气泡生长分为晶核形成、球形气泡、受侧壁挤压的气泡、沿微通道生长的气泡4个阶段,建立了矩形微通道内微气泡控制生长物理模型;采用Level Set Method模拟了矩形微通道内微气泡控制生长过程,获得了微气泡生长特性。数值模拟结果表明：微气泡初期生长速率较快,后期由于凝结率增大使生长速率减缓;液体温度、微通道宽度、微加热元宽度、加热电压等均对气泡生长始点和生长速率有显著影响。     

改性TiO2及其纳米流体的核沸腾传热特性

采用全氟癸基三甲氧基硅烷对纳米TiO₂进行改性,有效改善了纳米粒子的团聚性能。利用两步法分别制备相同浓度范围的改性前/后TiO₂纳米流体,考察了2组纳米流体的热导率、静态接触角对核沸腾传热特性的影响。结果表明,在质量分数为0.05%时,未改性纳米流体沸腾传热系数较去离子水最大提高16.3%,改性纳米流体传热系数可提高28.57%,且改性纳米流体临界热通量较未改性纳米流体降低11.68%。热导率和接触角是影响不同润湿性纳米流体具有不同沸腾传热特性的主要因素,润湿性较差纳米流体易提前起沸和形成核化点,但随热通量的增加会造成气泡合并、恶化传热。     

微小通道内液体扰流微孔逸出气泡串的迁移行为

对微小矩形通道内,液体扰流下微孔孔口逸出气泡串的迁移行为进行了可视化实验。基于图像处理方法,分别研究了液体流量和通道倾斜角度对气泡串尺寸、运动速度以及运动轨迹的影响。发现了不同工况下通道内的微气泡堆积现象,并对其进行了分析。结果表明：增大液体流量或者通道倾角,使得形成的气泡尺寸减小,气泡运动轨迹与通道壁面夹角减小,气泡运动速度增大。液体流量与通道倾角影响了通道内微气泡堆积的位置及数量。     

方肋微通道内流动沸腾的气泡动态与传热特性分析

为揭示方肋微通道热沉内流动沸腾的传热传质机理,本文基于耦合VOF方法与“饱和界面”相变模型对微通道内单个气泡绕流加热方肋的传热传质过程进行了数值研究。通过分析该过程中气泡增长速率与方肋壁面传热系数的变化,重点讨论了初始气泡体积和入口雷诺数Re对相变传热效率和流动结构的影响。结果表明：在气泡流经加热方肋过程中,气泡与方肋表面之间形成一层薄液膜,该薄液膜的相变蒸发极大强化方肋表面的换热效果,换热系数较相同条件下的单相流动提升6倍以上。此外液膜厚度随Re增大而变厚,液膜热阻相应增大,液膜蒸发对换热的促进作用随Re增大而降低。最后考察了气泡体积对方肋壁面换热的影响,结果表明：初始体积大的气泡具有更薄的液膜厚度及更大的蒸发面积,表现出更高的相变传热效率;而小气泡对壁面温度影响较小。     

壁面润湿性对微通道内Taylor流动特性的影响

壁面润湿性不仅影响着Taylor气泡的形状,同时对通道内流体流动、相变换热等有着关键的作用。采用VOF模型对T型微通道内气液两相Taylor流动进行三维数值模拟,重点研究了接触角改变对Taylor气泡流体动力学特性的影响。模拟结果与他人实验数据对比基本吻合,验证了模型的有效性。结果表明：随着接触角增大,气泡周围液含量逐渐降低,相界面也由外凸形变为内凹形。壁面越接近润湿（或疏水）状态,气液接触面的曲率就越大;当120°≤θ≤150°时Taylor气泡稳定性变差。当θ≥150°时“拖曳流态”出现,分析指出在大接触角下气体更易贴附壁面导致接触区内流场发生变化,形成的涡流减弱了水对气相的水平剪切作用,进而引起流型转变。接触角对通道内压力有着重要影响,通道中心轴向压力曲线以θ=90°为过渡,润湿状态下呈凸函数递减且p _G>p _L,疏水状态下气液进口处的压力分配改变,曲线趋势相反。     

Infrared thermometry study of nanofluid pool boiling phenomena

Infrared thermometry was used to obtain first-of-a-kind, time- and space-resolved data for pool boiling phenomena in water-based nanofluids with diamond and silica nanoparticles at low concentration (<0.1 vol.%). In addition to macroscopic parameters like the average heat transfer coefficient and critical heat flux [CHF] value, more fundamental parameters such as the bubble departure diameter and frequency, growth and wait times, and nucleation site density [NSD] were directly measured for a thin, resistively heated, indium-tin-oxide surface deposited onto a sapphire substrate. Consistent with other nanofluid studies, the nanoparticles caused deterioration in the nucleate boiling heat transfer (by as much as 50%) and an increase in the CHF (by as much as 100%). The bubble departure frequency and NSD were found to be lower in nanofluids compared with water for the same wall superheat. Furthermore, it was found that a porous layer of nanoparticles built up on the heater surface during nucleate boiling, which improved surface wettability compared with the water-boiled surfaces. Using the prevalent nucleate boiling models, it was possible to correlate this improved surface wettability to the experimentally observed reductions in the bubble departure frequency, NSD, and ultimately to the deterioration in the nucleate boiling heat transfer and the CHF enhancement.     

CONVECTIVE HEAT TRANSFER CHARACTERISTICS OF GAS IMPINGEMENT ON SMALL HEATED DEVICE IN LIQUID COOLANT BATH

Heat transfer characteristics of a small heated device have been investigated in a liquid bath with gas jetimpingement as function of gas flow rate,coolant temperature,liquid phsicochemical properties,heat flux,heat source size,ambient pressure and the distance between jet and heated wall.The experimental results show that the agitation of liquid caused by gas jet bubbles increases greatly therate of heat transfer,and the evaporation of coolant near the wall,which was due to the concentration differencebetween gas-liquid interface and bulk gas phase,gives additional enhancement of heat transfer.The rate ofevaporation related to the bubble growth was mathematically formulated.By using the simultaneous heat and mass transfer model,the convective heat transfer coefficient and masstransfer coefficient can be deduced from the experimental results.In addition,the local heat transfer coefficient and the distribution of evaporation heat flux on the smallheated surface are investigated mathematically and experimentally.     

不同润湿表面液体沸腾的分子动力学模拟

采用分子动力学方法研究纳米尺度下液氩在过热基板上的沸腾过程。通过调节固液间相互作用的方式改变壁面润湿性,模拟并分析了壁面润湿性对沸腾过程中能量传递和液体运动情况的影响。结果表明：不同润湿性表面均会发生固液分离的现象,但是固体表面附近吸附的氩原子数密度随润湿性增强而增大;润湿性较强时,液体的能量上升快,热通量高,液体内部温度梯度大,发生固液分离时间早,系统中氩的温度和能量低,上升过程中液氩密度、厚度变化小;润湿性较弱时,液体的能量上升慢,热通量小,液体内部温度梯度小,发生固液分离时间延后,系统中氩的温度、能量更高,上升过程中液氩密度、厚度变化较大。下部气体压力整体上大于上部气体压力,发生固液分离时润湿性越强的表面上液体上下压差越大,首次上升过程能达到的高度越高,所需时间越短。     

超疏水/亲水性结构表面流动沸腾传热实验研究

用激光烧蚀方法在抛光后的铜上制备出四种无需涂覆修饰即可获得超疏水/亲水性的规则微阵列结构表面。基于流动可视化与温度数据结果,分析了表面浸润性和过冷度对流动沸腾传热性能的影响,与经典汽化核心密度关联式进行了对比。结果表明：疏水表面可削弱单相对流传热,大幅强化沸腾传热,最大传热系数提高了75.5%,沸腾起始点提前3.5 K,且汽化核心数目较裸铜表面提高了5倍以上,但有较低的临界热通量。超亲水表面可增强单相对流传热、小幅度提升流动沸腾传热。对比亲水表面与疏水表面的气泡生长过程,发现疏水表面尾端气泡容易汇聚,生长周期较长;而亲水表面没有发生明显的气泡汇聚行为,气泡生长周期较短。     

A study of bubble nucleation in a mixture of molten polymer and volatile liquid in a shear flow field

Bubble nucleation in a mixture of volatile liquid and polymer melt under shear flow conditions was investigated, using a light scattering technique. In the study, a mixture of polystyrene and trichlorofluoromethane was extruded through a slit die having glass windows and bubble nucleation in the flow channel was observed optically. A He-Ne laser was used to illuminate the nucleating and growing bubbles. The light flux scattered by the growing bubbles at a fixed angle was detected by a photomultiplier with the aid of a high-voltage power supply. The bubble nucleating site in the flow channel was located using a computer controlled tracking system, which was designed to move the entire optical system automatically in the three dimensional space, and also had the ability to follow the software control command and cooperate with the data acquisition system. When the site of bubble nucleation was located, the coordinates of this site in the flow channel and the experimental conditions were automatically recorded on a floppy diskette by entering a software command. The pressure profile along the flow channel was measured by pressure transducers, with the aid of a microprocessor-based pressure reading system. It has been found that the site of bubble nucleation varies with the position in the direction perpendicular to the flow direction, which is attributed to the nonuniform velocity and stress distributions in the slit flow channel. The present investigation suggests that bubble nucleation can be induced either by flow and/or shear stress; specifically, flow-induced bubble nucleation is the dominant mechanism at positions near the center of the die opening, and shear-induced bubble nucleation is the dominant mechanism at positions near the die wall. It should be mentioned that the bubble near the die wall may also be generated by cavitation brought about by the surface roughness of the wall and also by thermal fluctuations due to the heat transfer between the metal (die wall) and the mixture of polymer and volatile component. The present study indicates that bubble nucleation in a shear flow field can occur at an unsaturated condition. This is in contrast to bubble nucleation under static conditions, where supersaturation is necessary.     

垂直管内弹状气泡上升中壁面传递的实验研究

垂直管内弹状流壁面传递是诸多工业应用中需研究的重要问题之一。今用极限扩散电流技术,对弹状气泡上升时瞬时壁面剪应力和传质系数进行了测定,结果显示:当基于表观气速的Froude数FrG < 0.74时,壁面剪应力随弹状气泡和液塞的到来呈现方向相反的交替变化,壁面传质系数亦相应变化;而当FrG > 0.74时,剪应力方向一直向下,说明液膜向下流动,且弹状气泡和液塞的到来对壁面传质系数的影响很小。这说明下落液膜射流穿透了液塞段,控制了整个壁面传递过程。研究还对下落液膜区、尾迹区及液塞段的不同传递特征及机理进行了分析, 并结合气泡塔熔融结晶器中弹状气泡上升时的传热,对结晶操作条件的合理选择进行了讨论。