【作者】 王鑫；

【导师】 陈振乾；

【作者基本信息】 东南大学 ， 工程热物理， 2022， 博士

【摘要】 液滴动力学和冷凝传热广泛存在于自然界和工业应用中,如能源利用、海水淡化、发电、石油化工、微电子冷却、航天热管理系统、医药等各行各业。近年来,随着微纳米制造技术的快速发展以及流体力学、界面力学、传热传质学、界面材料学等多个学科的交叉,各式各样的先进功能性表面的制备与设计有助于微液滴的局部调控（包括液滴生长、弹跳及脱落等）以及滴状冷凝传热的快速发展,加深了对固液作用机制及冷凝强化策略的理解。借助复杂的微纳结构及亲疏水特性对微液滴局部调控进而衍生到冷凝强化传热,是目前传热传质学研究的热点方向之一。因此,研究微纳结构表面上的液滴动力学及滴状冷凝传热过程具有重要的科学意义和工程应用价值。众所周知,滴状冷凝是一个涉及复杂的多尺度液滴动力学演化的相变传热问题。21世纪以来,虽然大量的学者对滴状冷凝传热及强化问题进行了广泛地研究,但仍存在着诸多难解决的问题,如气液相界面演化、多尺度特征、跨界面的热质传递、复杂的固液作用机制等。本文将从数值模拟、理论分析及实验探索三个方面对液滴的自发运动及冷凝传热问题进行全面研究。采用格子Boltzmann方法着重研究了液滴的定向迁移、合并诱导的弹跳行为以及冷凝传热,并对其中的表面能变化、能量转化效率、热流密度进行了定量分析。制备了纳米结构/微纳结构表面,对冷凝过程中的液滴动态演化进行了可视化观测,探索了多种因素对液滴多尺度演化及整体传热性能的影响规律,揭示了界面结构强化滴状冷凝传热的机理。本论文的具体研究内容及结论如下:（1）采用改进的三维格子Boltzmann伪势模型对复合楔形表面/微结构多润湿梯度表面上液滴的自发迁移进行了数值研究,分析了表面结构参数、润湿性及重力等因素对运动液滴形变、速度和表面能的影响规律,探究了毛细力诱导的微液滴自发运动的物理机制,通过优化表面结构参数提升了液滴的运动速度。结果表明:液滴在楔形轨道上的自发运动是由净表面张力驱动的。在净表面张力和固液粘附力的共同作用下,液滴先加速运动再减速运动。尽管顶角的增加会加速液滴的运动,但会减少液滴的输运距离。对比模拟结果,顶角为30°时是保证液滴运动速度和输运距离的优化方案。对于阵列微结构多润湿梯度表面,较大的润湿梯度和固体分数提升了液滴的运动速度和表面自由能。虽然分层润湿梯度表面能够阻止液滴浸入微柱,但会对液滴的运动速度造成负面影响。当Bo数在0.0126至0.063之间时,微液滴沿斜坡的向上爬升过程应考虑重力的影响。当Bo≤0.0126时,表面倾斜角对液滴的爬升运动几乎无影响。（2）采用二维格子Boltzmann方法对微结构阵列表面上冷凝液滴合并诱导的自发弹跳行为进行了数值研究,分析了润湿性、表面结构参数、重力、表面倾斜角及半径比对液滴弹跳速度及能量转化效率的影响,同时考虑了重力对液滴弹跳的影响。结果表明:模拟得到的液桥宽度和弹跳速度与已发表的实验和模拟结果吻合得很好,验证了该模型模拟液滴弹跳的适用性和正确性。微柱阵列的宽度和间距对合并液滴的弹跳能力有显著影响,但微柱高度对其影响不大。对比模拟结果,a^*=0.467、w^*=0.067和h^*=0.5的微柱结构是合并液滴（r₀=48.6μm）自发弹跳的最优结构。倾斜表面上的弹跳液滴在切向重力驱动下可以成功跳离表面而不返回原点。两尺寸不匹配液滴合并诱导的弹跳会产生一个切向速度,促使其从表面脱离或与其他液滴合并。当Bo>（?）0.00764/r时,应考虑重力对合并液滴弹跳行为的影响。弹跳速度和能量转化效率随半径比的减少和重力系数的增加而降低。此外,当能量转化效率低于2.0%时,液滴合并后不会跳离表面。（3）采用改进的混合热格子Boltzmann方法建立了蒸汽冷凝传热的微观模型。对单液滴蒸发和疏水表面上的单液滴冷凝过程进行了模拟,并与理论模型和实验结果进行了比较,验证了该模型模拟冷凝传热的适用性和正确性。结果表明:对于均质润湿性表面,随着接触角的增加,冷凝液滴的初始成核时间有所延长,而冷凝液滴的脱落时间明显减少。由于冷凝过程中相变潜热的释放,气液界面附近的温度,尤其是三相接触线区域附近的温度要比其余地方高很多。冷凝液滴的合并和脱落行为造成平均热流密度随时间的变化而局部波动。对于混合润湿性表面,存在一个最佳的疏水-亲水区域比例使得传热性能达到最优,最佳疏水-亲水比例会随着润湿差的减小而减小。对于梯度润湿表面,由毛细力引起的液滴定向迁移有利于冷凝液滴的去除,进而提升冷凝传热性能。此外,较大的润湿梯度有利于进一步提升传热性能。对于微柱阵列表面,冷凝液滴首先在微柱间形成。超疏水表面的冷凝量和冷凝率最终将超过疏水和亲水表面。此外,接触角为θ_a=150.7o表面的热流密度比接触角为θ_a=164.5o表面的略高。较小的微柱间距有利于增加冷凝液滴成核数量,而较大的间距则有利于增加合并液滴的弹跳高度。三角形微柱结构增强了合并液滴的弹跳能力,而平均热流密度却明显低于方形和半圆形微结构表面。随着过冷度的降低,成核时间有所延长。较小的过冷度既提高了最大弹跳高度,也减少了合并液滴的弹跳半径。对于竖直放置的冷凝表面,随着表面润湿性的增强,冷凝液滴的脱落模式由自发弹跳变为重力驱动的滚动。与微槽结构相比,微脊结构能改善冷凝液滴的弹跳能力且强化冷凝传热。从液滴成核至跳离表面,热流密度随时间的变化可分为三个阶段:a)液滴成核后热流密度逐渐增加,b)合并过程中热流密度略有减少,c)跳离壁面后热流密度急剧减少。较小的微槽宽度和高度有助于提升弹跳速度,但减小了临界脱落尺寸。（4）制备了纳米结构铜基超疏水表面、微纳结构硅基超疏水表面和梯级多孔-纳米结构铜基超疏水表面,搭建了纯蒸汽冷凝传热实验平台,研究了不同影响因素对冷凝液滴动力学特性及传热性能的影响,揭示了微纳结构强化冷凝传热的机制。结果表明:由表面过冷度增加引起的液滴润湿状态从悬浮的Cassie态过渡至刺穿的Wenzel态,阻止了合并液滴的弹跳。当过冷度从0.5 K增加到3.5 K时,液滴的弹跳频率从173 cm^-2·s^-1明显减小到36 cm^-2·s^-1,液滴平均直径增加了3倍。与此同时,液滴的弹跳直径范围为20～280μm,最大弹跳高度减少了约60%。随着倾斜角从0°增加到90°,过冷度为2.0 K时的平均和最大液滴直径分别下降了约63%和33%。随着表面过冷度的增加,仅有垂直取向的冷凝表面才能保持稳定高效的弹跳频率。此外,当过冷度为5.0 K、6.5 K和8.0 K时,30°倾斜表面上的临界液滴脱落直径分别是垂直表面上的1.5、1.4和1.5倍。与水平表面相比,当过冷度为2.0 K时,冷凝传热在30°时增强了21.1%,在60°时增强了49.2%,在90°时增强了72.4%。较大的微柱间距会恶化冷凝传热性能。在大过冷度下（～25 K）,冷凝液膜覆盖并淹没微柱阵列,但液膜的厚度与微柱高度处于同一数量级。与纳米结构表面相比,S10R30表面的传热系数提高了26.4%。相比较纳米结构表面,梯级多孔-纳米结构表面上的液滴弹跳频次更高,液滴的最大直径和平均直径更小。在较大过冷度下（ΔT≤16 K）,梯级多孔-纳米结构表面上的临界液滴脱落直径比纳米结构表面低一个数量级,传热系数至少增加了26.4%。本工作系统地研究了微纳结构表面上的液滴动力学特性以及冷凝传热性能,不仅为液滴动力学问题的数值模拟开拓了新思路和新的研究方向,而且还通过理论分析、实验和模拟手段探索了强化蒸汽冷凝传热的方法,为进一步推动液滴动力学及冷凝传热在各领域的潜在应用提供了理论支撑和技术支持。更多还原

【Abstract】 Droplet dynamics and condensation heat transfer widely exist in nature and industrial applications,such as energy utilization,seawater desalination,power generation,petrochemicals,aerospace thermal management systems,pharmaceuticals and various other industries.In recent years,with the rapid development of micro-and nano-fabrication technologies and the intersection of fluid mechanics,interfacial mechanics,heat and mass transfer,interfacial materials science and other disciplines,the preparation and design of various advanced functional surfaces have contributed to the local modulation of microdroplets（including droplet growth,jumping and shedding,etc.）and the rapid development of dropwise condensation heat transfer,which deepened the understanding of solid-liquid interaction mechanisms and condensation enhancement strategies.The local modulation of microdroplets by means of complex micro/nano structures and hydrophobic/hydrophilic properties,which can be derived to condensation heat transfer enhancement,is one of the current hot topics in heat and mass transfer science.Therefore,it is of great scientific significance and engineering value to investigate the droplet dynamics and dropwise condensation heat transfer on the micro/nano structured surfaces.It is well known that dropwise condensation is a phase change heat transfer problem involving complex multiscale droplet dynamic evolution.Although a large number of researchers have extensively studied the dropwise condensation heat transfer and enhancement since 21^st century,there are still many difficult problems such as gas-liquid interfacial evolution,multiscale dynamic characteristics,heat and mass transfer across the interface and complex solid-liquid interaction mechanisms.In this work,the spontaneous movement of droplets and condensation heat transfer are comprehensively investigated by numerical simulation,theoretical analysis and experimental exploration.The lattice Boltzmann method is used to emphasize the directional migration of droplets,coalescence-induced jumping of droplets and condensation heat transfer.Variation of the surface energy,energy coversion efficiency and heat flux are quantitatively analyzed.Nanostructured and micro/nano structured surfaces are fabricated to enhance condensation heat transfer.The influence of various factors on the multiscale evolution of droplets and heat transfer performance is explored,and the mechanism of interfacial structure to enhance dropwise condensation is revealed.The detailed research contents and conclusions of this work are as follows:（1）An improved three-dimensional lattice Boltzmann pseudopotential model is applied to numerically study the spontaneous movement of droplet on the composite wedge shaped surface and microstructured multi-wetting gradient surface.The influence of structure parameters,wettability and gravity on the deformation,velocity and surface energy of moving droplets is analyzed.As well,the physical mechanism of capillary force induced self-propelled movement of microdroplets is investigated.The velocity of droplet movement is improved by optimizing the surface structure parameters.The results demonstrate that the spontaneous movement of droplet on the wedge track is driven by the net surface tension.Under the combined effect of net surface tension and solid-liquid adhesion force,the droplet velocity increases and then decreases with time.Although the increase of vertex angle accelerates the droplet movement,it reduces the transport distance of droplet.In comparison with the simulation results,the vertex angle of 30°is an optimal solution to ensure the droplet velocity and transport distance.For the microstructured multi-wetting gradient surface,the larger wetting gradient and solid fraction improve the droplet movement and surface free energy.Although two-layer wetting gradient surface prevents the droplet from penetrating into the micropillars,it has a negative effect on droplet velocity.For the Bo number ranging from 0.0126 to 0.063,the climbing-upward movement of microdroplet on the inclined surface should take the gravity into account.For Bo≤0.0126,the inclined angle has almost no effect on the climbing-upward motion of droplet.（2）The self-propelled jumping behaviors induced by coalescence of condensate droplets on the microstructured surfaces are studied numerically using the two-dimensional lattice Boltzmann method.The effects of wettability,surface morphology,gravity,inclined angle and radius ratio on the jumping velocity and energy conversion efficiency of droplet are analyzed.The results show that the width of liquid bridge and jumping velocity simulated by LBM agree well with the published experimental and numerical results,which verifies the applicability and accuracy of the model for simulating the droplet jumping dynamics.The width and spacing of micropillar arrays have a significant effect on the jumping ability of coalesced droplets,while its height has little impact on it.Comparing the simulation results,a^*=0.467,w^*=0.067 and h^*=0.5 are the optimal structures for the spontaneous jumping of merging droplets（r₀=48.6μm）.The jumping droplets on the inclined surface can successfully jump off without returning to the original spot when driven by tangential gravity.The spontaneous jumping induced by the coalescence of two mismatched droplets generates a tangential velocity that drives them to detach from the surface or merge with other droplets.For Bo>（0.00764/r）^0.5,the effect of gravity on the jumping behaviors of merging droplets should be considered.The jumping velocity and energy conversion efficiency decreases with the decreasing radius ratio and increasing gravitational coefficient.In addition,as the energy conversion efficiency is less than 2.0%,the droplets fail to jump off the surface after coalescence.（3）A microscopic model of vapor condensation heat transfer is developed using an improved hybrid thermal lattice Boltzmann method.Single droplet evaporation and single droplet condensation on the hydrophobic surface are modeled and compared with the theoretical and experimental results to verify the applicability and accuracy of the model for simulating condensation heat transfer.The results show that for homogeneous wettable surfaces,the initial nucleation time of condensate droplets is prolonged with increasing contact angle,while the shedding time of condensation time is significantly reduced.Due to the release of latent heat during condensation,the temperature near the gas-liquid interface,especially near the three-phase contact line region,is much higher than elsewhere.The merging and shedding behaviors of condensate droplets cause local fluctuations in the average heat flux with time.For the hybrid wettability surface,there exists a hydrophobic-hydrophilic ratio to optimize the heat transfer performance.As well,the optimizal hydrophobic-hydrophilic ratio decreases as the wetting difference decreases.For the wetting gradient surface,directional migration of droplet caused by the capillary force facilitates the removal of condensate droplets,which in turn improves the condensation heat transfer performance.In addition,a larger wetting gradient is beneficial to further enhance the heat transfer performance.For the micropillared surface,condensate droplets initially form between the micropillars.The condensate mass and condensation rate of superhydrophobic surfaces eventually exceed those of hydrophobic and hydrophilic surfaces.In addition,the heat flux ofθ_a=150.7osurface is slightly higher than that ofθ_a=164.5osurface.The smaller spacing facilitates increasing the nucleation sites,while the larger spacing increases the jumping height of coalesced droplets.The triangular microstructure enhances the jumping ability of merging droplets,while the average heat flux is significantly lower than that of the square and semicircular microstructured surfaces.As the surface subcooling decreases,the nucleation time of droplet is prolonged.Smaller surface subcooling not only increases the maximum jumping height but also decreases the jumping radius of coalesced droplets.For the vertical condensing surface,the shedding mode of droplets changes from spontaneous jumping to gravity-driven rolling as the surface wettability strengthens.Compared with the microgrooved structure,microspined structure improves the jumping ability of condensate droplets and enhances condensation heat transfer.The evolution of heat flux with time from droplet nucleation to jumping off is divided into three stages:gradual increase after droplet nucleation,slight decrease during merging and sharp decrease after jumping off.Smaller width and height of microgroove contribute to improving the jumping velocity as well as reducing the critical departure size.（4）Nanostructured copper superhydrophobic surface,micro/nano structured silicon surface and hierarchical microporous and nanostructured copper superhydrophobic surface are fabricated to conduct the pure vapor condensation heat transfer experiments.The effects of different factors on the droplet dynamics of condensate droplets and heat transfer performance are investigated to reveal the mechanism of enhanced heat transfer by micro/nano structures.The results indicate that the transition of droplet wetting state from suspended Cassie state to impaled Wenzel state caused by the increase of surface subcooling prevents the jumping of merging droplets.As the subcooling degree is increased from 0.5 K to 3.5 K,the jumping frequency of droplet is reduced significantly from 173 cm^-2·s^-1 to 36 cm^-2·s^-1 and the average droplet diameter rises by～300%.Also,the jumping diameter of droplet ranges from 20μm to280μm,with a～60%reduction in the maximum jumping height.As the inclined angle increases from 0°to 90°,the average and maximum droplet diameters atΔT=2.0 K decline by about 63%and 33%,respectively.As the surface subcooling increases,a stable and high jumping frequency can only be maintained at the vertical orientation.Furthermore,the critical sliding diameter at a 30°inclination is 1.5,1.4 and 1.5 times higher than on the vertical substrate when the subcoolings are 5.0 K,6.5 K and 8.0 K,respectively.Compared to the horizontal surface,condensation heat transfer is enhanced by 21.1%atα=30°,49.2%atα=60°,and 72.4%atα=90°atΔT=2.0 K.The larger micropillar spacing can worsen the heat transfer efficiency.At large subcoolings（～25 K）,although the condensate film covers the micropillar arrays,the thickness of condensate layer is the same order of magnitude as the height of micropillars.The heat transfer coefficient of S10R30 surface is enhanced by 26.4%compared to the nanostructured surface.Compared to the nanostructured surface,the jumping frequency of droplet is greater and the maximum as well as average diameter is smaller on the hierarchical microporous and nanostructured surface.At larger subcoolings（ΔT≤16 K）,the critical shedding diameter of droplet on the hierarchical microporous and nanostructured surface is one order of magnitude lower than that on the nanostructured surface,and the heat transfer coefficient is increased by at least 26.4%.This work systematically investigates the droplet dynamic characteristics and condensation heat transfer performance on the micro/nano structured surfaces.It not only develops new ideas and research interests for the simulations of droplet dynamics,but also explores the methods to enhance condensation heat transfer via theoretical analysis,experiments and simulations.As well,it provides theoretical and technical support to further advance the potential applications of droplet dynamics and condensation heat transfer in various fields.更多还原