低压蒸汽滴状冷凝传热微观机理及强化

发布时间：2018-04-13 00:39

本文选题：低压蒸汽冷凝 + 液滴动态特性　；参考：《大连理工大学》2015年博士论文

【摘要】：蒸气冷凝作为典型的相变现象,普遍存在于自然界与日常生活中,同时作为高效的传热形式被广泛应用在工业生产中。其中,低压蒸汽冷凝在低品位余热回收领域具有重要的应用,如低压蒸馏、低温多效海水淡化、低温热泵和热管技术等。随着蒸汽压力下降,界面传递阻力显著升高,降低了冷凝过程的传热性能。与膜状冷凝相比,滴状冷凝可以有效地降低表面凝液引起的导热热阻,而且液滴动态行为促进汽液界面的传递过程,成为了低压蒸汽冷凝传热的理想强化策略。本文结合实验观测、数值模拟和理论分析,系统地研究了低压蒸汽滴状冷凝过程液滴动态特性、尺寸分布及演化规律和表面温度演化特征,揭示了蒸汽滴状冷凝传热微观机理；利用超疏水表面界面效应促进液滴合并弹跳,实现了低压蒸汽冷凝传热强化。设计搭建了低压蒸汽冷凝传热实验系统,考察了蒸汽压力对滴状冷凝传热性能和液滴动态行为的影响规律。结果表明,随着蒸汽压力下降,滴状冷凝传热系数先缓慢降低而后迅速下降。结合液滴传热模型,分析了低压蒸汽冷凝传热控制机理,表明液滴生长受汽液界面传递过程的影响随着压力降低而加强。实验发现了低压蒸汽冷凝中存在液滴脱落滞后现象,延缓了表面更新频率,揭示了液滴运动与宏观传热间的内在关联。通过引入液滴动态特性,修正了经典滴状冷凝传热模型,准确地预测了蒸汽压力和表面过冷度对滴状冷凝传热特性的影响规律。实验观测了不同蒸汽压力下液滴瞬态尺寸分布演化与稳态尺寸分布规律,揭示了蒸汽压力对液滴生长特性的影响规律。初始冷凝液滴呈现出由正态分布到双峰分布再到指数分布的瞬态尺寸演化特征,随着蒸汽压力降低,核化点密度减小且液滴生长速率减慢。与常压蒸汽相比,低压条件下冷凝表面小液滴密度减小,而大液滴出现频率升高,增加了冷凝表面液滴的平均尺寸。利用液滴脱落动态滞后模型,解释了液滴动态行为降低动态滞后角的现象；从局部能垒角度出发,阐明了液滴非连续性脱落行为的物理机制。利用红外热成像技术观测了低压蒸汽冷凝中液滴表面温度分布与演化特征,直观地获得了液滴运动引起的微细传热现象。结果表明,冷凝液滴表面温度呈现中间高边缘低的分布特征。蒸汽冷凝过程与液滴生长行为和界面演化密切相关,与微液滴的传热不同,大液滴通过运动更新汽液界面促进蒸汽发生快速冷凝,揭示了液滴行为引起的界面演化对宏观传热影响的内在机制。与超疏水表面相比,疏水表面更有利于液滴核化生长,表面温度分布更不均匀,液滴运动引起的表面温度波动更剧烈。结合模型分析、SE模拟和可视化实验,从蒸汽初始核化入手,考察了表面结构和冷凝条件对初始核化液滴及微液滴润湿模式的影响规律。根据冷凝液滴的跨尺度生长特性和超疏水表面结构特征,提出了表面结构对液滴生长的空间限制效应。利用V形纳米结构控制液滴初始核化形态和位置,揭示了超疏水表面上冷凝液滴的润湿转变机理。利用浸润因子来描述冷凝液滴的浸润程度,模型计算结果很好地预测了液滴表观接触角和滞后角的变化规律。结果表明,当液滴初始核化尺寸与纳米结构尺寸具有可比性时,核化倾向于在靠近纳米柱顶端的位置发生,从而形成悬挂模式的液滴,有利于合并诱导弹跳；随着表面过冷度增加,液滴核化尺寸大大减小,蒸汽在纳米结构间随机发生核化,形成了浸润模式的冷凝液滴,导致表面的超疏水性失效。通过控制氧化刻蚀时间制备了四种不同纳米结构的超疏水表面,实验考察了表面结构和表面过冷度对液滴合并弹跳行为的影响规律。随着纳米结构长度和间距的增加,液滴弹跳尺寸增加,而弹跳频率下降。随着表面过冷度提高,冷凝液滴在纳米结构内的浸润程度增加,合并初始弹跳速度减小,最佳合并弹跳尺寸增大。将冷凝液滴的润湿特性引入液滴合并诱导弹跳模型,分析了纳米结构尺寸和表面过冷度对液滴合并初始弹跳速度的影响机理,结果表明,蒸汽冷凝环境中液滴的动态行为由纳米表面结构、冷凝条件和液滴尺寸共同决定。实验发现了超疏水表面冷凝液滴润湿模式转变的不可逆性和传热性能随过冷度变化的单向性。利用超疏水表面实现了蒸汽在低过冷度范围内的冷凝传热强化,传热系数明显高于相同条件下膜状冷凝,甚至超过了光滑疏水表面的滴状冷凝传热性能。通过对比三种超疏水表面的传热性能可以看出,合理地设计和优化表面结构可以拓宽超疏水表面的强化传热区间,为低压蒸汽冷凝传热强化提供了实验基础和指导原则。
[Abstract]:As a typical vapor condensation phase transition phenomenon, widely exist in nature and daily life, at the same time as the heat transfer efficiency is widely used in industrial production. The low pressure steam condensate has an important application in the field of low grade waste heat recovery, such as low pressure distillation, low-temperature multi effect desalination technology, low temperature heat pump and heat pipe with the steam pressure drop, interfacial transfer resistance increased significantly reduces the heat transfer performance of the condensation process. Compared with the film condensation, dropwise condensation can effectively reduce the thermal resistance of the surface caused by the condensate, and the droplet transfer process to promote the dynamic behavior of the liquid vapor interface, a low-pressure steam condensation heat transfer enhancement ideal strategy. Based on the experimental observation, numerical simulation and theoretical analysis, systematic study of the low pressure steam dropwise condensation process of droplet size distribution and dynamic characteristics, evolution and table The surface temperature evolution of steam dropwise condensation heat transfer mechanism was revealed; promote the droplet with bouncing through the super hydrophobic surface interface effect, a low pressure steam condensation heat transfer enhancement. Design of low pressure steam condensation heat transfer experiment system, this study investigated the influences of steam pressure on dropwise condensation heat transfer performance and droplet dynamic behavior. The results show that the steam pressure drop, dropwise condensation heat transfer coefficient decreases slowly at first and then decreased rapidly. The combination of droplet heat transfer model, analyzed the control mechanism of low pressure steam condensation heat transfer, showed that the droplet growth affected by the liquid vapor interface transfer process enhanced with lower pressure. The experimental results showed that the droplet falling behind the phenomenon of low pressure steam in the condensing surface, delaying the update frequency, reveals the inherent relationship between the macroscopic droplet movement and heat transfer. By introducing the droplet dynamic characteristics, modified by Classic dropwise condensation heat transfer model, accurately predict the steam pressure and surface subcooling on dropwise condensation heat transfer characteristics. The influence of different steam pressure drop size distribution and transient steady state size distribution of the observation experiment, the influences of steam pressure on the growth characteristics of droplets. The initial droplet condensation by showing a normal distribution to Shuangfeng and then to the size distribution transient exponential distribution characteristics of evolution, with the steam pressure decreases, the nucleation site density decreases and the droplet growth rate slowed down. Compared with the normal pressure steam, reducing liquid surface condensation density drops under low pressure, and large drop frequency increased, increased the average size the condensation droplet surface. The droplet dynamic hysteresis model, explain the droplet dynamic behavior to reduce the dynamic lag angle; the energy barrier from the local perspective, the non continuous droplet removal The physical mechanism of falling behavior. Observation of low pressure steam condensation on droplet surface temperature distribution and evolution characteristics of the infrared thermal imaging technology, intuitive access to micro droplet movement caused by heat transfer phenomena. The results show that the condensation of the droplet surface temperature distribution showed the middle edge of the low. Closely related to the steam condensation process and droplet the growth behavior and interface evolution, different heat transfer and micro droplet, droplet movement by updating the liquid vapor interface promote rapid steam condensation, the internal mechanism of evolution reveals the droplet behavior caused by interfacial effects on macroscopic heat transfer. Compared with the super hydrophobic surface, the hydrophobic surface is more conducive to the growth of droplet nucleation, surface the temperature distribution is more uniform, the surface temperature fluctuation of the droplet movement caused more severe. Combined with the model analysis, SE simulation and visualization experiments, starting from the initial nuclear steam, the effect of surface structure and Influence of condensing conditions on initial droplet nucleation and droplet wetting mode. According to the cross scale and growth characteristics of super hydrophobic surface structure characteristics of condensing droplets, the surface structure on the droplet growth space limiting effect. Use of V shaped nano structure control of droplet initial nucleation over the shape and position. Hydrophobic condensation on the surface of droplet wetting transition mechanism was revealed. The infiltration factor to describe the infiltration degree of condensing droplets, the calculation results of the model well predicted the droplet apparent contact angle and hysteresis angle variation. The results show that when the initial droplet nucleation size and nano structure size comparable when the nucleation tend to occur near the nano column top position, thereby forming a droplet suspension mode, to merge with the surface induced bounce; increasing the degree of supercooling, droplet nucleation size is greatly reduced, the steam in nano structure Random nucleation, formation of condensate liquid infiltration mode, leading to failure of super hydrophobic surface. The super hydrophobic surface of four different nanostructures were prepared by controlled oxidation etching time, the influences of surface structure and surface subcooling on the droplet with bouncing behavior. With the increase of nano structure the length and spacing of the droplet size increases and bounce, bounce frequency decreased. With the increase of surface subcooling and condensation droplets in the nano structure within the degree of infiltration increased with initial jumping speed decreases with increasing of the size of the spring. The best wetting characteristics of condensing droplets into the droplet coalescence induced spring model, analysis the influence mechanism of nano structure, size and surface subcooling on droplet with initial hopping velocity. The results show that the dynamic behavior of steam condensation environment drops by nano structure on the surface of a liquid, condensation and Drop size determined. The experimental results showed that the one-way irreversibility and heat transfer performance change of super hydrophobic surface condensation droplet wetting mode varies with the degree of supercooling. The super hydrophobic surface to achieve the steam condensation heat transfer at low undercooling range enhanced heat transfer coefficient was significantly higher than that in the same conditions of film condensation. Even more than the smooth hydrophobic surface of dropwise condensation heat transfer performance. By comparing the three kinds of super hydrophobic surface heat transfer performance can be seen, the reasonable design and optimization of surface structure can broaden the super hydrophobic surface to enhance the heat transfer range, provides the experimental basis and guiding principles for low pressure steam condensation heat transfer enhancement.

【学位授予单位】：大连理工大学
【学位级别】：博士
【学位授予年份】：2015
【分类号】：TQ021.3

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