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水蒸气在超疏水表面上的冷凝传热 总被引:1,自引:0,他引:1
用高温裂解法在紫铜基底上制备了疏水性碳纳米管膜,通过对此碳纳米管膜进行氟化处理,改善了表面的疏水性.在室温下,实验测得水在这种表面上的接触角在90°~130°之间.以水蒸气为冷凝介质的冷凝传热实验表明,水蒸气在超疏水纳米材料表面上能形成较好的滴状冷凝,冷凝传热膜系数可达40000 W/(m2·K).与纯粹膜状冷凝相比,冷凝传热系数提高3~4倍.分析表明,此碳纳米管膜所产生的附加热阻只占冷凝传热热阻的千分之一,对冷凝传热膜系数的影响可以忽略. 相似文献
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根据相变过程的微观物理机理和热力学特性,提出了冷凝传热过程中,近壁面蒸汽分子经由团聚阶段进而冷凝成宏观液滴的物理模型.并将团聚体分布与滴状冷凝传热性能相联系,从而研究不凝性气体对滴状冷凝传热过程的影响.在改进的Dillmann和Meier(DM)模型基础上,将分子团聚过程中的临界过饱和度与冷凝过程中的过冷度相联系,以及将团聚体的能量特性与液固界面物理化学特性相联系,将团聚模型与考虑固液界面效应的滴状冷凝传热模型相联系,建立了近壁面条件影响的分子团聚模型.利用模型计算了近壁面蒸汽中团簇体尺寸和分布,以及不凝性气体存在导致的蒸汽冷凝团聚体分布的变化,并结合滴状冷凝传热模型,定量解释了少量不凝性气体的存在,极大影响了冷凝传热性能的现象.模型计算结果与实验结果及文献中含不凝气的蒸汽冷凝传热实验数据进行了比较,两者符合较好,验证了所提出模型的合理性. 相似文献
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对于滴状冷凝过程及其传热强化机理, 一般通过分析冷凝壁面上液滴分布和运动规律进行研究, 并且将单个液滴视为稳定的个体, 很少涉及液滴内部运动特征. 本文通过红外热像仪观测了纯蒸气滴状冷凝过程中, 液滴运动时自由表面温度场的演化过程. 发现在疏水壁面上, 液滴由于合并或脱落而发生移动过程中, 其自由表面温度先降低, 而后升高并高于移动前温度. 通过分析疏水表面上液滴移动过程的物理模型, 认为液滴移动时表面液膜发生履带式滚动现象, 或者发生液滴内部与自由表面附近的液体间形成对流和掺混现象. 对液滴运动时表面温度演变规律的分析表明: 触发液滴表面发生持续冷凝可能需要克服一个临界过冷度, 当气液间温差超过该临界值时才诱发冷凝; 液滴合并或脱落等整体运动过程, 导致了液滴内部的运动特征, 并促进了较大尺寸液滴表面发生直接冷凝, 这为强化冷凝传热的研究提供新的思路. 相似文献
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This article describes an experimental investigation of the condensation heat transfer of steam on dropwise and filmwise coexisting (DFC) surfaces, on which dropwise and filmwise exist simultaneously at adjacent positions. A fluor-containing organic film with thickness of more than 1 μm was coated on the dropwise regions of the external surface of a brass tube to promote dropwise condensation. The surfaces were divided horizontally into many regions according to the designed dropwise and filmwise condensation area ratio. The area ratio of dropwise regions and filmwise regions in the present study was fixed at 50%:50% for all the six surfaces, while the numbers of dropwise and filmwise regions ranged from 2 to 16. Experiments were conducted at atmospheric pressure and the treated tube was oriented vertically in the condensing chamber. It was found that the heat transfer performance for DFC surfaces increases with increasing number of dropwise and filmwise regions, and an enhancement ratio of 1.27 to 1.96 is realized compared with the results for bare surface. Visual observation revealed that the appearance of condensation near the boundary region between the dropwise and filmwise regions was dependent on the relative positions of the two condensation regions. The condensate flowed smoothly across the boundary for dropwise condensation in the upper region. With filmwise condensation in the upper region, a condensate ring was formed at the interface and was retained at the interface for a short period of time before collapsing and then continued to flow downward through the dropwise condensation region. The condensate ring made a remarkable contribution to the condensation heat transfer enhancement for DFC surfaces. Finally, the results also showed that the heat transfer enhancement for dropwise and filmwise coexisting surfaces depends not only on the area ratio on DFC surfaces, but also on the surface subcooling degree. An optimal coordinating condition between these two factors might realize a maximum heat transfer enhancement ratio. 相似文献
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This article describes work aimed at obtaining higher filmwise condensation heat flux by distributing dropwise condensation surfaces of optimal width promoted by an organic coating among filmwise surfaces, and to get higher mean overall heat transfer coefficients in condensing systems. Several different spacings were examined for the horizontal orientation, arranging a dropwise section above a filmwise one, to make clear the effect of the drops falling down to the filmwise section. The heat flux of the filmwise part increased with increasing the height of the dropwise part up to 2 mm, but then decreased above that. The extent of the filmwise part that was augmented by drops was also tested by changing the width of the filmwise section sandwiched between dropwise sections of constant width. The heat flux of the filmwise part increased abruptly at a width between 5 and 3 mm. Consequently, it was shown that there exists an optimum width for each section for enhancing condensation heat transfer in the filmwise sections. 相似文献
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Application of dropwise condensation to utility turbine condensers is investigated by comparing the thermal performance of dropwise and filmwise bundles at industrially relevant conditions. Steam and steam-air mixtures were condensed on bundles of in-line, titanium tubes. The row-by-row heat transfer coefficients are presented against bundle position. They show the expected behavior for filmwise condensation but demonstrate a different one for dropwise. In air-free steam, the dropwise heat transfer coefficients are much larger and do not vary significantly with bundle position. In air-steam mixtures the dropwise values decrease similarly to their filmwise equivalents. The findings are in accord with those found for other geometries. The findings indicate that significant reductions in condenser size can be obtained if permanent dropwise condensation can be produced at industrially relevant conditions. 相似文献
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蒸气凝结相关问题探讨 总被引:3,自引:1,他引:2
讨论了几个与蒸气凝结相关的问题,指出壁面上球冠形液滴的内外压差和临界半径同样遵循经典的Laplace公式和Kalvin公式;蒸气在冷壁上的冷凝形态主要由后退接触角决定;空气中的水蒸气在换热器表面呈膜状冷凝时换热器的性能优于呈滴状冷凝时换热器的性能。 相似文献