Dropwise condensation heat transfer of fluids with low surface tension

蒸汽的冷凝是许多技术应用的一部分，例如制冷循环、热泵和动力循环。它的特点是传热系数大，因为释放的汽化热可以传递到冷却介质，驱动温差小。在金属冷凝器壁处，通常发生膜状冷凝，其中热传递受到液膜的传导阻力的限制。相比之下，由于频繁的液滴脱落，暴露于冷凝蒸汽的几乎无冷凝物的表面积导致滴状冷凝期间明显更大的传热系数。为了实现这一点，冷凝器壁与冷凝物的润湿性必须通过适当的表面改性充分降低。90多年来，人们对工作流体水进行了广泛的研究。然而，旨在实现低表面张力流体滴状冷凝的研究很少，并且在2014年之前不包括与上述应用相关的流体。如今，已知几种类型的表面改性通过结合低表面自由能和低表面粗糙度来允许在不存在不可冷凝气体的情况下滴状冷凝流体如乙醇、正己烷或正戊烷。文献中报道的相应传热测量大多与仅略微变化和/或模糊描述的边界条件相关。这不仅限制了同一种流体不同表面改性所得传热系数的可比性，而且也限制了对影响滴状冷凝传热因素的准确分析。这个问题在拟议的研究项目中得到解决。对于选定的流体与低表面张力相结合的建立和有前途的表面改性实现滴状冷凝，系统的润湿行为的调查接近冷凝条件和传热系数的准确测量应进行。冷凝实验涵盖了广泛的饱和温度和表面过冷度，后者导致了广泛的变化的热流密度和冷凝率。该研究应显示传热系数如何受到冷凝物性质的影响，例如，导热性和密度，以及通过在滴状冷凝期间存在的变化的润湿状态，直到达到向膜状冷凝的转变。研究结果将用于现有的传热模型，特别是滴状冷凝的低表面张力的流体的进步。此外，它们应该有助于更好地理解这种冷凝形式如何实现特别有效的传热。

The condensation of vapors is part of many technical applications such as refrigeration cycles, heat pumps, and power cycles. It is characterized by large heat transfer coefficients as the released heat of vaporization can be transferred to the cooling medium with small driving temperature differences. At metallic condenser walls, filmwise condensation usually occurs, where heat transfer is limited by the conduction resistance of the liquid film. In contrast, the nearly condensate-free surface areas exposed to the condensing vapor as a result of frequent droplet shedding lead to distinctly larger heat transfer coefficients during dropwise condensation. To achieve this, the wettability of the condenser wall with the condensate has to be sufficiently reduced by appropriate surface modifications. This has been extensively investigated for the working fluid water for more than 90 years. However, studies aiming at the realization of dropwise condensation of fluids with low surface tension are rare and did not include fluids relevant for the aforementioned applications before 2014. Today, several types of surface modification are known to allow dropwise condensation of fluids like ethanol, n-hexane, or n-pentane in absence of noncondensable gases by combining low surface free energy and low surface roughness. Corresponding heat transfer measurements reported in the literature are mostly associated with only slightly varied and/or vaguely described boundary conditions. This limits not only the comparability of heat transfer coefficients obtained for the same fluid with different surface modifications, but also an accurate analysis of the factors influencing dropwise condensation heat transfer. This problem is addressed in the proposed research project. For selected fluids with low surface tension combined with established and promising surface modifications for the realization of dropwise condensation, systematic investigations of the wetting behavior close to condensation conditions and accurate measurements of the heat transfer coefficient should be performed. The condensation experiments cover wide ranges of saturation temperatures and surface subcoolings, where the latter lead to a broad variation of heat flux densities and condensation rates. The study should show how the heat transfer coefficient is affected by the condensate properties, e.g., thermal conductivity and density, and by the varying wetting states being present during dropwise condensation until transition to filmwise condensation is reached. The findings will be employed for the advancement of existing heat transfer models specifically for dropwise condensation of fluids with low surface tension. Furthermore, they should contribute to an improved understanding of how particularly efficient heat transfer can be achieved for this condensation form.