Dropwise condensation heat transfer of fluids with low surface tension

蒸汽的冷凝是制冷循环、热泵和动力循环等许多技术应用的一部分。它的特点是换热系数大，因为释放的汽化热可以传递到驱动温差较小的冷却介质中。在金属冷凝器壁面上，通常发生膜状冷凝，其换热受液膜传导阻力的限制。相反，由于液滴频繁脱落，几乎没有冷凝水的表面积暴露在冷凝蒸汽中，导致滴状冷凝过程中的换热系数明显较大。为了实现这一点，必须通过适当的表面改性来充分降低冷凝器壁与冷凝液的润湿性。90多年来，人们对工作液水进行了广泛的研究。然而，旨在实现低表面张力流体的滴状冷凝的研究很少，2014年前不包括与上述应用相关的流体。今天，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，允许乙醇、正己烷或正戊烷等流体在不存在不可凝结气体的情况下进行滴状冷凝。文献中报道的相应的换热测量大多只与微小变化和/或模糊描述的边界条件有关。这不仅限制了具有不同表面改性的相同流体的换热系数的可比性，而且还限制了对影响滴状冷凝换热的因素的准确分析。这一问题在拟议的研究项目中得到了解决。对于选定的低表面张力的流体，结合公认的和有希望实现滴状冷凝的表面改性，应对接近冷凝条件的润湿行为进行系统的研究，并准确测量换热系数。冷凝实验涵盖了很大范围的饱和温度和表面过冷，后者导致热流密度和冷凝速率的广泛变化。这项研究应该说明冷凝物的性质如何影响换热系数，例如导热系数和密度，以及在向膜状冷凝过渡之前的滴状冷凝过程中存在的不同润湿状态。这些发现将被用于改进现有的传热学模型，特别是用于低表面张力流体的滴状冷凝。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热传递。

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.