Dropwise condensation heat transfer of fluids with low surface tension

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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.