Abstract
We developed hybrid organic–inorganic sol–gel silica coatings with good durability in harsh environment (high temperatures, high vapor velocities) and with slightly hydrophobic behavior, sufficient to promote dropwise condensation (DWC) of pure steam. DWC is a very promising mechanism in new trends of thermal management and power generation systems to enhance the heat transfer during condensation as compared to film-wise condensation (FWC). The sol–gel coatings have been prepared from methyl triethoxy silane (MTES) and tetraethyl-orthosilicate (TEOS) and deposited on an aluminum substrate. The coatings were optimized in terms of precursor ratio and annealing temperature highlighting potentials and limits of such mixtures. A comprehensive surface characterization before and after saturated steam condensation tests has been performed and related to the thermal measurements for evaluating the heat transfer augmentation as compared to FWC obtained on untreated aluminum surfaces. The results showed that the developed hybrid organic-inorganic sol–gel silica coatings are promising DWC promoters.
Highlights
In the heat exchange industry, metals are the most widely used materials because of their thermomechanical characteristics, such as sturdiness, high strength to weight ratios and thermal conductivity
Several studies are related to the production of superhydrophobic surfaces using sol–gel coating [10,11,12,13,14,15,16], very limited studies rely on the quasi-hydrophobic surface [7], and, those treatments are not tested in harsh environment, e.g., high temperatures and high vapor velocity
According to the literature [20], the composition of each film was labeled with alphanumeric code: Mx Ty,_number, where x is the molar fraction of methyl triethoxy silane (MTES), y is the molar fraction of tetraethyl orthosilicate (TEOS), and the number corresponding to the baking temperature of the sample
Summary
In the heat exchange industry, metals are the most widely used materials because of their thermomechanical characteristics, such as sturdiness, high strength to weight ratios and thermal conductivity. For instance, the mutual effect of oxygen, magnesium, calcium, high level of chloride, combined with the presence of organic compounds, pollutants and biological activity creates very hostile conditions In these environment corrosion mechanism must be deeply understood to guarantee the metal’s strength [1]. Several studies are related to the production of superhydrophobic surfaces using sol–gel coating [10,11,12,13,14,15,16], very limited studies rely on the quasi-hydrophobic surface [7], and, those treatments are not tested in harsh environment, e.g., high temperatures and high vapor velocity Both quasi-hydrophobic and superhydrophobic surfaces allow the DWC mode, but research on superhydrophobic ones is very far from obtaining satisfactory results in terms of surface treatment life [17]. Different reagent concentrations and baking temperatures were analyzed in order to understand the limits and the advantages of the sol–gel MTES/TEOS hybrid coatings for the promotion of DWC
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