Thin non-perfluoroalkoxy superhydrophobic coatings are desirable for heat exchangers because of their lower thermal resistance and reduced environmental concerns. Coatings requirements must also include robustness and longevity and facilitate high defrosting rates in refrigeration applications to warrant their adoption and use. Methylfunctionalized silica nanosprings (SN) possess water droplet static contact angles above 160°with contact angle hysteresis values as low as 6.9°for a sub-micrometer-thick coating. The methyl functional groups render the silica surface hydrophobic, whereas the geometrical and topographical characteristics of the nanosprings make it super-hydrophobic. Results show that SN are capable of removing 95% of the frost from the surface at a lower temperature than the base aluminum substrate. The sub-micrometer SN coating also decreases the time to defrost by ≈1.5 times and can withstand more than 20 frosting− defrosting cycles in a high humidity environment akin to real working conditions for heat exchangers.
In
this work, the water condensation performance of methylated silica-nanospring
(SN)-coated horizontal aluminum tubes is assessed. Coated samples
with varying nanospring mat thicknesses, from 784 to 2902 nm, were
studied, which exhibited static contact angles and CA hysteresis values
of 155° and 16°, respectively. Dropwise condensation and
increased droplet shedding were observed on these coated tubes. Video
analysis determined that tubes with 15 and 20 min SN growth times
experienced an 84% increase in the condensate removal rate over the
baseline. Moreover, with a hybrid wettability consisting of alternating
regions of SN and bare aluminum, a 96% increase in condensate removal
was experienced. Additionally, the average droplet departure size
was reduced on these SN-coated tubes. SEM imaging and XEDS analysis
were also performed on the tubes and revealed that the coating was
reasonably durable having withstood the condensation environment.
Moreover, the coated tubes were shown to exhibit the same XEDS spectra
both before and after testing.
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