defrosting, superhydrophobic, nanosprings, coating, drainage
The overall aim of this work was to study the defrosting performance of functionalized heat transfer surfaces containing a novel, silica nanosprings coating combined with preferential microstructural roughness. In doing this, differences in drainage rates and defrosting effectiveness were explored both on patterned and non-patterned surfaces. To date, ten different surfaces have been examined— an uncoated, untreated aluminum plate (S1), plates containing a silica nanospring (SN) coating of varying thickness (S2-S6), a plate containing evenly-spaced microchannels both with and without the SN surface coating (S7, S8), and then finally a plate containing a microstructural roughness gradient both with and without the SN surface coating (S9, S10). Cyclical tests containing both frosting and defrosting periods were conducted on each sample. For these experiments, the frost layer was grown inside a controlled environmental test chamber where the relative humidity (RH) was held constant (i.e. 60%, 80%) while the temperature of the ambient air inside the enclosure was monitored to ensure consistency. The surface temperature of the plate was fixed using a thermoelectric cooler (TEC) typically at -8°C, -10°C or -12°C. The TEC unit was placed on an electronic balance within the test chamber, which permitted the frost mass to be recorded continuously during testing. Overall, the defrosting effectiveness varied from 56-96% across all the surfaces depending on the test conditions. For the tests performed at 60% RH, the uncoated baseline surfaces tended to have defrosting efficiencies in the range of 59-75%, while the nanospring-coated surfaces tended to have defrosting efficiencies in the range of 66-96%. Different nanospring mat thicknesses were also explored as part of this work, which showed that an optimum thickness likely exists with shorter overall mat thicknesses being preferred. The microstructural surface gradient pattern included in this work was designed to create “preferential lanes” on the surface for drainage. The surface which yielded the highest overall defrosting efficiency during testing was the surface with the uniformly-spaced microchannels and nanospring coating (S8), while the gradient surface design with nanospring coating (S10) also generally performed well versus the baseline surface, especially at lower plate temperatures (i.e. Tw = -10 and -12°C).