frost, surface wettability, microchannels, hydrophilic, hydrophobic
In this work, differences in drainage rates and defrosting effectiveness were explored for surfaces of differing wettability. Both patterned and non-patterned surfaces were explored. To date, six surfaces have been examinedâ€” an uncoated, untreated aluminum plate (Sample 1), an identical plate treated with a hydrophilic coating (Sample 2), a plate containing evenly-spaced micro-channels with no surface coating (Sample 3), a plate containing evenly-spaced micro-channels with a hydrophobic coating (Sample 4), and a surface containing a microstructural roughness gradient both with and without a hydrophobic surface coating (Samples 5 and 6). Cyclical tests containing frosting periods and defrosting periods were conducted on each sample. Each cycle consisted of one hour of frost growth, followed by ten minutes of defrost and drainage. For these experiments, the frost layer was grown inside a Plexiglas environmental test chamber where the relative humidity was held constant (i.e. 60%, 80%) for the duration of the experiment using an ultrasonic humidifier. The temperature of the ambient air inside the enclosure was recorded to ensure that it remained constant throughout the experiment, and the surface temperature of the plate was fixed using a thermoelectric cooler (TEC). 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 surface defrosting effectiveness varied from 52-77% across all surfaces depending on the test conditions, with one test showing slightly lower percentages. Our data show that only small differences were observed in the defrosting effectiveness between the samples. The gradient surfaces however did remove slightly more water from the surface during defrosting (as compared to the baseline) when the frost was grown at colder surface temperatures. The average increase in defrosting effectiveness at Tw = -12Â°C was 2-4% for Surface 6 versus Surface 1. Interestingly, when the frost was grown at warmer surface temperatures, the gradient surfaces did not perform as well. In almost all cases, however, the defrosting effectiveness increased as the surface temperature was decreased during the frost growth period. This finding suggests that defrosting effectiveness is intrinsically linked to the thermophysical properties of the grown frost layer. Additional research is needed to investigate this phenomenon more fully. The overall aim of this work is to study the effects of surface wettability and micro-structural roughness on the defrosting performance of functionalized heat transfer surfaces.