Defrosting performance on hydrophilic, hydrophobic, and micro-patterned gradient heat transfer surfaces

Abstract

In the current article, differences in drainage rates and defrosting effectiveness were explored for surfaces of differing wettability. Both patterned and nonpatterned surfaces were explored. Seven surfaces were examined in all—an uncoated, untreated aluminum plate (Sample 1), an identical surface treated with a hydrophilic coating (Sample 2), a surface containing evenly spaced microchannels with and without a hydrophobic coating (Samples 3 and 4), and a surface containing a microstructural roughness gradient with and without a hydrophobic coating (Samples 5 and 6). Cyclical tests consisting of three frosting/defrosting events were performed on each sample. Each cycle consisted of 1 hour of frost growth, followed by 10 minutes of defrost and drainage. The frost layer was grown on the surface inside an environmental test chamber under controlled operating conditions. The surface temperature, air temperature, and relative humidity were recorded to ensure that constant conditions were maintained during each experiment. Overall, the surface defrosting effectiveness varied from 52%–77% across all surfaces depending on the test conditions, with one test showing slightly lower percentages. The present data show that only small differences were observed in the defrosting effectiveness between the samples. The gradient surfaces did, however, 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 was 2%–4% for Surface 6 versus Surface 1 at Tw = −12°C. Interestingly, when the frost was grown at warmer surface temperatures, the gradient surfaces did not perform as well. However, in almost all cases the defrosting effectiveness increased as the surface temperature during the frost growth period was decreased. This finding suggests that defrosting effectiveness is intrinsically linked to the thermophysical properties of the grown frost layer.