Condensation, Droplets, Superhydrophobic Surface, Frost Dynamics, Heat Exchanger Design
Superhydrophobic surfaces, which promote the efficient removal of condensing droplets prior to supercooling and freezing through coalescence induced jumping, have been shown to delay frosting significantly. The performance of fin and tube heat exchangers used in refrigeration and heat pump applications has the potential for improvement when coated with suitably designed superhydrophobic coatings. Currently, state-of-the-art (SOA) fins and their spacing are designed carefully to optimize pressure drop and heat transfer. The interaction amongst condensate droplets on adjacent fins can enhance the rate of condensate removal, and contribute to frost formation delay. Therefore, understanding the dynamics of droplet jumping within the confined space between fins can help in optimizing the design of superhydrophobic heat exchangers. In our work, condensation/frost interaction between two parallel superhydrophobic surfaces was studied experimentally. Frost growth on two aluminum superhydrophobic surfaces (150 mm × 90 mm) was tested under different conditions: surface temperature Tsurface = 0°C, -5°C, -10°C and -15°C; surface spacing of 1mm, 2mm, 4mm, 6mm, and 8mm; Tambient = 25°C, relative humidity ≈ 50%. Frost and condensate growth was recorded via high speed imaging from the side and top of the gap between the surfaces. Average droplet size and frost thickness was measured visually by analyzing high resolution and high contrast images of the leading edge of the cold plates. The variation of frost density on the surfaces during various stages of frosting was measured by continuously recording the mass of frost on the cold plates. Tests were also performed for bare aluminum surfaces under the same conditions for comparison. Results showed the normalized frost growth rate for untreated surfaces was the same regardless of the gap between the fins. The identical growth rate occurred due to the reduced access to moisture during the later stages of the frosting process when the air between the two frost faces is very cold and dry. We also observed that the frost growth rates for the superhydrophobic surfaces are 3X lower than the untreated surfaces due to the presence of jumping droplet condensation. Defrost times and water retention were characterized and shown to be 50% lower on the superhydrophobic surfaces compared to the bare aluminum surfaces. Additionally, high speed imaging showed that the droplets that do coalesce and jump from one surface travel to the adjacent surface, causing additional coalescence and jumping, and increasing the condensate removal rate. Our work not only contributes valuable data that can be used to optimize the design of coated evaporators, it elucidates the complex thermodynamics governing the condensation frosting process on SOA and next-generation superhydrophobic heat exchangers.