Effect of aluminum heat exchanger surface wettability on condensation heat transfer and water harvesting performance

Water vapor condensation governs the efficiency of water harvesting and dehumidification. In air conditioning and refrigeration systems, condensed water accumulating on external heat exchanger surfaces decreases the overall thermal-hydraulic performance. The decrease in performance arises due to the considerable heat transfer resistance formed by the liquid that forms and resides on the surface of the heat exchanger. As a mitigation strategy, engineers have developed coatings using low-energy non-wetting materials that promote the formation of discrete droplets, enabling rapid shedding from the surface. This mode of condensation, termed dropwise condensation, enhances heat and mass transfer as well as water harvesting rates due to efficient condensate removal. In this work, we conduct rigorous condensation experiments on heat exchangers having five different surface wettabilities in a controlled environmental test facility. The surface fabrication procedures are done on decimeter-scale copper-tube aluminum-fin commercial heat exchangers with different geometrical dimensions and fins specifications. The study examines a range of surface functionalities, including uncoated surfaces, superhydrophobic aluminum oxyhydroxide (AlO(OH)) nanostructures, superhydrophilic AlO(OH) nanostructures, slippery liquid infused porous surfaces, as well as quasi-liquid surfaces. Test conditions included a range of relative humidities from 50 to 70 %, ambient air temperatures between 22 ⁰C and 27 ⁰C, and an inlet coolant temperature of 0 ⁰C. To compare performance, we quantify the amount of water collected from each heat exchanger during cooling and dehumidification and measure the steady state heat transfer rates during 2-hour condensation tests. Water retention tests were conducted to measure the quantity of water held by each heat exchanger to examine the impact of surface wettability on water retention. The results demonstrate that water collection is proportional to ambient relative humidity and highly dependent on surface wettability. Lower wettability surfaces (superhydrophobic and quasi-liquid surfaces) exhibit higher water collection rates, reaching up to 457 g/m² under moderate ambient conditions and 1 kg/m² under high ambient conditions. In contrast, higher-wettability surfaces (superhydrophilic and slippery liquid-infused porous surfaces) experience higher levels of liquid retention, limiting water collection to a maximum of 276 g/m² under moderate conditions and 400 g/m² under high ambient conditions. Heat exchangers with high fin density and louvered fins outperformed those with lower fin density and corrugated fins, achieving maximum enhancements in latent, sensible, and overall heat fluxes of 157 %, 419 %, and 369 %, respectively. Water retention tests showed that the quasi-liquid heat exchanger with higher fin density and louvered fins retained the least water (9.4 g/m²), followed by the SHP heat exchanger with lower fin density and corrugated fins (14.7 g/m²), highlighting the impact of fin density and design on minimizing water retention and enhancing thermal performance.