Enhancing heat-exchanger performance in frost conditions via superhydrophobic surface modification

Abstract

Frost accumulation significantly reduces the heat transfer efficiency of air source heat pump (ASHP) during winter operation, making frost suppression and defrosting critical issues that need to be addressed. This study presents a simple system optimization method that considers both active frost suppression and efficient low-temperature defrosting strategies to ensure prolonged efficient heat transfer. Heat exchangers with different surface properties (hydrophilic, hydrophobic, and superhydrophobic) were implemented, and the dynamics of frosting and defrosting processes were studied under high humidity conditions. The defrosting process employs low-temperature defrosting and quantitatively monitors heat transfer based on temperature differences, facilitating rapid defrosting in the event of a substantial decrease in heat transfer efficiency. Experimental results reveal distinct frost accumulation patterns between the fins, attributed to variations in freezing forms and surface properties of the droplets. The superhydrophobic heat exchanger ensures an effective heat exchange channel time of 25 min and 49 min longer than the hydrophobic heat exchanger and hydrophilic heat exchanger, respectively, showcasing excellent active frost suppression capability. After 90 min of frost accumulation, the defrost energy consumption of the superhydrophobic heat exchanger is 31.7% and 28.9% lower than that of the hydrophilic heat exchanger and hydrophobic heat exchanger, respectively. With the proposed defrosting strategy, the defrosting time of the superhydrophobic heat exchanger is reduced by 39% and 32.4%, and the heat gain is 11.7% and 11.3% higher than that of the hydrophobic and hydrophilic heat exchangers, respectively. Additionally, the superhydrophobic heat exchanger exhibits a 36.4% increase in heat gain compared to the conventional reverse cycle defrosting method. Additionally, in the cyclic frost-defrost study, the superhydrophobic heat exchanger maintained efficient heat transfer for 76.2% and 31.6% longer than the hydrophobic and hydrophilic heat exchangers, respectively. This prolonged efficiency is attributed to the effective drainage of water trapped on the fin surface. These findings underscore the potential of proposed defrosting strategies alongside dynamic studies of active surface frost suppression characteristics, offering significant energy-saving advantages.