Abstract
In order to solve the frosting problem of air source heat pump (ASHP) outdoor heat exchange under low-temperature and low-humidity conditions, a superhydrophobic aluminum (Al) surface with a contact angle (CA) of 158.3° was prepared by chemical etching. The microscopic characteristics of droplet condensation and the freezing process of a superhydrophobic surface were revealed through visual experiments and theoretical analysis. On this basis, the frost-suppression effect of a superhydrophobic Al-based surface simulating the distribution of actual heat exchanger fins was preliminarily explored. The results demonstrated that, due to the large nucleation energy barrier and the coalescence-bounce behavior of droplets, the condensed droplets on the superhydrophobic surface appeared late and their quantity was low. The thermal conductivity of the droplets on a superhydrophobic surface was large, so their freezing rate was low. The frosting amount on the superhydrophobic Al-based surface was 69.79% of that of the bare Al-based surface. In turn, the time required for melting the frost layer on the superhydrophobic Al-based surface was 64% of that on the bare Al-based surface. The results of this study lay an experimental and theoretical foundation for the application of superhydrophobic technology on the scale of heat exchangers.
Highlights
Electric air source heat pump (ASHP) have found worldwide applications due to their advantages of energy saving and environmental protection [1,2,3]
In order to clarify the effect of chemical etching on the contact angle (CA) and hydrophobic proper‐
Inlayer order to clarify the effect ofafter chemical etching on specimens the CA andafter hydrophobic proper‐
Summary
Electric ASHPs have found worldwide applications due to their advantages of energy saving and environmental protection [1,2,3]. When the heat pump operates under heating conditions in winter, at ambient air temperatures of −15 ◦ C to 11.5 ◦ C and relative humidity higher than 30%, coil pipes of heat pump outdoor units are prone to frosting [4]. In order to ensure the operation efficiency of ASHPs in winter, a variety of methods have been proposed to restrain and defrost the frost layer on the coil surface. Used defrosting techniques include electric heating defrosting [7,8], reverse-cycle defrosting [9,10], thermal storage defrosting [9,10], and hot-gas bypass defrosting [10,11,12,13].
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