Study On Frosting/Defrosting Characteristics Of Superhydrophobic Fins Under Cassie Wetting Mode

Air source heat pumps are widely used as cold and heat sources of building air conditioning systems because of their high efficiency,energy saving and environmental protection,both heating and cooling functions,low operating costs,and flexible installation and use.However,in practical applications,the frosting problem of air source heat pumps in winter has restricted the promotion and application of air source heat pumps.In order to ensure the efficient and stable operation of air source heat pumps in winter,the exploration of efficient frost suppression/defrost technology is imminent.In response to this problem.this paper proposes superhydrophobic modification of the surface of the air source heat pump fins.This technology has the advantages of low cost,high efficiency,energy saving and environmental protection.In this paper,through visual experiments and theoretical analysis,the frosting/defrosting characteristics of superhydrophobic fins under the Cassie wetting mode are studied,and the frosting/defrosting characteristics of superhydrophobic fins under the Wenzel wetting mode are compared and analyzed,The effect of the geometric characteristics of the surface microstructure on the frosting/defrosting of superhydrophobic fins under different wetting modes is revealed,the specific work content and results are as followsIn terms of frost visualization experiment,the frosting process on the surface of superhydrophobic fins under Wenzel wetting mode and Cassie wetting mode is compared,and microscopic images of droplet condensation,growth merger,bounce,freezing,and frost layer growth were obtained.Experiments find that Cassie fins have the characteristics of inhibiting frost formation and delaying the growth of the frost layer.Under the same experimental conditions,the surface of the Cassie fin is sparsely distributed with condensation droplets,the growth rate is slow,there are condensation droplets and self-bouncing phenomenon,and the condensation droplets start to freeze and complete freezing later,and the frost layer height is reduced compared with Wenzel fin About 50%.Based on this experiment,the effect of the wetting mode of the fin surface microstructure on the frosting characteristics of the superhydrophobic fins is clarified.Revealing that the Cassie wetting mode has the effect of suppressing frosting on the superhydrophobic fins.In the meantime,The freezing transfer behavior of condensed droplets at the initial stage of frosting on cold surfaces is studied.The study further shows that the freezing transfer of condensed droplets is affected by the lateral diffusion rate of frost crystals and the droplet spacing.Increasing the contact angle of the cold surface or reducing the distribution density of condensed droplets can reduce the freezing transfer rate and delay frosting.This is also one of the important reasons why the superhydrophobic surface has excellent anti-frost performance under the Cassie wetting modeIn terms of theoretical research,the heat transfer model,growth model.and Cassie w etting mode of superhydrophobic fin condensation droplets and self-bouncing models have been established for the condensation droplets on the surface of different fins,reveals the effect of surface microstructure on the growth of condensed droplets at the initial stage of frosting.The results show that:compared with ordinary fins and Wenzel fins,the condensed droplets on the surface of Cassie fins have greater heat transfer and thermal resistance,and the growth rate of droplets is slow.By increasing the surface contact angle,reducing the heat transfer temperature difference,and optimizing the design of the surface microstructure size,the growth of condensed droplets can be further delayed.Reducing the diameter of the nanostructure on the fin surface or increasing its height and spacing helps to slow down the growth rate of condensed droplets.The slow growth of the condensed droplets on the surface of the Cassie fin is mainly due to the continuous growth of the droplets and the combined release of surface energy,which drives the droplets to spontaneously bounce off the surface of the fin,thereby delaying the frosting process.Studies have shown that:a larger surface contact angle and a smaller solid-liquid contact area ratio can increase the remaining driving energy on the surface and promote the bounce of condensed droplets;The larger the droplet radius and the greater the number of droplets before merging,the easier it is to promote the bounce of the condensed dropletsIn the visual experiment of defrosting,the defrosting characteristics of the superhydrophobic fin surface in the Wenzel and Cassie wetting modes were compared,and the frost layer melting,shedding,defrosting water shrinkage were obtained.Studies have shown that:Compared with Wenzel fins,Cassie fins have higher defrosting efficiency and can be completely peeled from the surface of the fins,leaving relatively dry fins;In the Wenzel fin defrosting process,as the frost layer decomposes and melts and the melt water shrinks,the remaining droplets after the frost layer melts are not completely evaporated,and they will continue to grow into frost in the next frosting cycle.In terms of theoretical research,the Cassie wetting mode superhydrophobic surface frost shedding model is established.The results show that the key to frost shedding lies in the expansion of the air in the enclosed space between the microstructure of the fin surface and the frost layer.And the weak adhesion of the surface in Cassie mode.Reducing the diameter or increasing the height of the nanostructure on the surface of the fin is beneficial to reduce the heating temperature required for frost peeling,thereby reducing the energy consumption of defrosting.This article aims to explore the influence of the geometric characteristics of the surface microstructure on the frosting/defrosting characteristics of superhydrophobic fins under different wetting modes,and to provide theoretical guidance for optimizing the design of high-efficiency anti-frost/defrosting superhydrophobic fins.