Mechanism Study On Surface Hydrophobicity And Icing Inhibition Of Pre-cooler In Air-breathing Engine Of Wide Velocity Range

The present thesis work is to study the effects of monolayer micro-/ nano-structure and two-tier structures on wetting behavior and anti-freezing/frosting ability.The experimental systems include contact angle goniometer equipped with the temperaturecontrolled chamber,high-speed photography system and environmental scanning electron microscope(ESEM).Combined with the theories of classical wetting model,thermodynamics,phase change dynamics and solid surface adsorption,the wetting behaviors and wetting stability of different micro-and nano-structures,wetting modes and heat transfer characteristics during droplet evaporation,sessile droplet freezing and melting,droplet impact on low-temperature hydrophobic surfaces as well as condensation and frosting processes under micro-or macroscopic observations were studied in detail.The major works are summarized as follows.The surface wettability and wetting stability of three kinds of micro-and nanostructures were experimentally and theoretically investigated.The effects of surface structures,structural parameters and substrate materials on hydrophobicity were discussed.The correlation between pillar-structured surfaces and contact angle,contact angle hysteresis was established.It is found that two-tier structures were more hydrophobic than monolayer structures.Micro-nano-structures were more hydrophobic than micro-micro-structures.Micro-cubes were more hydrophobic than micro-cylinders.When the pillar height was certain,both micro-and hierarchical structures became more hydrophobic with interval-width ratio increased.The substrate had little effect on wetting behaviors.When the surface was under room temperature,hierarchical structures and monolayer nanostructures were more conducive to the complete rebound of the impinged droplets,implying that their wetting stability was stronger.The water droplet and ethanol droplet evaporation processes were studied.The results show that the surface structures affected the wetting behavior and evaporation rate during droplet evaporation.The Cassie-Wenzel wetting transition was more likely to happen on the micro-array-structure surfaces with a larger interval-width ratio.Cylinders were easier to achieve strong hydrophobicity and stability than cubes.Besides,the hydrophilic smooth silicon evaporated fastest,followed by the monolayer structures.Two-tier structures evaporated slowest.The nano-scale second-layer structure increased the evaporation rate,making the evaporation time of micro-nano-structures was shorter than that of the micro-micro-structures.Sessile droplet freezing was experimentally discussed.The freezing process can be divided into five stages,namely liquid cooling and supercooling,liquid metastable state,nucleation and surface recalescence,internal solidification and solid cooling.Micronano-structures had the longest time of remaining liquid of all structured silicon surfaces.When the crystal nucleus is formed,heat release made the droplet surface temperature rise sharply.During solidification,the initial period was affected by surface characteristics while the later period was mainly dominated by the solid-liquid contact area.The angle of ice cone formed on micro-and nano-structures was smaller than that on the smooth silicon,which was independent of the initial contact angles.In addition,hierarchical structures were the most proper surfaces to prevent the freezing of the impinged droplet and micro-nano-structures were more icephobic than micro-microstructures.The processes of frozen droplet melting and repeated freezing-melting were experimentally discussed.The results show melting on the hydrophilic surfaces needed a smaller degree of superheat than hydrophobic surfaces.Hierarchical structures can partly recover the hydrophobicity.Micro-nano-structures had the best durability after six repeated freezing tests.The effects of surface wettability and surface structures on condensation were studied theoretically.It is found that hydrophobicity mainly affects the formation and distribution of liquid nuclei.The larger the contact angle is,the more difficult it is to form liquid nuclei and the denser of liquid nuclei distribution is.Surface structures affect the propagation of condensed droplets and freezing waves.Three condensation modes were concluded based on the ESEM results.Condensation propagated slowest on two-tier structures while flooding was prone to happen on micro-cylinders.Under the macroscopic observation,frost layer structures on the hierarchical structures and uncoated silicon surface were similar,which were relatively short in shape and large in number.It should be noted that all structured surfaces can only delay frost formation in the early stage of frosting and the frost thickness difference gradually shrink lately.