Experimental Studies On Frost Suppression Performance Of Aluminum-based Super-hydrophobic Surface Of Evaporator In Air Source Heat Pump

In many China’s southern cities,due the humidity ratio of air is high in winter,the apparent temperature is very low.Although central heating is required,the policy doesn’t be implement and localized.As an environmental protection and energy-saving technology,air source heat pump(ASHP)is widely adopted as heating system in region lies in the south of Yangtze River.But there is an inevitable frost phenomenon on the surface of evaporator when the ASHP is operating for heating in those areas.And the frost formation will decrease the Coefficient of Performance(COP)of ASHP.Seriously it even leads to that the system can not run normally.At present,the several common defrosting modes for ASHP consume the indoor energy,and make a negative effect on the indoor thermal comfort.Therefore,this article tries a new method that applying the Super-hydrophobic to ASHP to suppress the frost phenomenon on the surface of evaporator.Theory of frost suppression of super-hydrophobic surface is studied in this paper.Thermal resistance increases by the growth of contact angle between the droplet and the hydrophobic surface.Therefor the vapor phase needs more energy to produce phase change.Thus the vapor in humid air is harder to take out phase change.The technology on fabrication of superhydrophobic surface is discussed.Considering the structure of finned-tube evaporator,the hierarchically structured micro-nano architecture is directly fabricated on sanded aluminum foils via a one-step solution-immersion process.Results show that the rough aluminum surface sanded by 120# sand paper exhibits super-hydrophobic surface with water contact angle as high as 155.4°,and the micro-nano architecture is similar to the lotus leaf.For the further research on phase change of vapor on a cold super-hydrophobic aluminum surface,a semiconductor refrigeration system and an air supply system are established in this paper.During the experiment,the temperature of the indoor air is 9 °C and the relative humidity is 70%.Results show that the frost formation of droplets on a cold surface generally tracks a course as the following: super-cooled droplets,ice-water mixed droplets and frozen droplets.The frozen order of different volume droplets reversed with the reduction of the cold surface temperature.At the range of-4 °C~-8 °C,the super-cooled droplets freeze earlier on the cold surface for smaller volume of droplets.At the temperature of-9 °C,the super-cooled droplets with different volume freeze at the same time.At the range of-10°C~-20 °C,the super-cooled droplets freeze earlier on the cold surface for bigger volume of droplets.The droplets on superhydrophobic aluminum surface freeze later than those on the bare aluminum surface.When the temperature of cold surface is-11 °C,droplets on the bare aluminum surface freeze at the time of 28 minutes,which is later than those on the super-hydrophobic aluminum surface.The frost suppression performance of super-hydrophobic surface is obviously better than that of bare aluminum surface.At 1 m/s humid air velocity,the phase change of water vapor in humid air is different with the change of cold surface temperature.At the range of-1°C~-3 °C,the vapor in humid air condensed into super-cooled droplets on the cold surface.At the range of-4 °C~-9 °C,the vapor in humid air freeze on the cold surface,and the type of frost crystals is beaded crystal.At the range of-10 °C~-15 °C,the vapor in humid air freeze on the cold surface,and the type of frost crystals is needle and pole-type crystal.The condensate droplets,beaded crystal and needle and pole-type crystal on the bare aluminum surface are less than those on superhydrophobic aluminum surface.The experimental results support that the super-hydrophobic aluminum surface can retard the frost formation.Compared with the bare aluminum surface,the super-hydrophobic aluminum surface has less melt-water.The melt-water on the superhydrophobic aluminum surface assembles together by itself and is blown away,so there is some area without melt-water on the super-hydrophobic aluminum surface.Compared with the bare aluminum surface,the super-hydrophobic aluminum surface is more suitable for restraining frost formation because of its preferable performance of retarding frost formation and drainage characteristics.