| In many China’s southern cities where is no central heating,the air is wet and cold and the apparent temperature is very low.Air source heat pump(ASHP)is a good heating equipment for energy saving and environmental protection.However 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 COP of ASHP.It can even leads to the system to stop running.Therefore,some measures should be taken to defrost the outdoor unit of ASHP to ensure the normal operation of the unit.At present,most of the defrosting methods for ASHP will affect indoor comfort or consume extra energy at home and abroad.Based on superhydrophobic theory and technology,our research group modified the surface of evaporator fins,and put forward a fundamental method of frost suppression,which not only did not affect the operation of ASHP system,but also delayed or inhibited the formation of frost layer.In this paper,the frost suppression performance was studied by theoretical mechanism analysis,experiments and simulation.Firstly,through theoretical analysis,it is concluded that when the surface contact angle is bigger,the heat transfer resistance between water droplets and cold surface is bigger,and frost layer is more difficult to grow.Then,the superhydrophobic aluminum surface with contact angle as high as 154.6° was prepared by using the existing superhydrophobic aluminum surface preparation technology of hydrofluoric acid and hydrochloric acid etching and fluorosilane surface modification,and the relationship between fluorosilane modification time and contact angle was studied.The results show that the optimal modification time is 40 min.the average contact angle of the prepared superhydrophobic aluminum surface is highest.The labyrinth micro-nano structure on the surface was observed by scanning electron microscopy,which was the key factor of hydrophobicity of superhydrophobic aluminum.In order to test the anti-frost performance of superhydrophobic aluminum surface,an experimental system was set up to study the frost and thawing process of vertical conventional aluminum surface and superhydrophobic aluminum surface.The temperature range of aluminum surface was 0℃ ~-15℃,the wind speed was 1 m/s ~ 3 m/s,the temperature of laboratory air is 10℃ and the relative humidity was 70%.The results show that when the temperature is 0℃ to-2 ℃,compared with conventional aluminum,the time of condensation water droplets on superhydrophobic aluminum surface is delayed by 150 s~ 200 s.When the temperature is-3℃ to-4℃,water droplets appear first,and then thecondensation water droplets frozen into ice beads.The ice beads on the surface of superhydrophobic aluminum are small and less while those on the surface of conventional aluminum are large and many.When the temperature is lower than-5℃,needle-like frost layer appears directly on the surface of this two kinds of aluminum,and the thickness of the frost layer on the surface of superhydrophobic aluminum is smaller,which indicates that the superhydrophobic aluminum has obvious anti-frost effect.The change of wind speed has a great influence on the formation of frost layer on the surface of conventional al-based alloy:the formation of frost layer will be accelerated when the wind speed is increased.But it has little effect on the formation of frost layer on superhydrophobic aluminum surface.In the defrosting experiment,it was found that the frost layer on the surface of the conventional aluminum surface appeared melting signs firstly,but the tiny water droplets formed by the melting of the super-hydrophobic aluminum-based surface frost layer are easy to evaporated,which can effectively avoid the re-freezing of the melted water.In order to prevent the fouling thermal resistance of outdoor evaporator fins,this paper set up a self-cleaning test rig of fins,and made an experimental test on the self-cleaning ability of superhydrophobic aluminum-based surfaces..It is found that when drops of water fall on the surface of superhydrophobic aluminum,they can easily tumble down and take away the dust on the surface effectively.In this paper,FLUENT is used to simulate the heat transfer process of water droplets on aluminum-based surface.The results show that when the contact angle of aluminum surface increases,the average temperature of water droplets increases during the whole heat transfer process,and the time for water droplets to reach freezing point temperature is prolonged.The simulation results demonstrate the frost suppression of superhydrophobic aluminum surface from the heat transfer point of view.Through the research in this paper,it is found that the application of superhydrophobic technology on the fins of outdoor heat exchanger of ASHP has great significance to solve the frosting problem of evaporator,which can greatly improve the performance of ASHP system and extend its application range. |
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