| The cities in the hot-summer and cold-winter zones of China generally show the characteristics of low temperature and high humidity in winter,and the indoor human body often has a strong feeling of wet and cold.There is still a need for heating,even if the cities in this zones do not implement central heating.Air source heat pump has been widely used in air-conditioning and heating because of its characteristics of dual functuion for heating and cooling,energy saving and environmental protection.However,when the air source heat pump is operated in a low-temperature and high-humidity environment,frosting cannot be avoided forming on surface of the outdoor heat exchanger,and the accumulation of frost will seriously affect the performance of the unit.And the defrosting process not only consumes extra energy but also has influence on the indoor comfort.From the perspective of frost restraint,this paper studies on the method and effect of anti-frosting of hydrophobic surface modification applied in finned-tube of heat exchanger.Firstly,the frosting area and process of heat and mass transfer on heat exchanger was analyzed,and the frosting simulation model of heat exchanger was established.Through the analysis of anti-frost mechanism,the influence of surface characteristics-contact angle on frost formation was considered in the model.The simulation results showed the variation of frost layer’s thickness and mass of bare aluminum fins and hydrophobic fins with running time under different working conditions.And the thickness and mass of the hydrophobic are lower than those of the bare aluminum.In order to verify the rationality and accuracy of this model,the results were compared with experimental test results.The trend between them was consistent and the errors was less than 20%,which indicated that the model could predict the frosting process and reflect the influence of surface characteristics on frost formation to some degree.Then,in order to explore the practical effect of hydrophobic surface modification applied in heat exchanger for anti-frosting,three split heat pump units were selected as test objects.And two of the outdoor heat exchangers’ finned-tube were hydrophobically modified with graphene and nano-inorganic coating.A visualized heat exchanger frosting performance test platform was established based on the enthalpy different laboratory.It simulated a variety of typical outdoor winter conditions in low temperature and high humidity areas.The process and performance of frosting,defrosting and drainage on different modified finned-tube were tested and analyzed under different working conditions.Finally,the experimental results were concluded.(1)The finned-tube modified by nano-hydrophobic coating had a good effect on anti-frosting at ambient temperature of2~5°C.The performance was as follows: in the early stage of frosting,the droplets froze slowly,and the formation of frost layer was delayed.In the later stage,the frost layer was thin and loose,and the frosting area was small.Compared with the hydrophilic bare aluminum fins,the frost layer was delayed by 535 seconds at 5 °C,and the mass of frost in45 minutes was a half.However,the effect of frost restraint on hydrophobic fins would be weakened with frosting running time increasing and ambient temperature decreasing.(2)Compared with the bare aluminum heat exchanger,the modification of the hydrophobic coating enhanced the heat transfer between heat exchanger and ambient air,and reduced the temperature of exhaust gas and the temperature required for defrosting.Besides,it could extend the heating time by up to 32% and shorten the defrosting time by 20% in a defrosting cycle,which improved the heating performance in a way.(3)The hydrophilic bare aluminum fins had an excellent drainage performance after melting,whereas droplets were retained on hydrophobic finned-tubes.But there was no phenomenon of "water bridge" or "secondary frosting".(4)The graphene coating made by graphene slurry in this experiment failed to perform hydrophobic characteristics.The above results indicate that the method of modifying by hydrophobic coating on heat exchangers for anti-frosting has certain feasibility and practical effects,and provides some reference for air source heat pump in application of anti-frosting. |
