Study On The Condensation Movement And Frosting Characteristics Of Cold Metal Surfaces

Air source heat pump systems have become one of the widely used air conditioning equipments in China due to the advantages of high energy efficiency and environmental friendly.However,there will be the phenomenon of condensation and serious frosting when they operate in areas with lower temperature and higher humidity.At present,the research of condensation water and frosting on fin surfaces of heat exchangers still have some shortcomings.In this paper,the condensation of single surface and double surfaces were studied.The movement of droplets on the vertical surfaces were analyzed and frosting and defrosting of double surfaces were also studied.The detailed contents are shown as follows:A single surface condensation experiments were carried out.Bare copper,bare aluminum surface,hydrophobic copper,hydrophobic aluminum surface,superhydrophobic copper,superhydrophobic aluminum surface and mixed wettability striped copper surface were prepared in different ways.The results showed that the liquid film on hydrophilic copper surface was closely distributed and the droplets on superhydrophobic surface copper were small and sparsely distributed.The laser machined area of the mixed wettability fringe surface can inhibit the retention and formation of large-size liquid film and droplet effectively.A vertical double surface condensation experiment system was established.Six different wettability combinations were selected to carry out the double surface condensation experiment.The results indicated that the large-size liquid film formed on the hydrophilic copper surface was easy to contact the liquid droplets on the other side of the surface and formed a liquid bridge.The formation of liquid bridge on superhydrophobic surface were inhibited because of the small size and low coverage of droplets condensed on the surface to a certain extent.Superhydrophobic and superhydrophobic combination have the best effect,the liquid bridge will not be formed when the cold surface spacing is 1.5mm and 2.0mm,and the liquid bridge will not be formed until 108 min when the cold surface spacing is 1.0mm.The simulation of droplet movement on the vertical walls were carried out.The user define function of dynamic contact angle was programmed and loaded to the wall boundary condition to simulate the droplet movement.The results showed that the coverage rate of droplets on the hydrophobic surface was smaller than hydrophilic surface and the movement speed was faster.The sliding time of droplet can be shortened by decreasing contact angle hysteresis,which worked for both hydrophilic and hydrophobic surfaces.Although increasing droplet mass can cause the increase of surface coverage rate,it can shorten the sliding time of droplets and reduce the retention time of droplets on the wall.Based on the condensation experiment,double surface frosting and defrosting experiment were carried out.The effects of cold surface temperature,surface wettability and spacing of cold surface on frost growth and defrosting water retention were investigated.We found that the lower the cold surface temperature is,the larger the cold surface spacing is,and the faster the frost growth rate is.Compared with the combination of hydrophobic and hydrophilic,the combination of hydrophobic and superhydrophobic have the best effect of delaying frosting among the all combinations.And the frosting time is delayed by 18 min when spacing of surface is 2mm.The defrosting time of the combination depended on the surface that defrosted more slowly.The decrease of the distance between cold surfaces increaseed the possibility of forming liquid bridges,while the superhydrophobic surface can avoid the formation of liquid bridges to a certain extent due to the small size of retained defrosting droplets.