Investigation On Behaviors Of Droplets On Hydrophobic Surfaces During Early Stagefrosting And Defrosting Process

Frost formation is a well-known and undesirable phenomenon incryogenics,refrigeration and air conditioningindustries, frost formation on heat exchangermakesboth thermalresistance and air pressure drop increase, which reduces therefrigeration system energy efficiency. In recent years, hydrophobic surfaces haveattracted special attention because they can retard frostformation without additionalenergy consumption,but frost formation can’t be eliminated completely, the drainagecharacteristics during defrosting directly affect hydrophobic surface comprehensiveperformance of retarding frost. Droplets behaviors on hydrophobic surfaceand barealuminumsurfaceduring early stage of frosting and defrosting process wereinvestigated.The main work of this paper is as follows:The growth process of condensed droplets during early stage of frostingprocesswere experimentalyinvestigated.The experimental conditions were73~145ostatic contact angle,11~1.4oC cold surface temperature,21~26oC humid airdry-bulbtemperature,60~78%relative humidity,0.5~5m/s humid air velocity. Results showedthat droplets are in regular sphere shape with a low growth rate on hydrophobic surfaces,the formation time of droplets is longer than that on bare aluminum surface. Diameter ofdroplets increases with the time but contact angle decreases with some fluctuation onhydrophobic surfaces. The growth rate of droplets increases with decreasing coldsurface temperature. The frost accumulationmassof145ohydrophobic surface increaseswith increasing humid air velocity, but when velocity is larger than5m/s, the frostaccumulationmassdecreasesbecause condensed droplets are blown away.Condensed droplets heat transfer and growth model considering the influence ofnon-condensable gas was built in natural convection conditions, and influences ofsurface characteristics and cold surface temperature on the droplet growth wereexplained by heat transfer model.The results showed that the diameter generated by thegrowth model is about1.5times of the biggest droplet diameter in experiments, heattransfer model explains the reason why droplets grow with a low growth rate onhydrophobic surfacesis that droplet’s interface and internal conduction thermalresistance are larger than that on bare aluminum surface.The lower the cold surface temperature,the higher the growth rate of condensed droplets.The influences of surface characteristics on the deforsting and drainage processwere experimentaly researched. The experimental conditions were73~145ostaticcontact angle,10~8oC cold surface temperature，20~21oC humid airdry-bulbtemperature,45~78%relative humidity. Results showed thatas compared with the barealuminum surface,145ohydrophobic surface has less residual meltwaterand dropletsmovement is very fast during drainage process,114ohydrophobic surface has moremeltwater after defrosting.145ohydrophobic surface is suitablefor in restraining frostformation because ofits perfect performance of retarding frost formation and drainagecharacteristics.The influence of surface characteristics on placed droplets was experimentalyinvestigated.Results showed thatas compared with bare aluminum surface,145ohydrophobic surface has the lowest roll angle while114ohydrophobic surface has thelargest roll angle with the same droplet. The high contact angle hysteresis leads to a lowdrainage rate of114ohydrophobic surface during defrosting process.Static and dynamic contact angle modelscombined with VOF model were used tosimulate droplet shape and analyse droplets movement. Results show thatstatic andmoving droplet contact angle difference between simulation and experiments are6oand5o, respectively. The reason that droplets movement is very fast during drainage processis that droplets roll from145ohydrophobic surface.