Experimental Study Of Frosting On Cold Surface Under Low Relative Humidity

Frosting phenomenon exists widely in industrial production and life.Frost of heat exchange equipment can produce a lot of harm,for example,reducing the heat transfer performance,blocking gas channels,increasing the resistance and energy consumption,etc.The occurrence of frost is often inevitable,so it is vital to understand the frost formation mechanism,explore the growth rule of frost layer and find an effective way to restrain the frost growth in industrial production and life.Scholars from domestic and overseas mainly research for wall temperature above-30? and relative humidity ranges from 40% to 80%.But in many practical industrial productions,the research for direct sublimation frosting phenomenon and mechanism under the conditions which below 40% of the relative humidity and the cold temperature below 30? are really scarce.The evolution of frost crystals on the surface of hydrophobic surface and its effect of suppress frost is not fully revealed.Micro visual observation method was used for low relative humidity conditions.The frosting on bare aluminum and hydrophobic surfaces has been studied comparatively.The bare aluminum which the contact Angle is 78°,hydrophobic which is 141°and hydrophilic water which is 26°,the initial frost crystals on these three different forms of surfaces are analyzed.To make up the insufficient research of the condition caused frost below-30?,another study content in this article focuses on exploring frost properties under low relative humidity.Different wind speed conditions and surface temperature were applied.The results showed that hydrophobic surface still has a good frost inhibition effect under the condition of low relative humidity,it can effectively delay the formation and growth of frost which formed is looser.Compared to the bare aluminum surface,hydrophobic surface delayed frosting for 30 min under the surface temperature of-10?.When the temperature is further reduced to-18?,hydrophobic surface frost layer thickness was only about 68% of the bare aluminum surface frost thickness after the experiment was carried for an hour.Under the same experimental condition,the subcooled water on the hydrophobic surface lasts longer.It was found that when the surface temperature is-26?,initial frost crystal shapes on the three different surface contact are various.Because contact angle has significant differences,hydrophobic surface tends to form denser dendritic frost crystals which grow along the horizontal direction and then transform to flake.Hydrophilic surface tends to form sparse needle frost crystals.This article also studied on surface temperature below-30?.The frost phenomenon was displayed in the experimental study,it was found that even in the case of low relative humidity,the initial frost crystals are approximate to the feather morphology,not needle or rod shape.The growth mode of the initial frost crystals mainly kept the initial shape of the frost crystals which grew up in one dimension,while almost no new frost crystals were formed on the cold surface.During the growth process of the initial frost crystals,their spacing does not decrease significantly.It was found that with the decrease of cold surface temperature,the frost layer thickness increased significantly,but the influence of cold surface temperature on the frosting amount was small.The frost thickness did not increase significantly with the increase of air velocity instead of the frost density.Even when the wind speed increased from 0.25m/s to 0.5m/s,the frost thickness did not increase significantly.This means that the critical air velocity for frost thickness growth is very small under this direct desublimation condition.