Study On Hydrophobic Frosting Performance Of Micro-nano Porous Aluminum

Frost phenomenon widely exists in refrigeration and cryogenic engineering,aerospace and other fields and the existence of the frost layer to bring many adverse effects.Lightly it can cause indoor power,air conditioning refrigeration,heating failure,and seriously it can cause collapsed transmission lines and the plane crash accident.As a result,improving the anti-frosting performance of the object surface is an effective way to ensure a safe and efficient operation.Hydrophobic surfaces can be prepared from the following two aspects: one is a method by certain physical,chemical or biological surface of the material having a micro-nano pore structure;on the other hand the material surface is coated with a low surface energy coating to change the surface properties of the material.The test samples in this paper,AAO(Anodic Aluminum Oxide,also known as porous anodized aluminum oxide)template with different pore size were prepared by the second-order oxidation method.A kind of ordinary polished aluminum plate(hereinafter referred to as sample 1)and the diameter of 30 nm,90nm,Nm,300 nm,400 nm(hereinafter referred to as sample 2,sample 3,sample 4,sample 5,sample 6)were used as test samples,and their contact angles were measured by surface contact angle tester.The research group developed the frost test device to observe the frost layer morphological of the sample surface.The test device consisted of semiconductor cooling plate,temperature and humidity environment control chamber,temperature and humidity test system,data acquisition system and digital microscope.The test specimen was placed in a constant temperature and humidity control chamber(Te = 22 ?,relative humidity ? = 60%),and no air flow in the chamber.The samples used in the first test were sample 1,sample 2 and sample 6,and the cold surface temperature of the cooling plate was set at-10 ?.Through the real-time observation of the surface frosting process and the morphological changes of the frost layer during the frosting process of differe nt samples were taken,got different surface condensation,frost crystal appear and frost crystal cover etc representative time of the time node.From the test results,it can be seen that the frost layer on the surface of the sample 1 is spherical ? needle? flake evolution;the evolution of the frost layer shape on the surface of the sample 2 is elliptical ? needle;the evolution of the frost layer shape on the surface of the sample 6 is bead-like? turbidity? irregular.The results showed that the evolution of the frost layer shape on the different sample surface is quite different,which is mainly due to the difference of microporous structure on the sample surface.From delaying the frost time,the sample 1 surface condensation,frost crystal appear and frost crystal cover time were 2min,3min,15 min,sample 2 surface condensation,frost crystal appear and frost crystal cover time than sample 1 was delayed for about 1 min,4 min and 3 min,while the delayed time of sample 3 respectively was 13 min,7 min and 15 min.This experiment mainly studied the air flow mode,temperature and humidity of the air,these three factors affect the anti-frosting performance of hydrophobic surface.This part experiment was completed in a box which has programmable constant temperature and humidity,with electronic analytical balance to measure the frost quantity of the samples.Experimental results showed that sample 5 has the best anti-frsoting effect under different working conditions,but the hydrophobic performance is not the best,which indicates that the surface structure of the sample has different influence mechanism on anti-frosting and hydrophobicity.The results showed that when the ambient temperature is in the range of-5 ? ~-25 ? and the relative humidity is ? = 60%,the average frost reduction rate of each sample surface relative to the surface of sample 1 respectively was 36.98%,33.83%,26.54 %,22.19% and 16.93%.When the ambient temperature is in the range of-5 ? ~-25 ? and the relative humidity is ? = 75%,the average frost reduction rate of each sample surface relative to the surface of sample 1 respectively was 37.24%,28.24 %,22.67%,21.04% and 14.53%.When the ambient temperature is in the range of-5 ? ~-25 ? and the relative humidity is ? = 90%,the average frost reduction rate of each sample surface relative to the surface of sample 1 respectively was 15.73 %,15.06%,9.57%,0 and 0.The results show that the amount of frost on each sample surface increases with the increase of relative humidity when the ambient temperature is the same.In the environment relative humidity of ? = 60%,wind speed of 0.25m/s,0.5m/s,0.75m/s and 1.0m/s conditions relative to the air flow(no wind speed)conditions of frost average reduction rates in the amount of frost at different temperatures respectively were 11.77%,23.24%,30.48% and 39.07%.The results showed that the amount of frost on the surface of each sample decreases with the increase of the ambient wind speed,which is mainly caused by the convective evaporation and heat transfer caused by the ambient air flow damaged the frost on the sample surface,thereby reducing the amount of surface frost..In this paper,according to the micro-nano porous aluminum plate to reduce the frosting time and anti-frosting performace,respectively from the droplet nucleation density and Gibbs free energy to analysis the hydrophobic surface of the anti-frosting principle,and useing surface water vapor condensation and adsorption theoretical to analysis the influence of surface structure on frost morphology.The micro-nano surface structure was analyzed by the fractal theory of porous media,and the smaller the fractal dimension of surface structure the anti-frosting performance is better.