Fabrication And Anti-frosting Property Of Low Adhesive Coatings On Thin Tubes

The quality of air pre-cooler,the core component of aerospace vehicle combined engine,has direct effects on the success or failure of the whole engine scheme.When hypersonic flight happens,the air needs to be cooled deeply(~1250K to ~120K)in a very short time(~0.01s),but the pre-cooler frosts immediately as soon as the water vapor,which is carried by air,contacts with the cryogenic pre-cooler thin tube.And this will cause the increase of mainstream pressure loss and the decrease of heat exchange capacity,the coefficient of thermal conductivity will be smaller after the formation of frost layer.In order to ensure the performance of the combined engine under the inspiration mode,the frosting phenomenon has to be controlled effectively.Overall,the frosting problem has become the essential problem at the field of deep precooling technology.To solve the frosting problem of air pre-cooler,the work was carried out from the following aspects:Superhydrophobic coating,containing primer bonding layer and topcoat functional layer,was fabricated with two-step method.The primer bonding layer was a mixture of fluorocarbon resin,polyurethane resin and particles.And the topcoat functional layer was a superhydrophobic coating with silica.Sol-gel dip-coating method was used to coat it on the thin tubes and the coating process of topcoat functional layer was investigated.After curing for 1 hour,the topcoating decreased by 3000 ?m/s and after 1 min dipping,it was pulled by 100 ?m/s,which cycle was 1 time.The water contact angle,sliding angle and ice adhesion strength were 159.6°,3.6° and 12.5 kPa,respectively.The coating exhibited excellent superhydrophobicity because of micro-nano composite porous structure.And it showed good freezing delay effect and perfect wear resistance,which was proved by the low ice adhesion strength,within 20 kPa,after 20 laps of wear.Also,its high temperature stability,low temperature stability and icing-melting stability were all good.In addition,during the condensation process,superhydrophobic surface had the least droplet coverage,and for droplets,it was easy to bounce off the surface and difficult to form a water film.Hydrophobic low-adhesion coating,including 41082 fluorocarbon resin,tin dibutyllaurate,propylene glycol methyl ether acetate,titanium dioxide and other additives,was prepared by one-step method and the optimum formulation was obtained by controlling variable method.Similarly,sol-gel dip-coating method was used to coat it on thin tubes and the process was: decreasing speed 3000 ?m/s,dipping time 30 s,pulling speed 3000 ?m/s and 1cycle.The water contact angle,sliding angle and ice adhesion strength were 105.1°,47.7° and 31.4 kPa,respectively.The coating exhibited low ice adhesion strength because of its regular structure and abundant fluorine.What's more,it had strong adhesion,high hardness and good oil resistance.However,its wear resistance was general and ice adhesion strength would increase sharply because of the destruction of external force.Meanwhile,the water condensation results were not much different from bare surface.The frosting characteristics of bare surface,superhydrophobic low-adhesion surface and hydrophobic low-adhesion coating surface were compared.It was found that under-20 ? static environment,superhydrophobic surface had the best frosting inhibition effect and the fastest defrosting speed,followed by hydrophobic surface and bare surface.The frosting time of them are 7 min,5 min and 3 min respectively.So superhydrophobic coating was chosen to dip-coated on precooler.Also,different environment parameters were studied.It was shown that humidity had little effect on frosting,but the speed of wind and methanol had serious influence on frosting.Finally,the frosting of the pre-cooler coated with and without superhydrophobic coating at-50 ? and-150 ? in static,70 m/s high-speed airflow shearing and 10 m/s methanol spraying environments were discussed.It was found that the superhydrophobic coating did not have obvious effect on inhibiting frosting at ultra-low temperature,but high-speed airflow shearing and methanol spraying could effectively inhibit the frosting of the pre-cooler module.This research is of practical significance for promoting the development of the deep precooling technology,widening the working range of aspirated combined engines and developing the reusable aerocraft engines.