Study On Synthesis Of Carbon Nanotube Arrays On Stainless Steel Surface And Dropwise Condensation Heat Transfer Performance

Steam condensation is widely used as an efficient form of heat transfer in waste heat recovery,power plants,desalination,aerospace and other industrial production.The enhancement of condensation heat transfer has become one of the research hotspots in related fields.Condensation heat transfer can be divided into droplet condensation and film condensation.Among them,drop condensing is the best way to use the surface energy of the liquid to enhance condensation heat transfer,and its heat transfer coefficient is 5 to 7 times that of film condensation.In industry,most of the current heat exchanger materials are metal,and the surface energy of common metal materials is large.Condensed steam is easily spread on the metal heat exchange surface to form a film condensation with large heat resistance.Vapor condensation on the surface of low surface energy materials tends to disperse into small droplets to form droplet condensation.Therefore,it is important to choose low surface energy materials to modify the metal surface to achieve droplet condensation.The development of superhydrophobic modification of metal surfaces using oriented carbon nanotube array films has provided new ideas for the study of droplet condensation.The method can effectively improve surface hydrophobic properties on the basis of almost no increase in surface heat transfer resistance,and achieve the purpose of enhancing condensation heat transfer.In this paper,as an entry point,the surface of stainless steel is hydrophobically modified by means of a floated catalytic chemical vapor deposition method using a carbon nanotube array film.The effect of preparation process parameters on the micro-morphology and contact angle of carbon nanotube arrays was investigated,and the condensation heat transfer effect of the modified metal surface was analyzed.The main research content is as follows:(1)Through a self-built chemical vapor deposition experimental bench,the effects of three different atmospheres of argon,hydrogen-argon,nitrogen,and two carbon sources of cyclohexane and xylene,on the growth of carbon nanotube arrays on the surface of 304 stainless steel were studied.influences.In addition,the common metal materials such as copper,brass,stainless steel 304,and iron were used as substrates to study the regularity and hydrophobic properties of carbon nanotube arrays grown on the surface of different substrates.It was found that when the substrate material is stainless steel 304,the carrier gas is pure nitrogen,and the carbon source is cyclohexane,it is an ideal condition for growing a carbon nanotube array film on the metal surface,and the surface has excellent hydrophobicity.(2)The six parameters(reaction temperature,growth time,catalyst concentration,feed rate of the solution,carrier gas flow,and substrate etch time)were analyzed by orthogonal test method to determine the height and orientation of the carbon nanotube array on the stainless steel surface.And the impact of the surface contact angle and other indicators.The most important parameters affecting the height,orientation and surface contact angle of the carbon nanotube array are obtained and the influence laws are obtained.The reaction temperature has the greatest effect on the height of the CNT arrays,and the substrate etch time has the least influence on the CNTs;the feed rate of the solution and the reaction temperature have the greatest influence on the directionality of the CNT array,and the catalyst concentration and substrate etch time have the least influence on the directionality;the solution feed rate has the greatest effect on the contact angle of the CNT array,and the substrate etch time has the smallest effect on the contact angle of the CNT array.(3)The effects of various conditions(reaction temperature,growth time,catalyst concentration,feed rate of the solution,carrier gas flow rate,and substrate etch time)on the growth of carbon nanotube arrays on the stainless steel surface were analyzed by the control variable method.The best conditions for the preparation of CNT arrays are explored.The results showed that the optimum conditions for preparing CNT arrays were as follows: growth time 1.5h,reaction temperature 780°C,solution injection rate 0.08ml·min-1,catalyst concentration 25mg·ml-1,carrier gas flow rate 100ml·min-1,base etching time 5min.(4)The wetting behavior of condensation droplets on the surface of carbon nanotube arrays was studied by a high-speed CCD camera.In the presence of a large amount of non-condensable gas,the condensed droplets on the surface of the carbon nanotube array will spontaneously bounce when combined.The heat transfer coefficient before and after the carbon nanotube array film was coated on the stainless steel surface was compared.Under the condition of low undercooling,the carbon nanotube array film had the effect of strengthening the condensation heat transfer.Through the self-built condensing heat exchange test bench,several different heat transfer surfaces were tested for heat transfer performance.The order of the enhanced condensation heat transfer effect was as follows: carbon nanotubes sparsely combined surface>fluorinated carbon nanotube array superhydrophobic surface>hydrophobic hydrophobic stainless steel hydrophobic surface>original stainless steel hydrophilic surface> carbon nanotube array modified superhydrophobic stainless steel surface.