Study On The Surface Of Polymer Hollow Fiber Heat Exchange Tube Promoting The Removal Of Condensation Droplets

Tubular metal heat exchanger is a kind of heat exchanger most used in modern industrial production,which is most widely used in petroleum,chemical industry,metallurgy and other fields.However,in practice,there are many problems such as easy corrosion,easy scaling and so on.Tubular plastic heat exchangers made of polymer heat tubes have great application prospects because they can better avoid the above problems of metal heat exchangers.However,its poor heat transfer performance hinders its popularization and application.In this paper,to solve the problem of poor heat transfer performance of tubular plastic heat exchangers,hollow fiber polyvinylidene fluoride（PVDF）microfiltration membranes with excellent mechanical properties and polyester lining ware used as base material,and sol-gel method was used to prepare hollow fiber composite hydrophobic heat exchangers I with porous structure internal and dense external surface.The influences of polydimethylsiloxane（PDMS）hydroxy-terminated concentration,additive content of nano-ZnO particles,coating time and times on contact angle on outer surface and pure water flux of heat exchange tubes were studied.The surface and section morphology,mechanical properties and heat transfer properties of heat exchange tube I were characterized.The results show that when PDMS concentration was 25 wt%,coating time was 45 min and addition content of ZnO was 4.0 wt%,the surface contact Angle of heat exchange tubeⅠcould reach125±1.0°.When flow rate of cold fluid was 2.5 m·s^-1,the K value of heat exchange tube I braided increased by 18.5%to 1552±54 W·m^-2·k^-1,compared with non-braided condition.On the basis of above research,the research on removal of condensation droplets to be promoted on outer surface of polymer hollow fiber heat exchange tubes,Thus heat transfer to be enhanced was focused.Firstly,the surface of heat exchange tubeⅠwas modified by sol coating method,and the super-hydrophobic heat exchange tubeⅡwith SiO₂micro-nano structure was prepared.The influence of super-hydrophobic modification conditions（including ethyl orthosilicate concentration,ammonia concentration,coating time,reaction temperature,etc.）on surface contact angle of heat exchange tubeⅡwere studied.The surface and section morphology,mechanical properties and heat transfer properties of heat exchange tubeⅡwere characterized.The results show that when the ethyl orthosilicate concentration was 25 wt%,oammonia concentration was 25 wt%,reaction temperature was 50℃,and coating time was 15 min,the surface contact angle of super-hydrophobic heat exchange tubeⅡcould reach 153±1.2°.The super-hydrophobic surface had micro-nano structures and low surface energy,which made condensation droplets slip of easily.Secondly,an asymmetric super-hydrophobic/hydrophilic combined surface for steam condensation was constructed by braiding a single super-hydrophobic heat exchange tubeⅡwith a single hollow fiber PVDF microfiltration membrane.Depending on the synergistic mechanism between super-hydrophobic and hydrophilic surfaces,nucleation time,merger time and migration time of droplets were shortened,and the droplets were quickly removed from the membrane surface.By utilizing the membrane’s mass transfer with high efficiency,the condensation droplets generated on the surface of heat exchange tubeⅡcould be quickly removed from the heat exchanger through the membrane,thus improving the surface renewal rate of the heat exchange tubes and strengthening heat transfer performance.The results show that when the flow rate reached 3.0 m·s^-1,the K value was 5150±96 W·m^-2·k^-1,which was 48%higher than that of heat exchange tubeⅠand microfiltration membrane braided.The super-hydrophobic modification on surface of hollow fiber PVDF heat exchange tubes studied in this paper,and the method of constructing an asymmetric super-hydrophobic/hydrophilic combined surface for steam condensation by braiding heat exchange tubes and microfiltration membranes,could significantly promote the removal of condensation droplets,thus enhancing surface condensation heat transfer.The research was of great significance for promoting the industrial application of tubular plastic heat exchangers.