Study On Heat Transfer And Tribological Behavior Of TiO₂ Nanofluids

Nanofluids,which were the suspensions that prepared by adding several metal or non-metal oxide to liquid medium,had drawn widespread attention from most scholars and scientists since the concept was proposed.It was reported that nanofluids possess excellent tribological and heat transfer properties,thus it was always considered a typical nano-lubricant to apply to mechanical even to metal-processing lubrication.However,most researches on nanofluids were superficial,several mechanisms related to nanofluids also kept obscure.Thus,it is necessary to make a further study on tribological and heat transfer performances and mechanisms,so as to promote people to understand nanofluids most conveniently,and make guidance for the application of nanofluids.For this purpose,this dissertation selected TiO2 nanoparticles as the research subject,it mainly focused on the preparation of TiO2 nanofluids and their physicochemical performance,tribological and heat transfer properties and so on.The primary work included four aspects as follows:Firstly,characterization on TiO2 nanoparticles by X-Ray Diffraction(XRD),Scanning Electron Microscope(SEM)and Differential Thermal Analysis(DTA)was done to ascertain the form of TiO2 nanoparticles existed.The dispersing effect of TiO2 in base fluid was also studied by sedimentation and Zeta potential methods.Stable TiO2 nanofluids were synthesized via physical dispersing combine chemical dispersing.The dispersing mechanism of TiO2 nanoparticles in aqueous medium was explored.Secondly,the wettability of nanofluids on metal surface was studied via contact angle tester,further confirmed wetting behavior of nanofluids,whereas when dispersant was added,the wetting state would transfer from Wenzel to Cassic.From several viscosity tests,it was also found that the increased viscosity of nanofluids was mainly attributed to the electroviscous effect of nanofluids.The deviation of experimental and theoretical data came mainly from primary electroviscous effect of nanofluids.This paper introduced Gouy-Chapman theory as well.New viscosity formula was proposed via theoretical computation and was proven to be well matched with experimental data.Thirdly,interface layer effect and Brownian movement were considered,two heat conductivity models that based on interface layer effect and Brownian movement were put forward,respectively.Effects of temperature,nanoparticle size,interface layer's heat conductivity and thickness on heat conductivity of nanofluids were examined.Contributions of interface layer and Brownian movement on the heat conductivity of nanofluids were evaluated according to new model built in this paper.Actual heat conductivity of nanofluids was measured via transient hot-wire method.Several hot rolling experiments revealed that nanofluids were good for improving the structure and graining the crystalloid of rolled strip.Finally,four-ball friction tests,universal friction and wear tests were conducted,effect of dispersant and nanoparticles on the lubricated properties of nanofluids was established.It was revealed that when sodium polyacrylate was used as the dispersant,superfluous sodium polyacrylate affected the Liquidity of nanofluids because of high viscosity.Nanoparticles can't easily enter into the wear surface due to higher viscosity of nanofluids,which eventually led to inferior lubricating effect.When sodium hexametaphosphate or oleic acid was used as dispersant,influence of dispersant could be negligible.Synergetic lubrication model was built according to follow explanation.TiO2 nanoparticles tended to deposit on the asperity and form a low shearing film,which made actual contact area comprise with nanofluids,low shearing film and asperities.Several asperities were replaced by nanofluids and low shearing film and thus reduced the friction.