Preparation Of High Stable Carbon Nanotube Nanofluidsand Its Thermo-physical Properties

In the world,the energy is one of the driving forces of human activities and economic developments.The process of heat transfer exists in the energy collection,conversion,storage and transportation etc.,where the efficiency of heat transfer is directly related to the efficiency of industrial production and the cost of production.However,the thermal conductivity of traditional heat transfer fluids is too small to meet the development of contemporary energy demand.With the development of nanotechnology,nanofluid was invented,whose unique characteristics of high thermal conductivity caused widespread concern.Among them,the carbon nanotube nanofluid is a very hot spot.However,the poor stability of carbon nanotube nanofluids limits its application.Therefore,with the purpose of improving the stability of carbon nanotube nanofluids,the cyclodextrin modified carbon nanotubes,CD-CNTs,were synthesized in this paper.And then the ethylene glycol based CD-CNTs nanofluids were prepared,and its thermo-physical properties were investigated.Some research findings are as follows:(1)In order to synthesize cyclodextrin modified carbon nanotubes,the silane coupling agent,KH560,was used as "bridging agent" to build a molecular bridge between carbon nanotubes and ?-cyclodextrins.First,the water soluble cyclodextrins was obtained by crosslinking reaction between ?-cyclodextrins and epichlorohydrin.The molar mass ratio of cyclodextrins to epichlorohydrin is 1:5,the reaction temperature is 30? and the reaction time is3.5 hours.After then,the target product was obtained by mixing the carbon nanotubes,KH560 and water soluble cyclodextrins in the dimethylformamide.The results of Infrared spectrum analysis,X ray photoelectron spectroscopy analysis,simultaneous thermal analysis and electron microscopy all showed that the cyclodextrin was successfully introduced onto the surface of carbon nanotubes.(2)The carbon nanotube nanofluids,EG/CD-CNTs,from 0 to 0.1%volume fractions were prepared by using the two-steps method.The ultrasonic processing time is 6 hours;Adjusting the pH value to 11 to achieve the desired steady state.(3)Qualitative and quantitative analysis methods were used to study the stability of EG/CD-CNTs nanofluids.The results of the static sedimentation method,the spectrophotometer method and the Zeta potential method together showed that,the EG/CD-CNTs nanofluids had high stability.It was analyzed that the high dispersibility of modified carbon nanotubes and the preparation method lead to the highly stable nanofluids.The stability of nanofluids decreases with increasing volume fraction of nanoparticles,increases with the ultrasonic processing time and increases with the increasing pH values.(4)The thermal conductivity of EG/CD-CNTs nanofluids increases with the increase of the volume fraction of nanoparticles.When the volume fraction is 0.1%,the thermal conductivity of nanofluids was 0.358 W/(m·K),which increased by 41.50%compared with that of the base fluid.The thermal conductivity of nanofluids is obviously affected by the ambient temperature and increases with the increase of temperature.The effective volume theory,molecular-level layering of the liquid at the liquid/particleinterface and the nature of heat transport in the nanoparticles were applied to qualitativelyanalyze the mechanism of heat transfer enhancement of nanofluids.In addition,the Brown motion of nanoparticles played an indirect role in enhancing the thermal conductivity.(5)Compared with the base glycol,the EG/CD-CNTs nanofluid has a higher specific heat capacity,and the specific heat capacity of nanofluids increases with the increase of volume fraction and temperature.When the temperature was 20?,the maximum specific heat capacity of nanofluids was increased by about 8%,which is very helpful to the application of heat transfer and heat storage.(6)Under the conditions of this work,EG/CD-CNTs nanofluids showed an interestingrheological behavior that it could be concluded into two groups.Thedilute nanofluids(?v(?)0.05%)with well dispersed nanoparticles behaved Newtonian behavior and there was no discernible shear-thinning behavior;The concentrated nanofluids(?v>0.05%)exhibited non-Newtonian properties with a clear shear-thinning behavior at the low shear rates(<200 s-1),but behaved Newtonian behavior at higher shear rates(>200 s-1).In addition,the viscosity of nanofluids increased with the increase of the volume fraction of nanoparticles,and decreased with the increase of temperature.The related suspension particle clusters theory explained the reasons for this appearance.(7)The overall work efficiency coefficients of EG/CD-CNTs nanofluids were scattered with volume fraction.However,the work efficiency coefficient of each volume fraction was less than 1,which revealed that the nanofluid has excellent comprehensive working efficiency.In addition,the emperature has a positive effect on the overall working efficiency of nanofluids.(8)The results of optical properties and photo-thermal conversion efficiency of EG/CD-CNTs nanofluids showed that the absorption properties of nanofluids increased with the increase of the volume fraction of nanoparticles,the solar absorption rate was up to 96.53%and the photo-thermal conversion efficiency was up to 41.5%.The above results showed that the EG/CD-CNTs nanofluid has great application value and application prospect in solar heat collection system.