Experimental Study On Convective Heat Transfer Characteristics Of Carbon Nanotubes Nanofluids

With the development of nanotechnology,nanofluids is becoming the hot topic of scholars at home and abroad for the excellent thermal physical properties and spectral absorption properties.Nanofluid refers to a stable uniform suspension and can be obtained through dispersing suitable amount of nanoparticles into liquid in a certain way.Due to the addition of nanoparticles,the nanofluids are significantly different from the traditional fluids in the properties of thermal conductivity,heat transfer capacity,kinematic viscosity and so on.Nanofluids are a new and efficient type of heat transfer medium.In the thesis,the carbon nanotube nanofluids were used as the object,and their flow-related viscosity,thermal conductivity,and other characteristic parameters were mainly studied,and their heat transfer performance was studied through insolation experiment.In this thesis,the carbon nanotube nanofluids were prepared by the two-step method and the suspension stability of nanofluids was improved by ultrasonic oscillation and dispersant addition.Suspension stability of nanofluids was characterized by using sedimentation method.The effects of different dispersants on the stability of nanoparticles were investigated and dispersants suitable for the preparation of carbon nanotube nanofluids were obtained.The viscosity of nanofluids with different volume fraction of carbon nanotubes was measured by NDJ-5S rotation viscometer.The influence of the volume fraction of nanoparticles,the particle size and the amount of dispersant on the viscosity was investigated.Combined with the classic viscosity calculation model,the experimental results were analyzed.The results showed that the viscosity of nanofluids increased with the increase of the volume fraction of nanoparticles and the amount of dispersant.At the same time,the smaller the particle size was,the larger the viscosity was.The thermal conductivity of carbon nanotubes nanofluids with different volume fractions and different types and amounts of dispersant was measured by Hot Disk thermal analyzer.The effects of the volume fraction of nanoparticles,the type and the amount of dispersant,the suspension stability of nanofluid and other factors on the thermal conductivity were investigated.The results showed that the thermal conductivity of carbon nanotubes nanofluids increased with the increase of the volume fraction and the suspension stability of nanoparticles,and decreased with the addition of dispersant.The photothermal conversion properties of carbon nanotube nanofluids and its base liquid were studied by the insolation experiment.The effects of the volume fraction of nanoparticles,the type of base fluid and other factors on the heat collection of nanofluids were investigated.The results showed that the heating rate of carbon nanotube nanofluids was faster than that of the base fluid.With the increase of the volume fraction of nanofluids,the heating rate of nanofluids was increasing and the temperature was correspondingly increased.At the same volume fraction,the MWCNTs-H₂O nanofluids had better heat collection efficiency than MWCNTs-EG nanofluids.The highest efficiency of MWCNTs-H₂O nanofluids was 55.59%,which was 12.98%higher than MWCNTs-EG nanofluids.Because of the excellent photothermal properties of the carbon nanotube nanofluids,it can be expected that using the carbon nanotube nanofluid as recycle fluids of the solar collector will significantly improve the heat collecting efficiency of the collector and realize the efficient use of solar energy.Therefore,nanofluids have broad application prospects in the field of solar thermal collector.