| At present,the development of enhancing heat transfer technology is mainly focused on the design and manufacture of compact heat exchangers with high efficiency and low resistance.However,because of the low heat conductivity coefficient of traditional liquid heat transfer medium(water,alcohol and oil),heat exchange system could hardly meet the requirements of high burden heat-transfer intensity.The self-performance of heat transfer medium gradually becomes the main restriction factor of enhancing heat transfer,while the nanofluids with high heat-conducting property provide the possibility of solving the technology bottleneck.Graphene has extremely high heat-conducting coefficient,and propylene glycol gradually replaces ethylene glycol as the green heat transfer medium with the odourless,non-toxic,biodegradable properties.Therefore,starting from high efficiency and environmental protection,a kind of high heat-conducting and green nanofluids,the propylene glycol based graphene nanofluids,is prepared,and then its thermal physical properties are studied in this paper.The concrete research contexts are summarized as follows:(1)The graphene oxide is synthesized by graphite in the condition of strong acidic condition using potassium permanganate as oxidizing agent,it is deduced by hydrazine hydrate in absolute ethyl alcohol to synthesize graphene,and then propylene glycol based graphene nanofluids and graphene oxide nanofluids are prepared via two-step method.The structures of objective products are characterized by Raman spectrum,X-ray diffractometer,scanning electron microscope and transmission electron microscope.Finally,the dispersion stability of propylene glycol based graphene nanofluids is studied via the precipitation method,Zeta potentiometric analysis,and dispersion stability analysis method.The results indicate that propylene glycol based graphene nanofluids had better dispersion stability than propylene glycol based graphene oxide nanofluids.(2)The heat-conducting coefficients of propylene glycol based graphene nanofluids are measured by KD2-Pro thermal analyzer.The experimental results show that its heat-conducting coefficients increase with the increasing of nanoparticle volume fraction.When the temperature increases,it decrease with low volume fraction,while increase with relatively high volume fraction.The heat-conducting coefficient of propylene glycol based graphene nanofluids with 1.0%volume fraction can reaches 0.276 W/(m·K)in the temperature of 40℃,which is increased by 28.37%comparing with base fluid.(3)The light-heat properties of propylene glycol based graphene nanofluids,including absorbance,transmittance,extinction coefficient and solar absorption fraction,are measured and calculated by ultraviolet-visible spectrophotometer and Matlab software.The results indicate that propylene glycol based graphene nanofluids can absorb light in the whole wavelength range,and its solar absorption capacity increases with an increase of volume fraction.The solar absorption fraction of propylene glycol based graphene nanofluids with 0.02%volume fraction can reach 90%with the incident optical path of 0.34cm.(4)The viscosities of propylene glycol based graphene nanofluids are determined in different conditions,and results reveal that its viscosities increase with an increase of volume fraction and decrease with the temperature rising.Propylene glycol based graphene nanofluids exhibit shear dilution performance with low shear rates while the characteristics of Newtonian fluid with higher shear rates.(5)The heat transfer process of propylene glycol based graphene nanofluids in the condition of natural convertion is simulated via Fluent Microsoft.It is found that graphene could improve Nusselt coefficient of base fluid and the propylene glycol based graphene nanofluids exhibit a good performance of heat exchange.Based on the research above,propylene glycol based graphene nanofluids could be considered as the working medium in the systems of heat-exchange equipment and solar heat collection. |
PDF Full Text Request