Experiment On Heat Transfer Characteristics And Viscosity Regulation Mechanism Of Glycerol-based Nanofluids

Nanofluids have received much attention for their ability to improve the heat transfer efficiency of heat transfer fluids,in which the base fluid of nanofluids plays an important role as the dispersion environment of nanoparticles,which affects the key thermal properties of nanofluids,including thermal conductivity（k）,viscosity,specific heat capacity（c_p）,freezing point and boiling point,etc.Conventional nanofluid base fluids such as water,kerosene and ethanol are unable to meet the operational requirements of heat transfer masses in extreme climatic conditions due to their short liquid ranges.As a byproduct of biodiesel manufacturing process,glycerol is a green,thermally stable,wide liquid range and good thermal conductivity,but its unique trihydroxy structure makes its intermolecular hydrogen bonding strong,which brings a large kinematic viscosity and limits its application as a heat transfer fluid.In order to reduce the viscosity of propylenol heat transfer agent while strengthening its k,this thesis intends to conduct a study on the heat transfer properties and viscosity regulation mechanism of glycerol-based nanofluids,which is divided into four parts as follows:（1）From the perspective of hydrogen bonding bond energy reorganization,three nanofluids filled with Ti O₂,Al₂O₃ and Fe₂O₃ nanoparticles were prepared with sodium acetate trihydrate as the hydrogen bonding acceptor and glycerol as the hydrogen bonding donor,and the mechanism of thermal conductivity change of the nanofluids was analyzed at the atomic and microscopic levels.The experimental results showed that the viscosity of the prepared propanetriol/sodium acetate trihydrate 2:1 deep eutectic solvent（DES）system decreased by 58.03%and k increased by 17.2%compared to propanetriol at 25℃.The effect of nanoparticles on nanofluid k and c_pwas also investigated,and the addition of nanoparticles Fe₂O₃ increased k compared to DES.Notably,the c_p of the nanofluid was increased by 121%after the addition of 0.8%Fe₂O₃ nanoparticles by mass fraction compared to propanetriol.（2）In order to investigate the convective heat transfer characteristics of Fe₂O₃nanofluid with different mass fractions and the effect of the flow rate of nanofluid on the heat transfer characteristics of the working mass,tests were conducted by building a nanofluid convective heat transfer test bench.The experimental results show that the mass fraction of nanoparticles has a large influence on the convective heat transfer coefficient of nanofluid,and the convective heat transfer coefficient is increased after the mass fraction is increased from 0.2%to 0.4%,with a maximum increase of 131.2%compared with DES.In addition,at the same concentration,the convective heat transfer coefficient increased from 27.3%to 96.3%for the nanofluid with a mass fraction of0.2%when the workpiece volume flow rate was increased from 3 ml/s to 12 ml/s.At the lower flow rate,the nanoparticles tend to settle,resulting in the increase of heat transfer thermal resistance and the smaller improvement of convective heat transfer effect,and the settling phenomenon improves with the gradual increase of the flow rate,and the heat transfer boundary layer between the nanofluid and the inner wall of the copper tube becomes thinner with the increase of the flow rate,which reduces the heat transfer thermal resistance and improves the heat transfer effect.（3）In order to comprehensively study the effects of mass fraction of nanoparticles,light intensity and liquid surface height on the photothermal conversion characteristics of nanofluid,Fe₂O₃ nanofluid was used as the research object and tested by building a nanofluid photothermal conversion experimental bench.The experimental results show that:the mass fraction of nanoparticles has a large influence on the photothermal conversion efficiency of the workpiece,and the highest photothermal conversion efficiency reaches 27.18%when filled with 0.4%nanoparticles,which is 74.8%higher relative to DES,which is related to its superior c_p and k;however,the photothermal conversion efficiency does not show the expected increase when the light intensity is increased,but the highest photothermal conversion efficiency is reached at 1500 W/m₂when the thermal conversion efficiency reached its highest value,which was analyzed to be related to the light transmission of the nanofluid.Collectively,the Fe₂O₃ nanofluid with a nanoparticle mass fraction of 0.4%has the most outstanding photothermal conversion characteristics.（4）To further reduce the viscosity of propanetriol by disrupting the hydrogen bonding interactions between intermolecular hydroxyl groups at the molecular level.We selected three nanoparticles and six aldehydes/ketones to successfully prepare low-viscosity and high-thermal-conductivity propylene glycerol-based binary hybrid nanofluids.The nanoparticles acted as nanoadditives to increase the k and c_p of the nanofluids,and also acted as catalysts for the acetal reaction to reduce the number of hydrogen bonds,leading to a decrease in the viscosity of the nanofluids.The study of thermal properties and viscosity characteristics mechanism of nanofluid showed that when Ti O₂ nanoparticles were selected,the k of the prepared nanofluid could be increased by 8.12%and the viscosity reduced by 47%compared with that of propanol.In addition,the results of Fourier transform infrared spectroscopy（FTIR）and proton nuclear magnetic resonance（¹H NMR）demonstrated the occurrence of the acetal reaction from a molecular point of view and clarified the changes in chemical bonding during the viscosity reduction of the malonatriol nanofluid.Meanwhile,X-ray diffraction（XRD）tests revealed that the nanoparticles were able to maintain their original structure before and after the reaction,maintaining the enhancement of the thermal conductivity of the nanofluid.