Effects of the Surface Properties of Nano-TiO2 and Surfactants on the Physical Properties of Nanofluids

Author:Han Xiao Xue

Supervisor:zhang sheng han

Database:Doctor

Degree Year:2019

Download:39

Pages:131

Size:8633K

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As a new type of high-efficiency heat exchange medium,nanofluid can not only enhance heat transfer,but also improve energy efficiency.Therefore,nanofluids have been used in many fields such as photothermal solar energy,electronic cooling,aero-space,nuclear energy,heat dissipation,metallurgy industry,etc.Both surface tension and viscosity are the physicochemical properties of nanofluids’important physical parameters.Surface tension affects the mass exchange between gas and liquid molecules.Viscosity directly affects the heat transfer and flow of the liquid.The stability of nanofluids is a necessary condition for the application of nanofluids.Surface modification of nanoparticles and addition of surfactants to nanofluids are two important means to enhance fluid stability.This paper takes nanofluids formed with hydrophilic or hydrophilic-lipophilic nano-Ti02 and deionized water(DIW)by two-step as the object,aims at reducing the surface tension and viscosity of nanofluids.The effects of nanoparticles with different surface properties on the surface tension and viscosity of nanofluids are discussed,and the same discussion on the effects of surfactants work on nanofluids’ viscosity.Regardless of the hydrophilic TiO2-DIW or hydrophilic-lipophilic TiO2-DIW nanofluids,the surface tension decreases with increasing temperature.With the addition of hydrophilic nano-TiO2,the surface tension of nanofluids is higher than deionized wa-ter,and increases with the increase of TiO2 mass fraction.With the addition of hydro-philic-lipophilic nano-TiO2,the surface tension showed the opposite result.This conclu-sion can be explained by Gibbs adsorption isotherm calculation of different surface properties of TiO2 adsorption excess.Hydrophilic nanoparticles are more likely to dis-perse inside the solution,allowing free surface water molecules to be pulled inwards more strongly,thereby increasing the surface tension.Hydrophilic-lipophilic nanoparti-cles accumulate on the free surface,reducing the attraction between the water molecules in the fluid and in the free surface area,thereby reducing the surface tension.The viscosity of hydrophilic and hydrophilic-lipophilic TiO2-DIW nanofluids de-creases with the increasing temperature,and increases with the increasing mass fraction of TiO2,both of which are higher than the viscosity of deionized water.However,at the same concentration,the viscosity of the hydrophilic TiO2-DIW nanofluid is greater than that of the hydrophilic-hydrophilic TiO2-DIW.The effects of nanoparticle hydrophobicity on the viscosity of nanofluids can be explained by the Stokes-Einstein diffusion equation.In polar solvents,the effective radius of the hydrophilic nanoparticles is larger than that of the hydrophobic nanoparticles,and the diffusion coefficient is small,thereby increas-ing the viscosity of nanofluid.Surfactants not only improve the stability of nanofluids,but also affect their viscos-ity.The study found that the effect of sodium dodecyl benzene sulfonate(SDBS)on the viscosity of deionized water is consistent with the effect of SDBS on the viscosity of TiO2-deionized water nanofluids.Both showed a trend of decreasing and then increasing with the increase of surfacetants’ concentration.For deionized water,surfactants can weaken the hydrogen bonding between water molecules and increase the interaction be-tween molecules or ions in the fluid.For nanofluids,surfactants not only reduce the ag-gregation effect,but also reduce the effective diameter of the aggregate,while the effec-tive volume fraction of the aggregate increases with the increase of the surfactant.Therefore,as the concentration of surfactant increases,the viscosity of deionized water and nanofluid decreases first and then increases,and the viscosity of hydrophilic TiO2-water nanofluid is higher than that of hydrophilic lipophilic TiO2-water nanofluid.It is found that for a suitable fractal dimension(Df),the trend of model fitting data is con-sistent with the experimental data,but the optimal fractal dimension of the hydrophilic and hydrophilic lipophilic TiO2-DIW nanofluids is different,is 2.4 and 2.7,respectively.The experimental data are fitted with the modified Smoluchowski viscosity model,and the effective volume fraction of the nanoparticles is reliable.Moreover,whether it is hydrophilic nano-TiO2 or hydrophilic-lipophilic nano-TiO2,after considering the elec-tro-viscous effect and using the fractal theory to treat the agglomeration effect of the na-noparticles,the relationship between the viscosity of the nanofluid and the effective volume fraction of the nanoparticles is in accordance with the Einstein equation.There-fore,it can be speculated that the fractal agglomeration of nanoparticles in nanofluids is responsible for the complex relationship between nanofluid viscosity and nanoparticle volume fraction.In addition,it is found that zero-charge potential of nanofluids decreased with the increasing amount of surfactant SDBS.They meet a certain exponential law.When the nanofluid is at a zero charge potential,its viscosity is low.