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Thermal conductivity properties of water-based nanofluids at low concentrations are being presented in this paper. Nanoparticles at low volume concentration in the range of 0.01 to 1% is added to water improve the properties of the base fluid. The properties are estimated at temperatures between 21°C and 45°C. The results indicate an increase of enhancement of thermal conductivity with temperature and are found to be in agreement with different models as well as the data published in the literature. Empirical equations to calculate thermal conductivity and density for varied particle concentration and temperature are established. The measured and predicted values of thermal conductivity and density have a maximum deviation of 8% and 6% in the working range, respectively.

Turbulent fully developed flow heat transfer coefficient and friction factor of Al2O3 nanoparticles are dispersed in water and ethylene glycol in circular tube is discussed in this paper. In order to validate the heat transfer coefficient and friction factor of nanofluid in circular tube commercially available CFD software FLUENT 6.0 is used. To achieve the fully developed flow condition, the aspect ratio (L/D) of the test section is equal to 94. The thermo-physical properties of the Al2O3 nanofluid are estimated by using the equations available in literature. Thermo-physical properties of the nanofluid are considered for heat transfer coefficient and friction factor by assuming nanofluid is a single-phase fluid. Constant Wall Heat Flux (CWHF) boundary condition is incorporated for heat transfer analysis and adiabatic boundary condition is incorporated for friction factor analysis. The analysis is conducted in the volume concentration range from 0.1% to 4%. A maximum of 2.25 times heat transfer enhancement and 1.42 times of friction is obtained by using nanofluid as working medium.

Convective heat transfer coefficients with Al2O3 nanofluid under turbulent flow in a circular tube fitted with twisted tape insert is estimated. The thermo-physical properties such as thermal conductivity and viscosity are established experimentally in a wide range of volume concentration and validated with values obtained by others. The heat transfer coefficient obtained with Al2O3 nanofluid having volume concentration of 0.02% is 10% higher when compared with water flowing in the tube at Re = 22,000. Tapes of different twist ratios in the range of 5 < H/D < 15 are used as tube inserts to evaluate nanofluid heat transfer and pressure drop. Nanofluid heat transfer coefficient is enhanced by 23% and maximum pressure drop by 1.7 times with tape insert in the range of H/D tested when compared to flow in a plain tube.

This paper provides an overview of the important physical phenomena necessary for the determination of effective thermal conductivity of nanofluid. Increase in the specific area as well as Brownian motion are supposed to be the most significant reasons for the anomalous enhancement in thermal conductivity of nanofluid. Usual heat transfer fluids with suspended ultra fine particles of nanometre size are named as nanofluids, which have opened a new dimension in heat transfer processes. This work presents the increase of thermal conductivity with temperatures for nanofluids with water as base fluid and particles of Al2O3 or CuO as suspension material. Transient hot-wire method is used for the measurement of thermal conductivity. Heat retaining time is introduced and estimated experimentally. The heat retaining time of 0.8% volume fraction of Al22O3 nanofluid increases up to 47.05% of the base fluid and 0.8% volume fraction of CuO nanofluid increases up to 61.7% of the base fluid. The results indicate an increase of enhancement of thermal conductivity with temperature found to be agreeing excellently with a wide range of different models as well as the data published in literature.