Nanofluid is a new class of heat transfer fluids engineered by dispersing metallic or non-metallic nanoparticles with a typical size of less than 100 nm in the conventional heat transfer fluids. This article aims to investigate the overall and convection heat transfer coefficient and Nusselt number of Al2o3-water nanofluid flowing in a horizontal double pipe heat exchanger under turbulent flow (14000 £Re£ 26500) conditions. Al2o3 nanoparticles with diameter of 20 nm dispersed in Deionized water with volume concentrations of 0.1% £ j £ 0.3% vol. are used as the test fluid. The results show that the overall and convection heat transfer coefficient and Nusselt number of nanofluid were approximately 15% -21% greater than that of pure fluid. Additionally, the heat transfer coefficient and Nusselt number increase with an increase in flow rate, Reynolds number, nanoparticle concentration and nanofluid temperature. Finally, the new correlations were proposed for predicting the Nusselt number of the nanofluids, especially. Employing particles of nanometer dimension suspended in solution as nanofluid shows considerable increase in the nanofluid thermal conductivity and heat transfer coefficient which result in increasing heat transfer and decreasing operational cost.

In this investigation the silicate nano structure of MCM-41 has been used for the production of nanofluid. The particles have negligible heat conductivity and therefore by their dispersion in a base fluid like water, it is possible to study the increase of heat conductivity due to the Brownian motion effects. In this work a suitable apparatus for the measurement of heat conductivity has been built and calibrated and then using the apparatus and preparing suitable nanofluids, the conductivity and the volumetric heat capacities have been measured. Experimental results show that the preparation time of the nanofluid using the ultrasonic method has pronounced effect on the increase of the conductivity. A twenty four hours preparation time for the nanofluid containing 2.5% (vol) of the particles results in a 7% increase in the conductivity of the base fluid while showing no significant increase in the volumetric heat capacity. Investigating the effect of increasing the temperature and volumetric percentage of the particles it can be deduced that Brownian nano particles movement is one of the main factors in increasing the thermal nanofluids conductivity.

The influence of rheological behavior on the natural convection in a dielectric nanofluid with vertical AC electric field is investigated. The rheology of the nanofluid is described by Maxwell model for calculating the shear stresses from the velocity gradients. The employed model introduces the combined effects of movement of the molecules of the fluid striking the nanoparticles, thermophoresis and electrophoresis due to embedded nanoparticles. The exact solutions of the eigen model value problem for stress-free bounding surfaces are obtained analytically using one term Galerkin method to find the thermal Rayleigh number for onset of both non-oscillatory (stationary) and oscillatory motions. It is found that the oscillatory modes are possible for both bottom and top-heavy distributions of nanoparticles. It is observed that the value of critical Rayleigh number is decreased by a substantial amount with the increase in electric field intensity, whereas role of viscoelasticity (time relaxation parameter) is to hasten the occurence of oscillatory modes appreciably. The thermal Prandtl number is found to delay the occurence of oscillatory modes. These results are also shown graphically.

In the present paper laminar flow forced convective heat transfer of CuO/water nanofluid in a triangular duct under constant wall temperature condition is investigated numerically. Sometimes, because of pressure drop limitations the need for noncircular ducts arises in many heat transfer applications. We used nanofluid instead of pure fluid because of its potential to increase heat transfer of system. In this paper, the effect of parameters such as nanoparticles diameter, nanoparticles concentration, type of nanoparticles and heat transfer comparison between nanofluid and pure fluid is studied. Comparison of convective heat transfer of nanofluid in isosceles triangular ducts with various apex angles is also presented. In this study, for the presence of nanoparticles, the dispersion model and for solving differential equations, the finite difference method is used. Numerical results indicate an enhancement of heat transfer of fluid with changing to the suspension of nanometer-sized particles in the triangular duct. Results also defined that equilateral triangular duct has a maximum heat transfer in comparison with other types of isosceles triangular duct.