Development of new correlations for the Nusselt number and the friction factor under turbulent flow of nanofluids in flat tubes

A three-dimensional turbulent flow and heat transfer of two different nanofluids, containing aluminum oxide (Al2O3) and copper oxide (CuO) nanoparticles, dispersed in ethylene glycol and water mixture (EG/W) in the flat tubes of an automotive radiator have been numerically studied to evaluate their performance. Computations have been carried out for nanoparticles volumetric concentrations up to 6% and over a Reynolds number range typically encountered in automobile radiators. Appropriate correlations for density, viscosity, specific heat and thermal conductivity of nanofluids as a function of particle volume concentration and temperature, developed from experiments have been used in this study. Numerical results have been first validated for the flow of single phase liquids, such as water and EG/W by comparing the computed values of Nusselt number and friction factor with those given by accurate correlations available in the literature. Inside the flat tube continuous reductions in the local heat transfer coefficient and wall shear stress are observed around the periphery of the flat tube, starting from the mid-point of the flat-wall and proceeding to the center of the curved wall. For the same Reynolds number, computations with nanofluids show an increase of friction factor and heat transfer coefficient with an increase in the particle volume concentration. The study reveals that under the basis of equal pumping power, Al2O3 and CuO nanofluids up to 3% and 2% particle volumetric concentrations respectively provide higher heat transfer coefficients than that of the base fluid. From the present study, several new correlations to determine the Nusselt number and friction factor for the nanofluids flowing in the flat tubes of a radiator have been proposed for the entrance as well as the fully developed regions.