Experimental study of heat transfer and friction factor of Al2O3 nanofluid in U-tube heat exchanger with helical tape inserts

Turbulent forced convection heat transfer and friction of Al2O3-water nanofluid flowing through a concentric tube U-bend heat exchanger with and without helical tape inserts in the inner tube were studied experimentally. The experiments were conducted in the Reynolds number range from 3000 to 30,000, volume concentrations of 0.01%, 0.03% and helical tape inserts of p/d = 5, 10, 15 and 20. The results indicate that an increase in Reynolds number and Prandtl number yields to an increase in the average Nusselt number, and augmentation of thermal conductivity in the nanofluid contributes to heat transfer enhancement. The Nusselt number of entire pipes for 0.03% concentrations of nanofluid with helical tape inserts of p/d = 5 shows an enhancement of 32.91%, as compared to water. The friction factor for the entire inner tube for 0.03% concentration of nanofluid with helical tape inserts of p/d = 5 has increased by 1.38-times, as compared to water; in general and consistent with theory, the pressure drop in the inner tube increases with an increase in nanoparticle volume concentration and aspect ratio of the inserts. The empirical correlations for the Nusselt number and friction factor are obtained as functions of the Reynolds number, Prandtl number, volume concentration and aspect ratio. (C) 2014 Elsevier Inc. All rights reserved.