An experimental comparison of convective heat transfer and friction factor of Al2O3 nanofluids in a tube with and without butterfly tube inserts

Nanofluids have acquired comprehensive attention due to their role in enhancing the efficiency of thermal systems. The present study reports a further enhancement in Nusselt number along with the modification of flow system structures, namely, the addition of butterfly tube inserts (BTI) with an inclined angle of 90 degrees in a tube at constant heat flux conditions. Experiments are performed to determine Nusselt number and the friction factor of Al2O3/water nanofluid up to 2.5% nanoparticle volume concentration (NVC) and in the Reynolds number range of 750-8500. Results showed that by using BTI, nanofluids friction factor increased by an average 2.6% relative to the plain tube. Also, Nusselt number increased with Reynolds and along the tube axis for laminar regime. A considerable enhancement of about 345% in the Nusselt was observed at 0.027% NVC and Re = 1500 for the flow in a tube with its BTI relative to plain tube (PT). A 2.5% increase in the NVC decreased Nusselt number for the flow in a tube with BTI. Finally, two correlations for the friction factor and Nusselt number of water and alumina nanofluid in a tube with BTI based on the measured experimental values were developed. (C) 2015 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.