Natural convection heat transfer enhancement in horizontal concentric annuli using nanofluids

Heat transfer enhancement in horizontal annuli using nanofluids is investigated. Water-based nanofluid containing various volume fractions of Cu, Ag, Al2O3 and TiO2 nanoparticles is used. The addition of the different types and different volume fractions of nanoparticles were found to have adverse effects on heat transfer characteristics. For high values of Rayleigh number and high L/D ratio, nanoparticles with high thermal conductivity cause significant enhancement of heat transfer characteristics. On the other hand, for intermediate values of Rayleigh number, nanoparticles with low thermal conductivity cause a reduction in heat transfer. For Ra=10(3) and Ra=10(5) the addition of Al2O3 nanoparticles improves heat transfer. However, for Ra=10(4), the addition of nanoparticles has a very minor effect on heat transfer characteristics. (C) 2008 Elsevier Ltd. All rights reserved.