Nanofluids are colloidal suspensions of nano-scale particles in water, or other base fluids. In this paper, the effect of natural convection on laminar flow of nanofluids in a horizontal tube has been addressed. The obtained experimental data could not be reconciled with existing correlations over a wide range of Prandtl number under laminar mixed convection. Three improved correlations have been derived by using single-phase fluid approach. These correlations fit our data to within ± 10 % and also agree with the data in literature quite well. Such results verify that nanofluids can be treated as a homogeneous mixture with effective thermophysical properties. Utimately, the new correlations have grasped the essence of natural convection and can reduce to both normal forced convection and pure natural convection equations at limiting cases.

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