This paper describes an experimental investigation into combined forced and natural convection heat transfer for large-Prandtl-number nanofluids flow in a horizontal tube at low Reynolds number (9 < Re < 450). By the inclusion of nanoparticles, the contribution of natural convection to the overall convective heat transfer can be either deteriorated under the same heat flux or enhanced under a given Grashof number. The huge increasing of the viscosity and Prandtl number were turned out to be the major reason for the observed deterioration and enhancement, respectively. Moreover, the measured heat transfer behavior of nanofluids was illustrated to be in good agreement with the single-phase-based evaluation. However, the experimental data obtained could not be totally reconciled with existing correlations, which relate mainly to specific pure liquids or relatively higher Reynolds number. Therefore, new correlations have been derived by using single-phase fluid approach. These correlations fit our data to within ± 10 percent 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. In addition, the new correlations grasp the essence of natural convection and can reduce to both normal forced convection and pure natural convection equations at limiting cases. Whether a flow can be treated as pure forced flow or not (i.e., natural convection effects cannot be neglected) is a crucial problem remains to be determined for the assessment of performance of nanofluids in low-Reynolds-number convection heat transfer application. Generally, the boundary curve function involves the variable parameter of forced main flow (Graetz number) and natural secondary flow (Rayleigh number), constituting a criterion suitable for defining transition of forced flow to mixed flow.

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