Laminar mixed convection of large-Prandtl-number in-tube nanofluid flow, Part II: Correlations

This is the second part of two related papers on mixed convection heat transfer of large-Prandtl-number nanofluids flowing in a horizontal tube. In the preceding experiments, it was observed that nanoparticles can either deteriorate the contribution of natural convection to the overall heat transfer under the same heat flux or enhance it under a given Grashof number. In this part a quantitative analysis of this effect is made to deepen insight of the ambiguous phenomena. First of all, by taking into account the effective thermophysical properties of nanofluids and using transition criterion for the onset of significant natural convection effects, the seemingly irreconcilable contradiction between experimental results has fallen to the power of dynamic viscosity. Furthermore, as the experimental data obtained could not be reconciled with existing correlations, 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, regardless of the diameter or the shape of the nanoparticles. Such results verify that nanofluids can be treated as a homogeneous mixture with effective thermophysical properties. Ultimately, 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.