Laminar mixed convection of large-Prandtl-number in-tube nanofluid flow, Part I: Experimental study

This paper describes an experimental investigation into combined forced and natural convection heat transfer of large-Prandtl-number nanofluids flowing in a horizontal tube at low Reynolds number (9 < Re < 450). SiO2 nanoparticles with diameters of 15 and 50 nm were dispersed in Ethylene Glycol (EG) or EG-Water mixture with volume concentrations of 0.2–2% to investigate the effects of size and loading of the nanoparticles on the convective heat transfer under laminar mixed flow regime. The effect of base fluid difference was involved by using two pure fluids of extremely dissimilar rheological behavior. The results show that the measured average Nusselt number is higher than that predicted by the pure forced convection heat transfer analysis due to the existence of natural convection, and increases with increasing the Reynolds number and Grashof number. By the inclusion of the 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; in fact, such seemingly paradoxical phenomena boil down to the different comparison criteria, i.e., heat flux and/or Grashof number. The huge increasing of the viscosity and Prandtl number were proposed to be the major reason for the observed deterioration and enhancement, respectively.