Heat transfer and pressure drop correlations of nanofluids: A state of art review

Nanofluids, a new class of thermo-fluids engineered by the stable suspension of nano-sized metallic and nonmetallic entities (particles, fibers, tubes, droplets) in base fluids with optimized thermal conductivity, demonstrate the advantages of efficient thermal management with miniaturization. However, thermal conductivity intensification is not the only mechanism responsible for the enhanced thermal efficiency of the nanofluids, other factors including gravity, inter-phase frictional force, sedimentation, dispersion, ballistic phonon advection, non-uniform shear rate, nanoparticle migration induced by viscosity gradient and layering at the solid-liquid interface also play a significant role. The hydrothermal characteristics of nanofluids are determined by the net influence of the relative modifications in the thermophysical properties of the nanofluids which are sensitive towards multiple parameters including particle morphology (size and shape), material and concentration, base fluid properties and pH value, fluid temperature and additives. Consequently, conventional correlations remain unsuccessful in explaining idiosyncrasies of nanofluids and a few studies contributed to the formulation of heat transfer and friction factor correlations for multifarious combinations of nanofluids and operating conditions. Although a group of researchers validated the applicability of the classical friction factor models for nanofluids, nevertheless, a few contradictory studies emphasized that penalty in pressure drop effectuated by nanoparticles is sufficiently large to be neglected. The primary objective of the present manuscript is to review the research progress in the development of heat transfer and pressure drop correlations for nanofluids under miscellaneous geometrical, operating and boundary conditions. Furthermore, a comprehensive comparison of the few heat transfer correlations proposed under identical construction and flow conditions has been also presented.