On turbulent drag and heat transfer reduction phenomena and laminar heat transfer enhancement in non-circular duct flow of certain non-Newtonian fluids

Abstract

The fascinating friction drag and heat transfer reduction phenomena associated with turbulent flows of so-called ‘drag-reducing fluids’ are not well understood. It is believed that elastic fluid properties are strongly related to these phenomena. However, not all drag-reducing fluids are viscoelastic, nor are all viscoelastic fluids drag-reducing, suggesting that drag reduction and viscoelasticity are probably incidentally accompanying phenomena. Furthermore, the limited research to date has revealed considerable heat transfer enhancement (virtually without friction drag increase) in laminar non-circular duct flows with certain polymer solutions, and has shown that all utilized fluids were indeed viscoelastic 1.It is argued here that turbulence suppression (i.e. flow laminarization), due to flow-induced anisotropic fluid structure and properties, is a determining factor for the reduction phenomena—not the fluid elasticity—while the latter may be a major cause for the laminar heat transfer augmentation. It is certain that many challenges in this interesting and useful area will keep researchers very busy well into the next century and beyond.