Pergamon Inl. zyxwvutsrqponmlkjihgfedcbaZYXWVUTS J 0017-9310(93)E0052-1 zyxwvutsrqponmlkjihgfedcbaZYXWVUTS HW1 Mfm Trans/er. Vol. 37, Suppl. I, pp. 133-147, 1994 Copyright 0 1994 ElsevierScienceLtd Printedin Great Britain. All rights reserved 0017-9310/94 S6.oO+O.CKl On turbulent drag and heat transfer reduction phenomena and laminar heat transfer enhancement in non-circular duct flow of certain non-Newtonian fluids M. KOSTIC Department of Mechanical Engineering, Northern Illinois University, DeKalb, IL 60115, U.S.A. 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! 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. 1. FASCINATING FLOW AND HEAT TRANSFER BEHAVIOR OF CERTAIN FLUIDS FEW DISCOVERIES in this century in the area of fluid flow have created such inquisitiveness as the drag- reducing effect of certain additives in common-fluid turbulent flows. Investigators have observed as much as 80% of friction drag reduction in turbulent pipe flow of rather very-dilute solutions (only a fraction of a percentile) of certain additives in water or other common (Newtonian) fluids. When these solutions have been tested in conventional viscometric (laminar) flow, non-Newtonian fluid properties have not been evident within the experimental capability. The density of these dilute solutions was virtually indistinguishable from that of the solvents, to many significant figures. On the basis of such measured phenomenological properties these fluids may have been classified as common, Newtonian fluids. The hydrodynamicists, who have regarded density and vis- cosity as the only relevent properties of the ‘common’ fluid flow, have been surprised to find that turbulent flows of such dilute soilutions (with virtually the same phenomenological properties as the solvents) could behave so differently from their Newtonian solvents. Also, no viscoelastic properties, such as the phase shift or the normal stress differences, have been exper- imentally detected for these dilute solutions. However, further concentration increase of some polymer addi- tives in solvent (e.g. 0.1% and higher) results in pseudoplastic and/or viscoelastic solutions. Ever since Toms’ discovery (1949) [I] that the fric- tion drag of some solutions under zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCB turbulent flow con- ditions is considerably smaller than the expected values, many researchers have been excited about the peculiar and often unexpected Aow and heat transfer behavior of these drag-reducing, so-called viscoelastic fluids. It is now well known that the pressure drop and heat transfer associated with the turbulent duct flow of certain fluids (see Table 1) are considerably lower than the corresponding values for Newtonian fluids. Excellent articles on the subject are presented by Dodge and Metzner [2], Metzner [3], Lumley [4], Virk et al. [5], Hoyt [6, 71, Cho and Hartnett [8], and Hartnett [9]. Hence, it is not the intention of this work to review existing literature, but to present the most peculiar behaviors and applications (see Tables 2 and 3), while interested readers are referred to the indi- cated articles, some of which [3, 681 cite extensive references on the subject. Although these ‘miraculous’ phenomena have been extensively investigated in recent decades, the underlying mechanism producing the drag and heat transfer reduction is not yet fully understood. Not surprisingly, Bird and Curtiss [lo] titled their paper ‘Fascinating Polymeric Liquids’, and even the New York Times wrote about these unusual and important phenomena in an article “‘Slippery Water' Mystery Seems FinaIly Solved” [I I]. Actually, the ‘mystery’ remains in clouds of hypotheses, far from resolution, primarily for two reasons : (1) the classical isotropic fluid mechanics approach does not work well for the very complex, flow- induced anisotropic fluid structure (even if the corresponding motionless fluid is isotropic) ; i.e. the constitutive equations are inadequate ; and (2) the turbulence itself is not yet well understood even for ‘common’ Newtonian fluids. 133