HYDRODYNAMIC AND HEAT TRANSFER STUDY OF DISPERSED FLUIDS WITH SUBMICRON METALLIC OXIDE PARTICLES

Abstract

Turbulent friction and heat transfer behaviors of dispersed fluids (i.e., uttrafine metallic oxide particles suspended in water) in a circular pipe were investigated experimentally. Viscosity measurements were also conducted using a Brookfield rotating viscometer. Two different metallic oxide particles, γ-alumina (Al₂O₃) and titanium dioxide (TiO₂), with mean diameters of 13 and 27 nm, respectively, were used as suspended particles. The Reynolds and Prandtl numbers varied in the ranges l0⁴-I0⁵ and 6.5-12.3, respectively. The viscosities of the dispersed fluids with γ-Al₂O₃ and TiO₂ particles at a 10% volume concentration were approximately 200 and 3 times greater than that of water, respectively. These viscosity results were significantly larger than the predictions from the classical theory of suspension rheology. Darcy friction factors for the dispersed fluids of the volume concentration ranging from 1% to 3% coincided well with Kays' correlation for turbulent flow of a single-phase fluid. The Nusselt number of the dispersed fluids for fully developed turbulent flow increased with increasing volume concentration as well as the Reynolds number. However, it was found that the convective heat transfer coefficient of the dispersed fluid at a volume concentration of 3% was 12% smaller than that of pure water when compared under the condition of constant average velocity. Therefore, better selection of particles having higher thermal conductivity and larger size is recommended in order to utilize dispersed fluids as a working medium to enhance heat transfer performance. A new correlation for the turbulent connective heat transfer for dilute dispersed fluids with submicron metallic oxide particles is given by the following equation: Nu = 0.021 Re^0.8Pr^0.5.