2018 Volume 35 Issue 2 Pages 121-
Transient two-dimensional numerical computations are carried out to clarify the laminar flow forced convective heat transfer characteristics of Al2O3-water nanofluids in a horizontal tube. A tube with the length of 50 times of a diameter is insulated from the inlet to the central part and is cooled isothermally from the central part to the outlet. The high-temperature Al2O3-water nanofluids flowing into a tube is assumed to be Hagen-Poiseuille flow. The thermophysical properties of Al2O3-water nanofluids, such as the effective density, the effective specific heat capacity, the effective viscosity, and the effective thermal conductivity, are estimated by the experimental correlation equations reported by Khanafer and Vafai. The present numerical computations are implemented under different particle diameters (dp = 25, 50, 100 nm), different volume fractions of nanoparticles (φp = 0.01 - 0.04), and different reference temperatures (θ´= 20, 30, 40 ℃). When the numerical conditions were dp = 25 nm,φp = 0.04, and θ´= 40 ℃, the ratio of the average Nusselt number of Al2O3-water nanofluids to that of water took a minimum value (= 0.929), the ratio of the average heat transfer coefficient of Al2O3-water nanofluids to that of water took a maximum value (= 1.136), and furthermore, the ratio of the pressure loss of Al2O3-water nanofluids to that of water took a maximum value (= 1.802). In addition, when the computed local Nusselt numbers of Al2O3-water nanofluids were estimated with the Pe number defined by the thermophysical properties of water, they were distributed below the Graetz solutions. On the other hand, when the computed local Nusselt numbers of Al2O3-water nanofluids were estimated with the Pe number defined by the thermophysical properties of Al2O3-water nanofluids, they almost agreed with the Graetz solutions.
