2017 Volume 34 Issue 3 Pages 167-179
Transient two-dimensional numerical computations are carried out to examine 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 are estimated by either the experimental correlation equations reported by Khanafer and Vafai or the conventional prediction equations for solid-liquid dispersed system. The numerical conditions are Re = 1700 or 2300, Pr = 5.85, and the volume fraction of nanoparticles is in the range of 0.00 to 0.04. The ratios of average Nusselt number of Al2O3-water nanofluids computed by applying the experimental correlation equations and the conventional prediction equations to that of water denoted less than unity. On the contrary, the ratios of average heat transfer coefficient of Al2O3-water nanofluids to that of water denoted more than unity. Specifically, the average heat transfer coefficient ratio of Al2O3-water nanofluids with 50 nm diameter particles computed by applying the experimental correlation equations took a maximum value (= 1.041) when the volume fraction of nanoparticles was 0.03. That of Al2O3-water nanofluids with 100 nm diameter particles took a maximum value (= 1.016) when the volume fraction of nanoparticles was 0.01. That of Al2O3-water nanofluids computed by applying the conventional prediction equations took a maximum value (= 1.071) when the volume fraction of nanoparticles was 0.04. The enlargement of the average heat transfer coefficient of nanofluids is due to the enlargement of effective thermal conductivity of nanofluids.
