A heat transfer study of SiO2 colloid flow in the thermal entry region of mini circular and square tubes.

摘要

Flow and heat transfer behavior of silicon dioxide (SiO2) nanofluid (9.58% by volume) was studied in three sizes of hydraulic diameters and two cross-sectional shapes. Altogether six test sections were used. The outer dimensions of these test sections were 1/8", 3/32", and 1/16"; and shapes were circular and square with respective hydraulic diameters of 0.097", 0.066", and 0.035" (2.46 mm, 1.67 mm and 0.88 mm). Rheological study of the fluid showed the SiO2 nanofluid to be a dilatant non-Newtonian fluid. The friction factor of SiO2 nanofluid was compared with the friction factor of water over a range of Reynolds number. In laminar regime the friction factor for both fluids was similar, and in post laminar regime SiO2 nanofluid had lower friction factor. There was no observed effect of hydraulic diameter on the friction. For the same hydraulic diameters, flow in channels with square cross-section had less friction factor than circular at same Reynolds number. The Nusselt number at the same Reynolds number for SiO2 nanofluid was higher than that for water. Further, the difference between the Nusselt number of SiO2 and water increased with Reynolds number. Also the local heat transfer behavior of the two fluids was compared along the length and this comparison revealed that improvement in heat transfer was higher in earlier parts of the entrance region. Thus better enhancement in heat transfer can be obtained in the turbulent regime and in the earlier parts of the entrance region. Heat transfer behavior of SiO2 nanofluid and water was also compared at the same pumping power. This comparison evinced that at the same pumping power, water is a better heat transfer medium than SiO2 nanofluid. There was no observed effect of the hydraulic diameter on the heat transfer, and for the same hydraulic diameters circular channels had better heat transfer behavior.