Turbulent heat transfer and nanofluid flow in an annular cylinder with sudden reduction

Turbulent heat and fluid flow of a nanofluid in an annulus with sudden area reduction are numerically and experimentally studied. Two different nanofluids are considered. Nanoparticles made of aluminum oxide (Al2O3) and titanium oxide (TiO2) with volume concentrations varying from 0.005 to 0.02 have been examined. Reynolds number and heat flux varied from 10,000 to 40,000 and 1000 to 6000 W m(-2), respectively. Area reduction ratios from 1 to 2 were studied in this research. For turbulence modeling, the SSTk-omega model in a 3D domain was utilized. In order to validate the numerical results, Al2O3-water-based nanofluid was tested, and a great agreement was observed. It was noted that the maximum thermal performance enhancement was about 194.7% in an annular channel with an area reduction ratio of 2 compared with a straight pipe using water. The heat transfer enhancement is attributed to the downstream flow separation and recirculation region. It was noted that rising the volume concentration of the nanoparticles improves the heat transfer coefficient; this enhancement was about 26.9% (Al2O3) and 5.5% (TiO2). Also, the influence of the Reynolds number on the surface heat transfer coefficient augmentation is quantitatively reported in this paper. The maximum pressure drop was about 7.5% for Al(2)O(3)and 5.9% for TiO(2)nanofluid, compared with pure water at a reduction ratio of 2 at the highest Re value considered here.