This study is concerned with the numerical investigation of the developing laminar mixed convection of a nanofluid which consists of water- γAl2O3 in a horizontal semicircular duct with the flat wall at a vertical position. The governing momentum and energy equations are solved numerically using a marching technique with the finite control volume approach following the SIMPLER algorithm. The properties of the nanofluid have been simulated by using a well known models and correlations from the literature. Results are obtained for the thermal boundary condition of uniform heat input axially with uniform wall temperature circumferentially, at any cross section (H1 boundary condition), different values of particles volume concentration and for two values of Grashof number 104 and 106. These results include the velocity and temperature distributions at different axial locations, axial distributions of local Nusselt number, and local average wall friction factor. It was observed that increasing the nanoparticles concentrations at low Gr, has a negligible effect on the developing of Nusselt number and friction factor. However, at high Gr it was found that increasing the particle volume concentration increases the Nusselt number in the developing and fully developed regions and reduces the friction factor at the developing and fully developed regions, if it is compared to the results obtained from the results obtained from the pure water at the same flow conditions. As an example for the case of Grashof number equals to 106 and particles volume concentration equals to 0.1, the enhancement of the Nusselt number and the reduction of wall friction factor at the fully developed region, are 17.5% and 6.4% respectively, if it is compared to that of the base fluid (water), at the same flow conditions.

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