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The Reynolds number in nanofluids studies depends on the inlet velocity and the kinematic viscosity of nanofluid. The nanofluid kinematic viscosity increases with an increasing in nanoparticles volume fraction while the inlet velocity should be increased to keep the Reynolds number constant. Therefore, it is not clear that either increasing the nanoparticles volume fraction or increasing the inlet velocity has major role on heat transfer enhancement in nanofluids flow studies which are done at constant Reynolds numbers. In this study, forced convection Al 2 O 3 -water nanofluid flows in two-dimensional rectangular microchannels have been investigated to study heat transfer enhancement due to addition of the nanoparticles to the base fluid especially in microchannels at low Reynolds number. Three

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Laminar mixed convection Al 2 O 3-Water nanofluid flow in elliptic ducts with constant heat flux boundary condition has been simulated employing two phase mixture model. Threedimensional Navier-Stokes, energy and volume fraction equations have been discretized using the finite volume method. The Brownian motions of nanoparticles have been considered to determine the thermal conductivity and dynamic viscosity of Al 2 O 3-Water nanofluid, which depend on temperature. Simulation effects of solid volume fraction, aspect ratio and buoyancy forced on thermal and hydraulics behaviors of nanofluid flow in elliptic ducts have been presented and discussed. The calculated results show good agreement with the previous numerical data. Results show that in a given Reynolds number (Re) and Richardson number (Ri), increasing solid nanoparticles volume fraction increases the Nusselt number (Nu) while the skin friction factor decreases. Increasing aspect ratio (AR=b/a) in elliptic tubes reduces the local skin friction factor whereas it does not have any specified effect on the local Nusselt number.