Laminar two dimensional forced convective heat transfer of Al_2O_3 -water nanofluid in a horizontal microchannel has been studied numerically, considering axial conduction, viscous dissipation and variable properties effects. The existing criteria in the literature for considering viscous dissipation in energy equation are compared for different cases and the most proper one is applied for the rest of the paper. The results showed that nanoparticles enhance heat transfer characteristics of the channel and inversely, viscous dissipation causes the Nusselt number and friction factor to decrease. The viscous dissipation effect may be emphasized by increasing Reynolds number and decreased by raising the exerted heat flux. Also, it was found that there is a critical Reynolds number below which the average Nusselt number of the nanofluid changes abnormally with Reynolds number as a result of variable properties effect.
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