Heat transfer enhancement technologies are adopted in several applications as heat exchangers for refrigeration, automotives, process industry, solar heaters. A possibility for increasing the convective heat transfer in a fluid is to employ rough surfaces or adopting additives. When a fluid flows in a channel, ribs break the laminar sub-layer and create local wall turbulence due to flow separation and reattachment between consecutive ribs, which reduce thermal resistance and greatly augment the heat transfer. This behaviour overcomes the effect linked to the increased heat transfer area due to the ribs. However, higher friction losses are expected. In this paper a numerical investigation is carried out on forced convection with nanofluids (water-Al2O3) in a ribbed channel with a constant heat flux applied on the external walls. Properties of fluid are considered constant and a single phase model is employed. Flow regime is turbulent; in fact, Reynolds numbers ranging from 20000 to 60000 are considered. Furthermore, different shapes, such as square, rectangular, triangular ones, and different dimensionless heights and pitches of elements are analyzed. Moreover, two volume particle concentrations are investigated. Results are presented in terms of temperature and velocity fields, average heat transfer coefficients and pressure drop profiles. The aim of this study is to find arrangement of ribs such to give high heat transfer coefficients and low pressure drops in presence of nanofluids.

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