Heat transfer enhancement technology has the aim to develop more efficient systems as demanded in many applications, like heat exchangers for refrigeration, automotives, process industry, solar heater etc.. Convective heat transfer may be enhanced passively by adopting different solutions. A possibility for increasing the heat transfer is to employ rough surfaces. When a fluid flows in a channel, ribs break the laminar sub-layer and create local turbulence, due to flow separation and reattachment between consecutive ribs, which reduce thermal resistance and augment heat transfer. This behaviour overcomes the effect linked to the increased heat transfer area due to the ribs. However, higher friction losses are expected and turbulence must be created only in the region very close to the heat transferring surface and the core flow should not be unduly disturbed. In this paper a numerical investigation is carried out on air forced convection in a rectangular channel with constant heat flux applied on the bottom and upper external walls. Properties of fluid are considered temperature-dependent and flow regime is turbulent. The investigation is accomplished by means of the commercial code Fluent. A three-dimensional model is developed in order to study the effect of the angle between the fluid flow direction and the ribbed surfaces. In fact, secondary turbulence is promoted in the orthogonal direction to the channel longitudinal axis. Three different inclination angles of the ribbed surfaces have been considered and the channel is provided with rectangular ribs. Simulations have shown that Nusselt numbers as well as the pressure drops increase as the inclination angles increase.

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