Conventional sources of energy have been depleting at an alarming rate, which makes future sustainable development of energy use very difficult. Thus, heat transfer enhancement technology plays an important role and it has been widely applied to many applications as in refrigeration, automotive, process industry and solar energy heater. Convective heat transfer can be enhanced passively by changing flow geometry, boundary conditions or by increasing thermal conductivity of the fluid. Another possibility for increasing heat transfer with gas is to employ extended surfaces. Transversal ribs in a channel work as fins, increasing the heat transfer area and break the laminar sub-layer creating local wall turbulence. The heat transfer rate is improved but pressure drops increased as well. In this paper a numerical investigation is carried out on forced convection in channels heated by a constant heat flux. Also conductive effects are taken into account. The fluid is air and properties are assumed as function of temperature and flow is turbulent. Ribs of the same material of the channel walls are introduced and several arrangements and shapes are analyzed. The investigation is accomplished by means of the Fluent code. Results are presented in terms of temperature and velocity fields, average heat transfer coefficients and friction factor profiles. The aim of this study is to find arrangement of ribs such to give high heat transfer coefficients and low pressure drops. The maximum Nusselt number and friction factor have been detected for dimensionless pitches equal, respectively, to 12 and 10 for square and rectangular ribs, 10 and 8, for triangular ones.

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