The main goal of this study is to analyze by a numerical simulation the performances of a solar heat exchanger constituted by a horizontal duct heated from below by a concentrated solar heat flux. Particularly, we focus on the effect of surface radiation heat transfer effect on the mixed convection. The code used solves the coupled Navier Stokes (non Boussinesq) and energy equation with radiation heat transfer. The effect of combined convection and radiation is highlighted and particularly the effect of inner emissivity of the duct surfaces on the heat transfer is evaluated. It is shown that there is an optimum inner emissivity, for particular conditions. Asymmetric emissivity modify the size and intensity of convection rolls but decrease slightly heat transfer. The importance of the heat loss by radiation and natural convection by the heated wall on the heat exchanger performance is also shown.
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