3D Numerical Investigation of Convective Heat Transfer and Friction in Solar Air Collector's Channel Roughened by Triangular Ribs

Solar air collectors are widely used in drying process of food and agricultural products. They are combined with dryer room to constitute the indirect solar dryer. In this type of solar dryer, the drying process efficiency depends on solar air collector's one. Heat transfer augmentation using artificial roughness have been studied by many authors both numerically and experimentally. Most of them are related to upward solar air heater ( the air flows above the absorber). In this work, we were interested in studying the effect of triangular cross-section ribs provided on the absorber of downward solar air heater ( the air flows under the absorber) on the increase of convective heat transfer coefficient in a differentially heated channel which represents solar air collector. 3D numerical study has been performed using CFD (Fluent Code) based on finite volumes method. The air flow is considered turbulent (Re=4000 to 14000), so RANS formulation were used. From the obtained results we recorded an improvement of Nusselt number (from 48 to 78) at Re = 14000. Also an augmentation in friction factor was observed (from 0.008 to 0.02) which still acceptable and do not penalize the collector thermohydraulic performance. A comparative study of three ribs with different top angles showed an optimum top angle of 22.5 degrees.basedon the obtained results, transverse triangular cross-section ribs can be used successfully to improve convective heat transfer and thermohydraulic performance of solar air collectors.