Numerical Analysis of Roughened Rib Solar Air Heater with Experimental Validation

The heat transfering rate between the air (as flowing media) and the absorber plate of a solar air collector is relatively low, due to very thin viscous laminar sublayer at surface of the absorber plate. This layer can be distributed by using an artificial roughness at the absorber plate surface and make a turbulent flow adjacent this sublayer region, hence the convective heat transfer can be increased. A new artificial roughened surface has been proposed for enhancing the thermohydraulic performance of the air collector. The proposed artificial roughened surface is in the form of a square cross section rib having relatively large width (25 mm) splits with a relatively large gap (20 mm). The study focused on the influence that coming from various Reynolds number and different solar interesting fluxes. The Reynolds number based upon volumetric air flow rates at the entrance of the collector and based on the hydraulic diameter of the solar air heater was in range of (5000 – 15000). The input solar intensity fluxes comes from solar radiation of 400, 600, 800 and 1000 W/m2 were depended. In order to simulate theflow and heat transfer numerically, a three dimensional (3D) and realizable k-ɛ (RKE) model is carried out then the results were validated against experimental data. The Reynolds number effect on Nusselt number, thermohydraulic and friction factor are obtained and analyzed. The thermohydraulic factor is determined and plotted for the range of Reynolds number under investigation at different solar intensity fluxes. The results show that the thermohydraulic more than unity for the cases studies. The proposed roughness geometry and according to the flow Reynolds number of approximately 8000 at 1000 W/m2 solar intensity flux yields to maximum thermohydraulic performance. The maximum value of the thermohydraulic factor was found to be 1.26. The present proposed artificial rib is a promising method for thermohydraulic performance at relatively low Reynolds number.