Effect of Nusselt Number Dependence on Reynolds Number in Solar Air Heaters of Varying Constructal Rib Geometries

The most serious challenge in solar air heater is that the thermal performance is generally poor due to low heat transfer coefficient between the absorber plate and air flowing in the channel. Thermal performance in solar air heater is a critical issue because of usual low heat transfer coefficient. Hence, artificial roughness is being employed to enhance the rate of heat transfer in solar air heater. This study involved simulation of radiant air transport with geometric roughness such as square, rectangle and semi-circle rib. The operating parameter considered are; relative roughness height (e/D) values of 0.0283, 0.0566, 0.08500; relative roughness pitch (P/e) values ranged from 3.3 to 10 and the Reynolds number ranged from 100 to 2000 for standard laminar flow in a solar air heater. The absorber plate is heated with a uniform heat flux of 1000 W/m2 for all the constructal ribs used in this study. The effect of geometric roughness investigated is based on both velocity and energy fields obtained via conservation principles of Navier-Stokes equations solved numerically on a finite element based platform. Thereafter, the results obtained for Nusselt number (Nu) dependence on Reynolds number (Re) for various constructal rib geometries at selected roughness pitch investigated are here presented. For P/e at 3.33, Re ranged at 100 to 2000, Nu was observed to be 5.82 to 11.88; 11.32 to 17.98; 5.94 to 20.81; 0.85 to 9.33 for square, rectangle, semicircular and smooth ribs respectively. At P/e of 5.0 and investigated Re values, Nu was reported as 5.37 to 12.59; 9.72 to 16.20, 5.49 to 19.05, 0.85 to 9.33 for the respective constructal geometries. The Nu values obtained for P/e at 10.0 for the respective Re and geometric shapes 4.94 to 17.21, 14.71 to 33.61, 5.09 to 19.57, 0.86 to 9.33. The present study observed that Nusselt number considerably increased with roughness pitch for square and rectangular shaped rib geometries while it decreases slightly for semi-circle ribs but remains unchanged in smooth or rib-less ducts. Therefore, it can be concluded that rectangular and semi-circle shaped ribs offer a promising alternative in the design of micro solar-air heaters.