Heat transfer and fluid flow characteristics of rib-groove roughened solar air heater ducts

Abstract

The performance of solar air heater is considerably low due to the low value of convective heat transfer coefficient between the flowing air and the absorber plate. This is due to the formation of thin viscous sub-layer at the surface of the absorber plate. Artificial roughness is used to break the laminar viscous sub-layer formed on the absorber plate for increasing the level of turbulence to enhance the heat transfer. In the present work artificial roughness in the form of integral wedge shaped rib with and without groove has been employed on one of the heated broad wall of the rectangular duct to enhance the heat transfer. The wedge angle (ϕ) was varied from 10 ° to 25° for relative roughness pitch (p/e) was kept as 8 and relative roughness height (e/D) was maintained as 0.033. The aspect ratio of the rectangular duct was maintained as 8. The Nusselt number and friction factor of the artificially roughened ducts were determined experimentally and the corresponding values were compared with that of smooth duct for the range of Reynolds number 3000-20000. The maximum enhancement in heat transfer was observed for the artificial roughness of wedge angle 15°. At the optimum angle of 15° a groove was incorporated. It was observed that wedge-groove roughened surface shows more enhancement in heat transfer compared to only rib roughened surface. The investigation revealed that Nusselt number increases 1.5-3 times of the smooth duct while the friction factor increases two to three folds that of the smooth duct in the range of operating parameters. The uncertainties in the measurements due to various instruments for the Reynolds number, Nusselt number and friction factor lies in the range of ± 3.8%, ± 3.54%, and ± 7.6%, respectively.