Experimental study of Nusselt number and Friction factor in solar air heater duct with Diamond shaped rib roughness on absorber plate

- Solar air heater is used to heat air but it has low thermal efficiency because of low thermal conductivity between air and absorber plate. Thermal efficiency of solar air heater can be improved by creating artificial roughness on absorber plate which causes higher temperature to absorber plate and hence maximum thermal losses occurs to atmosphere. There are number of parameters which enhances the thermal conductivity such as relative roughness height (e/Dh), relative roughness pitch (P/e), Reynolds number (Re), and angle of attack (α).Experimental investigations were carried out to study heat transfer enhancement using diamond shape rib on absorber plate of solar air heater. Absorber plate is heated with the solar radiation in outdoor experiment whereas electric heater is used for indoor experiment. Setup is isolated from the three sides with Thermocol. The relative roughness pitch (p/e) varies from 10 to 25 mm. The roughened wall has relative roughness height (e/Dh) of 0.023mm and 0.028mm, angle of attack (α) is 0° degree, rib height (e) is 1 mm and 1.25 mm. Duct aspect ratio (W/H=8), rate of air flow corresponds to Reynolds no. (Re) ranging from 3000-14000.Finally comparison of heat transfer and friction factor from both smooth and roughened plate under the similar condition of air flow is made.