OPTIMIZATION OF SOLAR AIR HEATER HAVING ARTIFICIAL ROUGHNESS

Abstract

Artificial roughness elements are employed in solar air heaters to enhance the thermal efficiency which is ordinarily quite low because of poor heat transfer between the absorber plate and air. However, in such systems the pumping power needed to overcome the friction losses can reduce the benefits of increased useful heat gain. Thus, the design of solar air heaters ( having artificial roughness) with high heat transfer rates and low friction losses is of particular interest. Both the heat transfer coefficient and friction factor are found to be strong functions of roughness parameters. The effective efficiency is a useful parameter to take into account the effect of enhanced useful heat gain and friction losses. By maximising the effective efficiency, the roughness parameters can be optimized with regards to -high useful heat gain and low-friction loss. In this work solar air heaters having absorber plates roughened with chamfered rib roughness and expanded metal mesh roughness are optimized for maximum effective efficiency with respect to roughness parameters like relative roughness height, relative roughness pitch, chamfer angle, relative shortway length and relative longway length of mesh. The results of optimization are presented in the form of design plots for solar air heaters for both types of roughness in terms of their respective roughness parameters. These plots can be used for selecting the optimal roughness parameters and predicting the thermal efficiency of these solar air heaters for a particular temperature rise and insolation