INVESTIGATIONS ON OPTIMISATION OF ARTIFICIALLY ROUGHENED SOLAR AIR HEATERS BASED ON SECOND LAW CONSIDERATIONS

Abstract

Solar air heaters have been observed to have generally poor heat transfer co-efficient from absorber plate to air. This low heat transfer co-efficient results in relatively higher absorber plate temperature leading to higher thermal losses to the • environment and hence low thermal efficiency. Thermal efficiency can be increased by employing artificial roughness in solar air heaters. However, in such systems the pumping power needed to overcome the friction losses results in decrease in net useful energy gain. So, while designing such an air heater our purpose is to attain high heat transfer rates with low friction losses. The net exergy is a useful parameter which takes into account the effect of enhancement in exergy of the absorbed solar radiation transferred to the fluid along with the exergy losses due to friction. The roughness parameters can be optimised on the principle of maximum net exergy gain. In the present work, solar air heater having artificial roughness has been optimised on the basis of maximum net exergy flow with respect to roughness parameters. Two types of roughness_geometries have been considered for the analysis, namely , transverse rib roughness and expanded metal mesh roughness. In the case of transverse rib roughness the optimisation has been carried out with respect to relative roughness height (e/D) and relative roughness pitch (p/e), while in the case of expanded metal mesh roughness, the relative roughness height (e/D), relative short way length (S/e) and relative longway length (L/e) have been considered. The results of optimisation have been presented in the form of design plots. The design plots can be used for selecting optimum roughness parameters for a particular' temperature rise parameter and insolation and the corresponding thermal efficiency can also be predicted.