Performance evaluation of solar air heater for various artificial roughness geometries based on energy, effective and exergy efficiencies

A comparative study of various types of artificial roughness geometries in the absorber plate of solar air heater duct and their characteristics, investigated for the heat transfer and friction characteristics, has been presented. The performance evaluation in terms of ηI, ηef and ηII has been carried out, for various values of Re, for some selected artificial roughness geometries in the absorber plate of solar air heater duct. The six roughness geometries as per the order of ability to create turbulence and a smooth surface have been selected. The correlations for heat transfer and coefficient of friction developed by respective investigators have been used to calculate efficiencies. It is found that artificial roughness on absorber surface effectively increases the efficiencies in comparison to smooth surface. The ηI in general increases in the following sequence: smooth surface, circular ribs, V shaped ribs, wedge shaped rib, expanded metal mesh, rib-grooved, and chamfered rib–groove. The ηef based criteria also follows same trend of variation among various considered geometries, and trend is reversed at very high Re. The ηII based criteria also follows the same pattern; but the trend is reversed at relatively lower value of Re and for higher range of Re the ηII approaches zero or may be negative. It is found that for the higher range of Re circular ribs and V shaped ribs give appreciable ηII up to high Re; while for low Re chamfered rib–groove gives more ηII.