Experimental investigations on a cross flow solar air heater having perforated circular absorber plate for thermal performance augmentation

Conventional solar air heaters are associated with low thermal and thermohydraulic efficiencies due to the formation of laminar sub-layer created in the region where the flowing air comes in contact with the absorber plate. Hence there is a need to break this laminar sub-layer to improve the convective heat transfer capability of the collector. In this study, a solar air heater with perforated circular absorber plate is adopted with cross-flow configuration. The thermal performance is investigated for 5, 8 and 10 mm perforation vent diameters as well as 24, 36 and 54 number of vents. The configuration with perforated absorber plate provides a better convective heat transfer and thereby leads to better thermal and thermohydraulic efficiencies. This is compared with the base model in the absence of perforated absorber plate for a wide operating range of Reynolds numbers from 3000 to 21000. It is found from the study that when there is an increase in the number of perforations, there is a significant increase in thermohydraulic efficiency, even though there is a marginal drop in thermal efficiency. Also, with an increase in diameter of the perforated vents there is a remarkable improvement in the thermohydraulic efficiency though there is a marginal drop in the thermal efficiency. The best operating range of Reynolds number for the perforated solar collector is found to be between 9,000 to 15,000 having 54 vent perforations corresponding to 8 mm diameter and offers an efficiency of 75.55%.