Effect of air bubble injection on the thermal performance of a flat plate solar collector

Air bubble injection has been proved to be a simple and effective heat transfer enhancement technique in liquid-to-liquid heat exchangers. This study aimed to investigate the technique experimentally as a possible method for augmenting the thermal performance of a flat plate solar collector in the climatic conditions of Najaf city, Iraq. Air was injected as small bubbles into the riser tubes of the solar collector with three different flow rates (Q(a) = 1, 1.5and2LPM) and was compared with no air injection (Q(a) = 0LPM). The flat plate solar collector had dimensions of 1000x2000x80 mm (length, width and thickness respectively) and contained seven riser tubes of 8 mm internal diameter and 0.6 mm thickness. A perforated silicon tube (with holes of 0.5 mm diameter) was used for the air injection. Tap water was used as a working fluid and the experiments were repeated three times for each air flow rate. The experimental results showed that the injected air significantly improved the thermal performance of the solar collector. Its efficiency was enhanced by about 16.5%, 69.2% and 68.8% for the injected air flow rates of 1 LPM, 1.5 LPM and 2 LPM respectively.