Experimental investigation of the effect of air bubbles injection on the performance of a plate heat exchanger

Injection of sub-millimeter scale air bubbles into liquid flows has been recognized recently as an effective method for thermal performance enhancement of heat exchangers. However, no studies have been reported so far on air injection into the cold liquid stream of plate heat exchangers (PHEX). This approach is investigated experimentally in the present study for the first time in PHEXs to map the energetic and exergetic performance characteristics of a typical PHEXs in terms of the number of transfer units (NTU), effectiveness, entropy generation number, augmentation entropy generation number, rational effectiveness, and Witte-Shamsundar efficiency. Air (150-840 L/h) and hot water (280-880 L/h) streams are mixed in a T-junction mixing chamber before entering the PHEX as a two-phase flow. The results show a remarkable increase in NTU by up to 12.4% due to injected air bubbles, whereas the effectiveness increases by up to 14.6%. On the other hand, considerable increments in entropy generation rate are noticed, reaching a maximum of 4.1 W/K, but this increment was less than 3 folds for most examined cases. Overall, the Witte-Shamsundar efficiency, representing the combined energetic-exergetic performance, reached a maximum of 96.9% at air and hot water flow rates of 350 and 280 L/ h, respectively.