Enhancing thermal performance in horizontal plate heat exchanger through air injection: an experimental study

Introducing air-bubbles into heat exchangers is an effective technique for enhancing heat transfer; however, to the best of the authors' knowledge, no previous study has investigated air-bubble injection in a horizontally oriented plate heat exchanger (HPHEs). This study experimentally investigates the thermal behavior of an HPHE with two-phase flow. The evaluation focused on effectiveness, the number of transfer units, and the overall heat transfer coefficient as key indicators of system performance. Air was introduced into the hot water line upstream of the HPHE using a T-junction mixer at air flow rates ranging from 0 to 2 L/min. Also, three intake temperatures of hot water (40 °C, 50 °C, and 60 °C) were tested. The results reveal that bubble injection consistently enhances the thermal performance under all tested scenarios. Compared with the single-phase baseline case, the overall heat transfer coefficient and the number of transfer units (NTU) increased by up to approximately 42.7%. The largest enhancement in effectiveness reached 14.98%. In absolute terms, the maximum overall heat transfer coefficient attained was 2.82 kW/m 2 ·°C, while the highest NTU and effectiveness values achieved were 2.43 and 0.835, respectively. However, the thermal enhancement was accompanied by an increase in pressure drop. At the optimum operating condition corresponding to the highest thermal enhancement (40 °C hot-water inlet temperature, 0.8 L/min hot-water flow rate, and 2 L/min air flow rate), the two-phase pressure drop was approximately 3.1 times the corresponding single-phase value, indicating that the hydraulic penalty should be considered alongside the thermal enhancement when evaluating the practical performance of the proposed air-injection technique.