Experimental study of improving the shell and tube heat exchanger by injection air bubbles

An experimental investigation examined the effect of air bubble injection as an active method in thermal performance enhancement of a counter flow horizontal shell and tube heat exchanger. The focuses on the impact of air bubble injection on the thermal characteristics of a heat exchanger equipped with three segmental baffles and 18 tubes were studied. The experimental setup includes a cylindrical shell with a length of 490 mm and an outer diameter of 120 mm. The hot fluid flows through a copper tube with a length of 590 mm and an outer diameter of 6.3 mm. A plastic tube with a diameter of 6 mm and 100 uniformly distributed holes was used in shell side to injection the air. Three different cold flow rates on the shell side (2, 4, and 6 LPM) were tested at an inlet temperature of 22℃ under laminar flow conditions (316≤Re≤1200). On the other side hot flow rate was varied between (1 and 1.5 LPM) with an inlet temperature of 50℃ through the tubes. Air bubbles are injected into shell side through three different injectors air flow rates of 2, 4, and 6 LPM. Key performance includes the effectiveness, overall heat transfer coefficient (Uo), and void fraction were invested. The experimental results show that the injection of air significantly enhances the performance of the heat exchanger. The effectiveness (ε) increased with air injection and enhancement ratio of the effectiveness (𝜀/𝜀𝑁𝐸) reaches 1.15 and 1.08 at hot flow rate (1 and 1.5) LPM respectively. The overall heat transfer coefficient (Uo) showed an increase with air injection and the percentage of improvement in performance reached to 1.47 at hot cold flow rate (1 and 6) LPM respectively compared to 1.22 at hot flow rate 1.5 LPM. Optimal improvement in heat transfer is observed within a void fraction range of 0.5 to 0.75.