The effect of injected air bubble size on the thermal performance of a vertical shell and helical coiled tube heat exchanger

In the present study, the effect of injecting air bubble size on the thermal performance of a vertical counter-current shell and coiled tube heat exchanger is experimentally investigated. The experiments were accomplished in a cylindrical shape heat exchanger with a 50 cm height and 15 cm outer diameter. Copper coil with 3.939 m equivalent length and 0.6 cm outer diameter was used to carry the hot fluid (water). Four different cold fluid (shell side) flow rates (Q_s = 2; 4; 6 and 8 LPM) under laminar flow conditions (316 ≤ Re ≤ 1223), constant hot (coil side) flow rate fluid rates (Q_h = 1 LPM), four different injected air flow rates (Q_a = 0:5; 1; 1:5 and 2 LPM), invariant temperature difference (ΔT = 20°C), and constant bubble’s number (1400) were tested. To demonstrate the effect of bubble size, a sparger with orifice diameters of 0.1, 0.8, and 1.5 mm was manufactured and used in the study. The overall heat transfer coefficient (U), NTU, effectiveness, and pressure loss were invested. The experimental results clearly showed that the heat exchanger’s thermal efficiency significantly improved with increasing the shell side flow rate and the injected air flow rate. The maximum improvement in U, NTU, and effectiveness was 153%, 153%, and 68%, respectively. The thermal performance of the heat exchanger was shown to be improved with increasing the bubble size. Although the latter finding agrees with recent CFD published results, more studies need to be confirmed.