Fin angle effect on heat transfer enhancement in double-tube heat exchangers

This study aims to experimentally investigate the effect of combining Fe 3 O 4 –water nano-fluid and longitudinal fins with different inclination angles on the thermal and hydraulic performance of a double-tube heat exchanger. The experimental system consists of 30 fins arranged in an annular configuration on a horizontally positioned inner tube. The fins are placed at inclination angles of 90°, 60°, 45°, and 30°, with 14 plate-type fins of 30 mm height in each ring. The study was conducted at inlet temperatures of 313.15 K and 323.15 K and flow rates ranging from 0.2–2 L/min (Re = 249–3000). Air was used as the external fluid at a constant flow rate of 1250 L/min. The nano-fluid was prepared to contain 0.5% by volume of Fe 3 O 4 nanoparticles. In the performance evaluation, the heat transfer coefficient, Nusselt number, friction factor, and thermal performance factor (TPF) were considered. The results show that the use of nano-fluid and fins significantly increases heat transfer compared to a smooth tube. Among all tested configurations, the 90° fin orientation yielded the highest performance improvement, followed sequentially by the 60°, 45°, and 30° arrangements. In conclusion, for temperatures of 313.15 K and 323.15 K, the overall heat transfer coefficient and Nusselt number increased by 246.26–250.94% and 233.55–253.91%, respectively, while the friction factor increased by 1.080–1.094. At an inlet temperature of 313.15 K, the thermal performance factor (TPF) values for the double-tube heat exchanger were calculated as 1.97, 3.147, and 3.376 for 0.2, 1, and 2 L/min, respectively, while at 323.15 K, the TPF values were calculated as 2.27, 3.19, and 3.25. These findings demonstrate that the combined use of nano-fluid and optimised fin geometry is an effective method for designing compact and highly efficient heat exchangers.