Performance evaluation of different fin geometries for heat transfer augmentation and flow characteristics in a turbulent double-pipe heat exchanger

One of the major problems is to improve the heat transfer in double-pipe heat exchangers without any excessive hydraulic penalties. Different passive enhancement techniques have been investigated in previous studies. However, a direct experimental comparison of various fin geometries under the same operating conditions is scarce. This study experimentally investigates the thermo-hydraulic performance of three annulus side fin configurations, circular fins, helical fins, and interrupted rectangular fins in a double pipe heat exchanger. The hot-side Reynolds number was maintained at ∼14,680, while the cold-side Reynolds number varied from 2,218 to 10,347. The effects of fin shape on Nusselt number, overall heat transfer coefficient, heat transfer rate, effectiveness, NTU, pressure drop, and thermo-hydraulic performance factor (THPF) were systematically evaluated in comparison with a smooth tube. The results show that the performance is strongly dependent on the standard fin shape. The rectangular fins gave the highest thermal enhancement with an increase in the Nusselt number of 121-295% and an effectiveness of 121-255%. However, the rectangular fins gave the highest increase in pressure drop of 19-245%. The helical fins enhanced the Nusselt number by 47–82% through swirl-induced secondary flow and achieved THPF values of 1.5–1.75. Circular fins provided more moderate heat-transfer enhancement but the lowest hydraulic penalty, with pressure-drop increases of 4–45%. Overall, the results show that rectangular fins maximize thermal performance, whereas circular fins provide a more balanced thermo-hydraulic response. The experimental comparison provides new insight into the trade-off between heat-transfer enhancement and hydraulic resistance and offers a basis for selecting fin geometry according to the desired thermal and hydraulic performance.