This study experimentally investigates the thermal-hydraulic performance of alternating flattened tubes in a double-pipe heat exchanger. The primary objective is to evaluate the effects of geometric modifications, including pitch length and degree of flattening, on heat transfer enhancement and pressure drop compared to conventional circular tubes. Four tube configurations were examined: alternating flattened tubes with pitch lengths of 12 cm and 28 cm (flattening of 1 cm), a completely flattened tube (100 cm pitch length, 1 cm flattening), and an alternating flattened tube with a higher flattening degree of 1.2 cm (pitch length 12 cm). Experiments were conducted over a range of Reynolds numbers, and the Nusselt number and pressure drop were measured and analyzedc. The results demonstrate that alternating flattened tubes significantly improve heat transfer compared to circular tubes. The AF_12_1 tube (12 cm pitch, 1 cm flattening) exhibited the highest Nusselt number, achieving 1.9 times that of the circular tube. However, this enhancement was accompanied by an increase in pressure drop, which was 1.22 times higher than the circular tube at higher Reynolds numbers. Increasing the pitch length or reducing the degree of flattening decreased both heat transfer and pressure drop. The completely flattened tube showed a more modest improvement (1.33 times the Nusselt number) with the lowest pressure drop increase. Overall, the alternating flattened tube with shorter pitch length and moderate flattening provides the best thermal performance at the cost of higher flow resistance.