This study investigates the hydrothermal performance of a horizontal double-pipe heat exchanger with a counter-flow arrangement. The system incorporates flower baffles at a relative angle of γ = 90° and segmental baffles, while Al2O3/water nanofluid is used in the annular side. The experiments evaluate the effects of nanoparticle concentration (0–1.5% by volume), annulus Reynolds number (2620 ≤ Reₐₙ ≤ 13110), and Prandtl number (3.91 ≤ Prₐₙ ≤ 7.48) under steady operating conditions. Hot water at 70°C is maintained at a constant flow rate in the inner tube, while cold water circulates in the annular region. Key findings demonstrate significant enhancements in the Nusselt number (¯Nu_an) and friction factor (f_an) with the addition of baffles and Al2O3 nanoparticles. Flower baffles outperformed segmental baffles, achieving increases in ¯Nu_an and f_an by 343.1% and 190.3%, respectively, at a nanoparticle concentration of 1.5%. The Heat Transfer Performance Index (HTPI) consistently exceeded unity across all test parameters, with the highest HTPI of 3.24 observed for flower baffles with δ = 16.7% and λ = 8.3%. Furthermore, empirical correlations for ¯Nu_an, f_an, and HTPI are proposed. The results highlight the combined potential of nanoparticle-enhanced fluids and innovative baffle geometries to improve heat exchanger performance, offering valuable insights for industrial applications.