<p>This study numerically investigates the thermohydraulic performance of Al₂O₃-CNT-graphene ternary nanofluid in a laminar counterflow double-pipe heat exchanger. A 2D axisymmetric laminar flow model is developed to evaluate the thermal and hydraulic behavior of this hybrid nanofluid. The governing equations are solved using the finite volume method. The model shows good agreement with published benchmark data for both base and nanofluid confirming its reliability. The analysis focuses on the average Nusselt number (Nu), pressure drop, performance evaluation criterion (PEC), outlet temperatures, and local contour distributions of pressure coefficient, velocity magnitude, and temperature. The results show that the average Nu increases with Reynolds number (Re) for all fluids, while the ternary nanofluid consistently provides higher values than base fluid (water) and Al₂O₃/water nanofluid. The pressure drop also increases with Re and particle volume fraction. Despite this hydraulic penalty, the thermohydraulic assessment indicates that the heat transfer enhancement remains greater than the associated hydraulic loss, with the highest-loading case yielding the best overall performance among the studied nanofluids. The outlet temperature trends and contour fields further support these findings and provide physical insight into the transport processes within the heat exchanger. Overall, the results demonstrate that Al₂O₃-CNT-graphene ternary hybrid nanofluid is a promising working fluid for heat transfer enhancement in laminar double-pipe heat exchangers.</p>