The present numerical study examines heat transfer performance of water, \({\text{Al}}_{2}{\text{O}}_{3}\) -water, and \({\text{Al}}_{2}{\text{O}}_{3}/\text{Cu}\) -water hybrid nanofluids flowing through a straight duct with rectangular cross-section. The duct, subjected to uniform wall heat flux \((\approx 7955\text{ w}/{m}^{2})\) , experiences symmetrical heating. The volume fractions taken for \({\text{Al}}_{2}{\text{O}}_{3}\) -water single-phase nanofluid and \({\text{Al}}_{2}{\text{O}}_{3}/\text{Cu}\) -water hybrid nanofluid are 0.5%, 1%, 1.5%, and 2%. For the hybrid nanofluid, the proportions of constituent nanoparticles are \({\text{Al}}_{2}{\text{O}}_{3}/\text{Cu }(1:1)\) , \({\text{Al}}_{2}{\text{O}}_{3}/\text{Cu }(1:2)\) , \({\text{Al}}_{2}{\text{O}}_{3}/\text{Cu }(2:1)\) , \({\text{Al}}_{2}{\text{O}}_{3}/\text{Cu }(1:3)\) , and \({\text{Al}}_{2}{\text{O}}_{3}/\text{Cu }(3:1)\) . For the present study, the Reynolds number (Re≈ 2000–12,000) varies with flow velocity. Among the selected grades, the 2% \({\text{Al}}_{2}{\text{O}}_{3}/Cu\) hybrid nanofluid with \({\text{Al}}_{2}{\text{O}}_{3}/\text{Cu }(1:3)\) exhibits superior heat transfer performance. The simulations also enable the derivation of an empirical correlation among the Nusselt number, Prandtl number, Reynolds number, volume fraction (φ), and the relative proportion of constituent nanoparticles. Moreover, the simulation results reveal that the growth in convective heat transfer coefficient (h) surpasses the improvement in thermal conductivity by a considerable margin. The significant enhancement of heat transfer rates in nanofluids has been examined in connection with several factors, which include thermal conductivities, energy transfer through nanoparticle dispersion, non-uniform shear rates, nanoparticle migration due to viscosity gradients, thermophoresis, and Brownian diffusion.