This work examines the thermal and flow characteristics of a heat sink equipped with twisted longitudinal fins, employing a Cu-water nanofluid as the working medium. Numerical simulations, carried out using the finite element method, were used to evaluate the system’s performance. The geometry is divided into three key sections: an inlet, a finned middle section subjected to a heat source, and an outlet region. Simulations covered a range of Reynolds numbers from 500-2000 and nanoparticle volume fractions, ϕ, between 0% and 4%. Analysis of the resulting temperature and velocity distributions shows that the presence of nanoparticles, along with the complex geometry of the twisted fins, leads to sharper thermal gradients and increased mixing within the fluid-flow, both of which contribute to enhanced heat transfer in the central region. Results show that increasing the Reynolds number alone enhances convective heat transfer by up to 42.8% for pure water and 36.4% for the 4% nanofluid. At a constant Re = 500, increasing the nanoparticle volume fraction from 0%-4% improves the average Nusselt number by 19.8%. When both parameters are increased simultaneously, from Re = 500 and ϕ = 0% to Re = 2000 and ϕ = 4%, the overall enhancement reaches approximately 63.4%. These findings confirm the effectiveness of the finite element method in capturing complex thermal-fluid interactions and highlight the strong potential of combining nanofluids with optimized fin geometries for advanced heat sink applications.