Synergistic thermohydraulic enhancement and correlation development for ZnO/H₂O nanofluid flow in an elliptical Dimpled Square tube

This study presents a comprehensive experimental and numerical investigation of the thermohydraulic performance of ZnO/H₂O nanofluid flowing through a square tube equipped with elliptical dimpled surfaces under laminar flow conditions. To the best of the authors' knowledge, this study constitutes the first experimental investigation of nanofluid flow in a square cross-section channel enhanced with elliptic dimple fins. A significant original contribution of this work lies in the development of novel empirical correlations as a function of fin geometry and nanofluid concentration, derived systematically from the experimental data. Moreover, the comparison of experimental results with numerical analyses performed under identical boundary conditions substantially broadens the scope of the present study. Experiments were conducted within a Reynolds number range of 500–2000 under a uniform heat flux of 2000 W/m2. Three different pitch ratios (P/b = 10, 15, and 20) were examined to clarify the influence of dimple density on flow structure, heat transfer characteristics, and pressure losses. Numerical simulations were conducted using the finite volume method with a laminar viscous model in ANSYS Fluent 2024 R2. Pressure–velocity coupling was handled via the SIMPLE algorithm, while the Second-Order Upwind scheme was applied for the discretization of momentum and energy equations. Gradient terms were evaluated using the Least Squares Cell-based method, and convergence was assumed when all residuals fell below 10−6. Among the tested configurations, the P/b = 10 geometry with 1 vol% ZnO/H₂O nanofluid demonstrated the best thermohydraulic performance, yielding a combined Nusselt number enhancement of approximately 27.9% over the smooth tube at Re = 2000, while the performance evaluation criterion reached a maximum of 1.25, confirming a net benefit despite a friction factor penalty of 5.92%. The developed empirical correlations for the Nusselt number and Darcy friction factor predicted the experimental data with an average deviation below 4%, and numerical results agreed well with experimental measurements, with deviations of 1.56–5.43% and 1.84–3.15% for Nu and f, respectively.