Numerical investigation of conjugate heat transfer in circular pipes considering wall thickness, material conductivity, and external convection

• A comprehensive CFD study of turbulent conjugate heat transfer in circular pipes was performed. • Reynolds number was identified as the dominant parameter governing internal convection. • Wall thickness, material conductivity, and external convection mainly affected the overall heat transfer rate. • A dimensionless Nusselt number correlation was developed based on 54 numerical cases. • The proposed correlation predicts numerical results within ±8% deviation. Conjugate heat transfer in circular pipes involves the coupled interaction of turbulent forced convection in the fluid, heat conduction through the pipe wall, and external convection to the ambient, which represents realistic conditions encountered in piping and heat-exchanger applications. However, the relative influence of Reynolds number compared with wall thickness, wall thermal conductivity, and external convection on internal convection and overall thermal performance has not been systematically quantified for turbulent conjugate conditions over a broad parameter space. In this study, a three-dimensional CFD model with fully coupled conjugate heat transfer was developed and 54 cases were simulated by varying Reynolds number (5000–20000), wall thickness ratio (0.02–0.08), wall thermal conductivity (0.2–400 W/mK), and external convective heat transfer coefficient (5–100 W/m²K). The bulk fluid temperature was evaluated using a mass-weighted definition extracted from the CFD field, and numerical reliability was ensured through grid-independence, near-wall resolution (y⁺) , and turbulence-model sensitivity checks. The results show that the internal Nusselt number is predominantly governed by Reynolds number, whereas wall thickness, wall conductivity, and external convection mainly affect the total heat transfer rate and wall temperatures through redistribution of thermal resistances. A power-law correlation for the Nusselt number was proposed and shown to predict the numerical dataset within ±8% over the investigated ranges, providing practical guidance for thermal design of circular pipes under conjugate boundary conditions.