Impact of shear and buoyancy on convective heat transfer in annular finned cylinders with internal turbulent flow

This paper numerically investigates the coupled problem of internal turbulent forced convection and external natural convection in a horizontal finned tube with diameter d . Using a realistic thermal model that avoids prescribed wall-temperature boundary conditions, the study analyses the relationship between heat transfer performance and the net vertical force arising from the combined effects of buoyancy and shear in the external flow. For all cases examined, a clear maximum in the total heat transfer rate per unit tube length ( $$q'$$ [W/m]) is observed, coinciding with a peak in the averaged net vertical force. This behaviour explains, under realistic boundary conditions, the emergence of an optimal fin spacing resulting from the balance between induced air motion and available heat transfer surface area. For a finned-tube consisting of thirty-five annular fins of diameter D , an optimal spacing of approximately Sₒₚₜ ~ 5 mm is identified for different diameter ratios ( D / d ), yielding heat transfer enhancements of up to approximately $$80%$$ compared to cases with significantly smaller or larger fin spacings. The results demonstrate a direct correlation between heat transfer rates, the global heat transfer coefficient, and the net vertical force, providing a physical explanation for the existence of an optimal fin spacing that maximizes heat transfer per unit tube length in these configurations.