Forced Convective Heat Transfer in Tubes and Ducts: A Review of Prandtl Number, Geometry, and Orientation Effects

This paper presents a comprehensive review of forced convective heat-transfer phenomena in fluids, emphasizing the influence of fluid properties, tube geometries, and flow orientations under varying Prandtl numbers. Key governing parameters—including velocity, viscosity, thermal conductivity, density, specific heat, surface area, and flow regime (laminar or turbulent)—are expressed through dimensionless groups such as the Nusselt (Nu), Reynolds (Re), and Prandtl (Pr) numbers. The review encompasses heat-transfer characteristics of low-, medium-, and high-Prandtl-number fluids flowing through circular, square, triangular, and elliptical tubes in both horizontal and vertical orientations, aiming to critically evaluate the effectiveness and trends reported in previous studies. Where applicable, symmetry correlations—based on equivalent thermal and hydrodynamic behaviour along geometrically symmetric boundaries—were considered to interpret flow uniformity and heat-transfer distribution across cross-sectional profiles. Analysis reveals that over 84% of the reviewed studies emphasize on horizontal configurations and 55% on circular geometries, with medium-Prandtl-number fluids dominating experimental investigations. While these studies provide valuable insights, significant research gaps remain. Limited attention has been given to vertical orientations, where buoyancy effects may alter flow behaviour due to temperature and pressure gradients arising from variations in fluid density and viscosity, to non-circular geometries that enhance boundary-layer disruption, and to extreme-Prandtl-number fluids such as liquid metals and heavy oils, which are vital in advanced industrial applications. Bridging these gaps presents opportunities to design and optimize diverse engineering systems requiring efficient convective heat transfer. Practical examples include coolant flow in nuclear reactors, heat dissipation in high-performance CPUs, and high-speed airflow over automotive radiators. This review therefore underscores the need for future research extending forced-convection studies beyond conventional configurations, with particular emphasis on vertical orientations, complex geometries, and underexplored Prandtl-number regimes.