Single-Phase Mixed Convection in a Vertical Heated Tube with Varying Flow Directions and Operating Conditions

For thermal energy storage and nuclear safety systems, single-phase mixed convection experiments are conducted in a 2.8 m vertical test section with upward and downward water flows to investigate the interaction of forced and natural convection in intermediate Prandtl (Pr) number fluids. Uniform wall heating is applied, and axial wall temperatures are measured to determine local Nusselt (Nu) numbers under laminar and transitional conditions. In laminar flow, the Nu number exceeds the theoretical value of 4.36 due to buoyancy-driven enhancement. Four parameters—flow direction, heat flux, mass flow rate, and inlet temperature—are varied to assess the competing effects of inertia and buoyancy. Downward flows result in higher Nu numbers than upward flows under identical conditions, as buoyancy assists the flow and creates sharper velocity gradients near the heated wall. Increasing heat flux strengthens buoyancy and raises Nu numbers, while higher mass flow reduces buoyancy effects due to dominant inertia. The effects of Pr number variation with inlet temperature conditions are also investigated through wall-to-bulk temperature differences, which are directly related to buoyancy-driven convection enhancement. These results highlight the influence of the balance between inertial and buoyant forces on the heat transfer mechanism.