Thermal and Hydrodynamic Behavior in the Transitional Mixed Convection Regime for Buoyancy‐Aiding and Opposing Flows

ABSTRACT In practical heat exchangers, where tube lengths are typically shorter, the flow is largely developed. In this numerical investigation, we examined the thermal and hydrodynamic characteristics within the transitional developing regime of mixed convection for buoyancy‐aiding and opposing flows through a vertical tube. Employing a two‐dimensional axially symmetric approach with a steady state, simulations were conducted for a range of Reynolds number (), Grashof number (), and Richardson number ( Ri ) of 0.1 for a nondimensional length ( L/D ) of 150 subjected to uniform wall heat flux condition. The study involved a comparative analysis of two transition models: transition and transition shear stress transport (SST) and the model with better accuracy was selected. The findings reveal the crucial role of buoyancy in the laminar‐turbulent transition for both assisting and opposing flows. In both flow scenarios, the coefficient of skin friction ( C f ) and Nusselt number ( Nu ) exhibit an increase with increasing Re at a constant Ri . Notably, under the same Ri conditions, the opposing flow exhibits higher pressure drop (assessed by C f ) and heat transfer (assessed by Nu ). For buoyancy‐aiding and opposing flows, the hydrodynamic entry length reduces from L/D ~ 70 to 60, while the thermal entry length extends with increasing Gr and Re , when Ri is fixed. Furthermore, the onset of transition in mixed convection is dependent on the wall heat flux and is earlier in opposed flow than assisted flow. Based on the intermittent behavior, the lower transitional zone (2000 < Re < 3000) and the higher transition zone (3000 < Re < 5000) have been demonstrated.