Flow regimes and heat transfer for opposing flow mixed convection in the thermal entry region of a vertical tube

This study focused on the thermal entry length problem for turbulent opposing flow mixed convection in a vertical tube. A Reynolds-averaged Navier–Stokes (RANS) simulation was performed using the v²–f turbulence model, and its results were compared with previous experimental data. The simulation results revealed several flow structures and heat transfer characteristics in the entry region, which varied depending on the competing strength of forced and natural convection. Flow regimes were classified based on their flow structures as follows: (a) Non-separating flow regime: When natural convection minimally influences the flow field, the Nusselt number in the entry region is higher than that in the fully developed region. As the influence of natural convection strengthens, the entry length decreases. (b) Separation bubble regime: As the influence of natural convection on the flow field increases, the velocity boundary layer along the heated wall in the entry region separates, leading to the formation of a recirculation region known as a separation bubble. Here, the depression in the Nusselt number distribution was observed owing to the thickening of the thermal boundary layer caused by the separation bubble. (c) Reverse flow regime: Further strengthening of the influence of natural convection leads to the formation of reverse flow along the entire heated wall. Here, the Nusselt number exhibits a nearly flat distribution in the entry region because the reverse flow thickens the thermal boundary layer. A flow regime map based on the results of the RANS simulation was created and proposed in this paper.