Numerical simulations of 3D-flow for laminar mixed convection in horizontal rectangular tubes

A mathematical model was established to simulate the three-dimensional laminar mixed convection in a horizontal rectangular duct. The program was developed based on SIMPLER algorithm and the coupled convection and conduction heat transfer was simulated by the thermal balance method. The results showed that the secondary flow in the horizontal tube appeared due to the buoyancy force, and the conduction heat flux across the circumferential direction of duct walls reached different maximum values at the bottom and top corners related to the heated plate. The local Nusselt numbers related to the bottom wall were larger than those related to other walls, and the local Nusselt numbers related to the top wall were smaller than those related to other walls. The mean Nusselt numbers increased with the increase of Re and Gr*, but when Re was larger than 1500 the mean Nusselt numbers were almost independent of Gr*.