Analysis of Developing and Fully Developed Turbulent Flow and Heat Transfer in a Square-Sectioned U-Bend Duct

Turbulent flow and thermal field were predicted in a square-sectioned 180° bend at a Reynolds number of 56000. Suga’s low-Re cubic k-ε model [5–6] and the RSM [7–8] were used. The results were compared to experimental data [1]. Identical inlet boundary conditions were used in both cases. The inlet length impact on the flow-heat transfer in the bend was investigated. The velocities are higher near the inner wall and lower near the outer wall when a short inlet section is used. As the inlet length increases, the boundary layer grows thicker and the pressure-driven secondary vortex near the side wall becomes stronger. This vortex contributes significantly to the mixing process and heat transfer. It also alters the velocity distribution to a higher velocity near the outer wall and a lower velocity near the inner wall. When using a very long inlet length the vortex grows so strong that it generates a second counter-rotating vortex which isolates the fluid near the inner wall and prevents from further mixing. Consequently the local Nusselt number decreases. Both models reproduced the experimental data fairly well. Suga’s model performed better and converged without problems. It is believed that Suga’s model would be more suitable for industrial applications.