Unsteady laminar flow and convective heat transfer in a sharp 180° bend

Unsteady laminar flow and heat transfer in a sharp 180° bend is studied numerically to investigate a convective heat transfer regime of especial relevance to electronic systems. Due to the high geometrical aspect ratios occurring in the practical application, two-dimensional unsteady simulations are considered. The two-dimensionality assumption adopted is validated by three-dimensional test simulations. Unsteady heat transfer simulations are performed for 50⩽Re⩽1000. Results show that the flow remains steady until Re≈600. In this steady regime, the re-attachment length increases gradually with the Reynolds number. For Re>600, the flow becomes unsteady with large-scale vortices emanating from the sharp edge dominating the flow field. Flow oscillation causes a substantial reduction in the re-attachment length and a dramatic heat transfer enhancement. As the vortices move downstream, the Nusselt number along the wall oscillates significantly. The correlation between the flow structure and the heat transfer is found to be strong.