Numerical prediction of flow and heat transfer in a rectangular channel with a built-in circular tube

Abstract

A numerical investigation of flow and heat transfer in a rectangular duct with a built-in circular tube was carried out for moderate Reynolds numbers and varying blockage ratios. Since the heat transfer in the duct is dictated by the flow structure, the study was directed towards characterization of the flow. To this end, the topological theory shows promise of becoming a powerful tool for the study of flow structures. The limiting streamlines on the tube and the bottom plate reveal a complex flow field. The separation lines and points of singularity (saddle points and nodal points) were investigated. The iso-Nusselt number contours and span-averaged Nusselt number distribution in the flow passage shed light on the heat transfer performance in the duct. The investigation was necessitated by the need to enhance heat transfer in fin-tube heat exchangers through identification of the zones of poor heat transfer. The predicted results compare well with the well documented experimental results available in the literature. In the range of Reynolds numbers considered for the present case, no need is felt to employ any turbulence model in order to describe the heat transfer behavior. Time series signals of the transverse velocity component in the wake zone are presented with their FFT and time-delay plots. The onset of turbulence is not observed up to the highest value of the Reynolds number considered in the present case. This confirms that the transition to turbulence is delayed in the present case compared with that observed for flow past a circular tube placed in an infinite medium. The reason may be attributed to the narrow gap between the no-slip channel walls.