Heat transfer enhancement in a swirling pipe flow at low Reynolds number
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Nowadays, there has been a growing interest in energy conservation from the viewpoint of sustainability as a countermeasure against the rapid progress of global warming. In particular, the improvement of turbulent heat transfer is required for industrial products such as heat exchangers and is actively discussed. Recently, the swirling flow is known to improve the heat transfer at the transient regime between the turbulence and laminar flow. [1][2] In this study, we investigate the effect of swirling flow on the heat transfer in the pipe flow using direct numerical simulations. All simulations start from a fully developed incompressible pipe flow. We compare two cases, i.e., the bulk Reynolds number of 5000 and 3000. The wall rotation starts at t= 0 to create the swirling flow and it stops at t=40. The speed of the wall is fixed at 30 % of the bulk mean velocity. The Prandtl number is 1 and a uniform heat generation condition is imposed. In both Reynolds number cases, at t<40, the skin-friction coefficient and the Nusselt number are decreased due to the swirling below the uncontrolled case. However, after stopping the wall rotation (i.e., t>40), they are suddenly increased. The near-wall streaky structures are tilted in the swirling direction. In the final paper and the presentation, we will compare these cases in detail and discuss the dissimilarity between the momentum and heat transfers.
