Numerical simulation on heat transfer enhancement in twisted-tape-inserted tubes

To clarify the mechanism of heat transfer enhancement in swirl tubes, especially the effect of secondary flow generated in a pipe cross-section on temperature field mixing, numerical simulation of swirl flow under the laminar condition including a mixed convection region is performed. As the result of the simulation, the generation and development processes of secondary flow are visualized clearly, and it is clarified that these transitional processes depend on a twist ratio of swirl-tape γ and Reynolds (Re) number. Furthermore, these behaviors of secondary flow also greatly contribute to the heat removal performance, and the Nusselt (Nu) number becomes higher, especially if the secondary flow has some instability such as that found in the transition from one to two vortices. Numerical data on the heat transfer performance as well as the flow patterns show good agreement with conventional experimental results. On the other hand, it is shown that the transition process of secondary flow under a high heat flux condition is strongly affected by gravity effect, and its flow pattern and the local Nu number change periodically in a streamwise direction. By summarizing all the numerical data, a new prediction formula for the Nusselt number is constructed in wide Re, Prandtl (Pr), Grashof (Gr), and γ ranges, and it is shown that this formula almost corresponds to the conventional experimental results.