07. Turbulence and Flow Instabilities

It has been confirmed previously that when swirling is applied to the flow in a pipe, heat transfer is significantly enhanced compared to pressure loss in the transition regime. One of the reasons for this is considered that in the laminar region (Re = 1000⁓2200), instability caused by the swirling promotes the turbulent transition, and heat transport becomes more prominent than momentum transport. It has also been suggested that in the turbulent region (Re ≈ 5000), centrifugal instability increases the near-wall flow velocity, leading to a dissimilarity between momentum and heat transport. In this paper, fluctuation pattern of the heat transfer measured using high-speed infrared thermography was observed carefully and considered the factors that enhance the heat transfer in the transition regime from the perspective of turbulent vortical structure. The observation revealed that the advection direction does not match the inclination angle of the streaky structure, whose inclination angle is around 5°⁓10° larger than the advection angle. The reason for this can be considered that when the longitudinal vortices are inclined in the swirl direction, the cyclonic vortices with larger inclinations become more stable. These cyclonic vortices may contribute to a dissimilar enhancement of the heat transport relative to momentum transport.