Interaction between near-wall turbulence structure and immiscible droplets falling with wobbling motion in upward water flow

Flow visualization and image processing have been carried out for turbulent upward water flow with falling immiscible droplets of perfluorocarbon or hydrofluoroether in a pipe. It is found that the decreases of the mean velocity and the increases of the turbulence intensities are caused by the droplet wake flow. The droplets of FC-72 fall near the axis with small-amplitude wobbling motion. On the other hand, the droplets of HFE-7200 fall near the pipe wall with noticeable wobbling motion. The near-wall turbulence structure is modified by the wake flow of the wobbling droplets. Also, direct numerical simulation has been carried out for understanding the modification of near-wall heat transfer by the droplet. It is found that the outward flow in the wake carries hot fluid into the central region and that the wallward flow induced by the main flow impingement onto the cap of droplet carries cool fluid into the near-wall region. These cause near-wall heat transfer enhancement. The ratio of the energy associated with the droplet and that of the equivalent fluid lump is effective for discussing the wobble motion and the wake flow of the droplets.