Direct numerical simulation of turbulent heat transfer in water flow with immiscible droplets

Direct numerical simulation has been carried out for turbulent upward water flow with two immiscible droplets in a vertical channel. The density of the droplets is higher than that of water. The flow is heated from the channel walls. The governing equations are solved with finite difference schemes. The grid is allocated nonuniformly, except for a region including the droplets. The grid is uniformly and densely allocated in the region. The grid is used as the overset grid so that it reduces the computational errors due to high interfacial tension and complicated flow near the interface. The interface is tracked with the modified version of the volume-of-fluid method. It is found that the computational errors are reduced by using the overset grid. A large number of small-scale vortices are generated around the falling droplets. The secondary flow in the wallward direction is induced below the droplets, and the secondary flow in the outward direction is induced above the droplets. These flows are effective for the enhancement of the near-wall heat transfer. These flows are attenuated by the adjacent droplet in the axial direction. The droplets deform large-scale streamwise vortices and attenuate small-scale streamwise vortices.