Heat transfer enhancement in a channel flow using two wall-mounted flexible flags with a confined cylinder

A heat transfer system to improve the thermal performance of a channel flow with two symmetrically wall-mounted flexible flags is proposed by arranging a confined cylinder in an upstream region of the flags. The effects of the horizontal gap distance between the cylinder center and the wall-mounted position of the downstream flags (Gx) and the bending rigidity of the flags (??) on the vortex dynamics are examined based on numerical simulations of the fluid-structure-thermal interaction using a penalty-immersed boundary method. Four distinct flapping modes of the two downstream flags are identified in the regime map of (Gx, ??): an out-of-phase flapping mode with symmetric motion of the flags (OS mode) and an out-of-phase flapping mode with non-symmetric motion of the flags (ON mode), and an in-phase flapping mode (I mode) and combined flapping mode in which the I mode and ON mode are presented alternately with time (ION mode). Heat transfer systems consisting of two flags under the OS and ON modes achieve improved thermal efficiency compared to that for a clean channel system (CS) due to vortex-to-vortex interactions promoting fluid mixing between the core cold fluid and near-wall hot fluid. However, the thermal efficiency for the systems in the I and ION modes decreases due to the small increase in the heat transfer rate for the I mode and a large deficit of the mechanical energy loss for the ION mode, respectively. When the values of Gx, ?? and the Reynolds number are optimal, the present system in the OS mode results in an increase of the heat transfer rate by approximately 177% with an accompanying increase of the thermal efficiency up to 24% compared to that for the CS (despite an increase of the mechanical energy loss of 1021%)