Heat transfer enhancement by a flexible inverted flag with an inclination angle

The heat transfer system, including an inclined inverted flag that plays a role of a vortex generator, is proposed in the present study. A two-dimensional simulation is performed to analyze the effects of the inclination angle and the bending rigidity of the inverted flag on thermal performance. To consider the fluid-flexible body–thermal interaction, an immersed boundary method is adopted. The four regimes are observed depending on the inclination angle and the bending rigidity, that is, large-amplitude flapping (LAF), small-amplitude flapping (SAF), deflected (D), and straight (S) modes. The SAF and LAF modes are observed to be favorable in terms of the heat transfer efficiency, which considers the heat flux and mechanical energy loss. A scaling analysis is performed to explain the correlation between the flapping kinematics and the thermal quantities. A scaling parameter is newly defined based on the momentum transfer to the inverted flag due to a vortical impulse and shows a proportional relation to the mean drag force with a slope of 0.166. The heat transfer efficiency is observed to be proportional and inversely proportional to the parameter in the SAF and LAF modes, respectively. The optimized heat transfer system is obtained at the angle of 12° and the bending rigidity of 0.7, where the efficiency is enhanced up to 112.8% over the baseline flow.