Heat transfer enhancement of turbulent channel flow using tandem self-oscillating inverted flags

Abstract

Heat transfer enhancement by tandem self-oscillating inverted flags was investigated for channel flow. An isolated flag was selected as the benchmark. The highly unsteady flapping motion of the inverted flag was captured using a high-speed camera and identified by the structure boundary detection algorithm. The resultant temperature fields were determined by temperature sensitive paint measurement. Three dynamic regimes were observed for the isolated inverted flag: biased, flapping, and deflected modes. The results show that the self-oscillating inverted flag in the flapping mode exhibited the most energetic motion, with a maximum amplitude of 1.75 times of the flag length and a Strouhal number 0.17. This significantly promoted heat removal from the heated wall, with a considerable local Nusselt number ratio exceeding 1.5. Tandem flags flapped synchronously with the same frequency at a low Reynolds number and a close distance G* between two inverted flags, and the phase difference depended linearly on the separation distance. Increases in the separation distance and Reynolds number led to decoupling behavior. Local Nusselt number ratio peak regions were found behind both the front and rear flags, and the heat transfer performance behind the rear flag was superior to that behind the front flag. The global Nusselt number ratio was not sensitive to the separation distance and Reynolds number for tandem flag configurations. Increasing Reynolds number reduced the mechanical loss, so operating the tandem inverted flags’ system at a high Reynolds number moderately improved the heat transfer efficiency.