Interaction between inverted flags: Application in heat transfer enhancement

Abstract

This paper investigates the dynamics of inverted flags and their effectiveness in enhancing heat transfer in a tube bank heat exchanger. Fluid-structure interaction and the interaction between two flags in one row lead to fascinating oscillatory behavior. Two inverted flags demonstrate a preferred flapping mode at different air speeds, with occasional switching between in-phase and out-of-phase vibrations. In-phase vibrations are favored at the onset of large-amplitude flapping, but the lock-in behavior weakens as the gap distance increases or the airflow speed rises. In terms of flapping frequency, constructive interference between the flags results in a 9 % enhancement, while out-of-phase flapping reduces the frequency by up to 13.5 %. The application of inverted flags placed in front of tubes in a tube bank was studied experimentally. The strong tip vortices generated by the free ends improve convective heat transfer by approximately 12 % for a single flag and for dual-flag and four-flag systems 20 % and 13 %, respectively. However, heat transfer enhancement diminishes in the fully deflected mode, approaching the performance of a tube bank without flags. Pressure drop measurements show <8 % increases for a single flag and almost 12 % and 16 % for dual-flag and four-flag systems, respectively. The volume goodness factor, assessing the hydrothermal performance of the heat exchanger, shows a 16.6 % improvement for longer flags and 13.7 % for shorter ones.