Flags as vortex generators for heat transfer enhancement: Gaps and challenges

The use of flexible plates or "flags" as vortex generators inside a channel was successfully demonstrated as an alternative heat transfer enhancement technique. This paper aims to present a brief review of flag vortex generators for thermal enhancement. Although flag dynamics is widely reported, the review reveals that this heat transfer technique is not widely explored, specifically on the heat transfer performance of flags. Extensive and intensive experimental results are lacking to validate numerical and theoretical predictions. This paper further provides a non-exhaustive list of existing gaps, challenges, and potential research areas in using flags as vortex generators for thermal enhancement, which aims to guide future research directions in this thermal fluid-structure problem. There is a need to conduct further investigations on this technique to fully establish its thermal characteristics.