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A comparative study on the enhancement mechanism by various twisted tapes (TTs) in association with the local flow pattern is investigated in detail via computational fluid dynamics (CFD). The effect of gap between TT and tube is further examined based on prior experimental validation. Result shows that the Nusselt number and friction factor will be increased by 12.6% and 18.1%, respectively, and the comprehensive thermal performance (CTP) will be improved from 1.05-1.07 to 1.12-1.15 when the gap is reduced from 1.0 mm to zero. The obtuse V-cut design in the experimental study shows superior performance, and the related enhancement mechanisms by windward, isosceles, and leeward V-cut configurations are numerically investigated. By comparing the local streamline at V-cut regions and the velocity distribution on the cross section, the windward V-cut might notably introduce fluid into the cut region and toward the tube wall subsequently, thereby resulting in effective heat transfer augmentation near the tube wall. Compared to the isosceles V-cut TT, the CTP of windward V-cut TT can be augmented from 1.15-1.18 to 1.16-1.21. However, the leeward V-cut TT will impair the CTP due to its inconsistent turbulence direction to the main helical flow. On the other hand, it is also indicated that fabricating three windward V-cuts at each helical pitch shows the highest improvement on CTP. Further increment of V-cut number may reversely reduce the CTP because of the rapid increase of flow resistance. Moreover, fabricating windward V-cuts on bilateral edges of the TT shows limited improvement. [ABSTRACT FROM AUTHOR]