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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.

In this paper, the influence of outlet arrangement and plenum structure on impingement cooling is experimentally and numerically investigated in a typical 1-U confined server space. Three outlets include Z-type, bilateral, and U-type arrangements, and the plenum configurations contain partially inclined, fully inclined, and staged layouts. As a result, using the U-type outlet or staged plenum may prominently compromise the impingement cooling performance on the target plates with lower pumping power. With numerical investigation, it is found that, for the case with Z-type outlet, the flowrate of jet impingement increases alongside the streamwise direction. Besides, the impingement stagnation region on target plates with the minimum thermal resistance may shift toward the outlet. Meanwhile, the uniformity of jet impingement can be improved by 10.7% and 50.3% when the bilateral and U-type outlets are applied, respectively, and the jet impingement is changed to perpendicular direction due to the opposite cross flow from the coming flow direction. On the other hand, by applying the inclined plenum and staged plenum, the uniformity of jet impingement can be dramatically improved by 113.9% and 215.1%, respectively. However, the local jet impingement velocity distribution is still nonuniform. Hence, a novel design of impingement plate based on the concept of Coanda effect is proposed. The peak value of the thermal resistance on target plate can be reduced by 21.8% and 16.0% at the center region and the fore part of the jet array.