|Abstract:||Offset-strip fins are used in many compact heat exchanger applications because of excellent thermalhydraulic
performance. Enhancing the air-side heat transfer performance for offset-strip fins can lead to smaller,
more efficient heat exchangers. In this research, an innovative concept of generating streamwise vortices in offsetstrip
fin arrays has been investigated experimentally to seek further air-side heat transfer enhancement. Flow
visualization, PIV, naphthalene sublimation, and pressure drop measurements were performed for a baseline offsetstrip
fin array and six arrays enhanced with delta wing vortex generators (VGs) over a Reynolds number range
(based on hydraulic diameter) from 400 to 3700, to obtain comprehensive heat transfer enhancement and pressure
drop results and to develop a clear understanding of associated flow field mechanisms.
Array-averaged heat transfer enhancement is present for all VG-enhanced arrays even at very low Reynolds
numbers, and it increases with increasing Reynolds number. The heat transfer enhancement reaches a maximum at
Re @ 1000, with the largest enhancement being 32% for the 8VG at Rows 1 and 5 - enhanced array. The overall
enhancement in this low Re range is caused by streamwise vortices only. As Re is increased beyond 1000, the arrayaveraged
enhancement starts to decrease and reaches a minimum at Re @ 1630 for all enhanced arrays. Streamwise
vortices suppress spanwise vortex shedding along the paths of their travel, and shedding is either delayed to higher
Reynolds numbers or weakened in the regions adjacent to the streamwise vortices. This behavior causes the
decreasing trend of heat transfer enhancement in this Reynolds number range. For Re = 2040, the enhancement
returns and increases as Re increases. The flow in the downstream part of the array becomes chaotic and exhibits
turbulent-like features for both the baseline array and the VG-enhanced arrays at these high Reynolds numbers.
However, the heat transfer enhancement for the first three rows increases with increasing Re, due to the stronger
streamwise vortices at higher Reynolds numbers, which contributes to the overall enhancement return and the
increasing trend with Re.
The largest array-averaged heat transfer enhancement for the 2VG-enhanced array, 4VG-enhanced array,
and 4VG at Rows 1 and 5 - enhanced array is 7.6%, 16%, and 22%, respectively, which is accompanied by a
pressure drop penalty of 0.4%, 25%, and 57%, respectively.
The re-generated streamwise vortices in the middle of the array are found to be weaker and decay more
quickly in the flow direction than the streamwise vortices generated at the inlet of the array.