Heat transfer and fluid flow behaviors in a tube with modified wire coils

Experimental turbulent heat transfer and flow resistance behaviors in a plain tube inserted with wire coils (WCs) were investigated by using air as working fluid. The thermal and pressure drop tests were performed respectively under uniform constant wall heat flux conditions and iso-thermal conditions with Reynolds number (Re) ranging from 6000 to 20,000. The various arrangements in WCs including WCs with uniform pitch (WCs similar to UP) at p/d = 0.172, 0.345, 0.517, 0.690, 0.862 and 1.034, WCs with varying pitch (WCs-VP) at p/d = 0.172-0.345, 0.172-0.517, 0.345-0.517, 0.172-0.345-0.517, 0.172-0.690-0.690 and 0.172-0.690 and WCs with gradually varying width (WCs-GVW) at Avid = 0.552-0.897-0.552 were studied. Effects of those parameters on heat transfer and flow characteristics in terms of Nusselt number (Nu), friction factor (f) and performance evaluation criterion (PEC) were investigated. The obtained results show that the WCs augment considerably the heat transfer and pressure drop over the plain tube and the Nu in WCs gives the range of 1.46-2.49 times that of the plain tube and the increased f ratio is in the range of 8.36-18.62, depending on varying Re and arrangements. The interesting heat transfer and flow behaviors are found in the WCs similar to UP with the largest Nu and highest f obtained respectively by p/d = 0.690 and p/d = 0.345 cases. The use of WCs-VP presents superior Nu than the WCs-UP with p/d = 0.172 while the adoption of WCs-GVW yields higher Nu than its counterpart of the WCs-UP with p/d = 0.345, however, both of these modified WCs exhibit lower Nu than the WCs-UP with p/d = 0.517. Due to the compound heat transfer and fluid flow mechanisms governed by the flow separation-reattachment, the second tube effect and the swirl flows, the WCs-UP with p/d = 1.034 gives the largest PEC of about 1.14 under considered operating and geometry conditions in the light of competition between heat transfer improvement and pressure drop enhancement. In addition, the use of the WCs-VP and WCs-GVW give no further improvement in PEC compared with the WCs-UP. Finally, the Nu and f empirical correlations within +/- 4.0% and +/- 7.0% from experimental values are constructed and agree well with some published data. (C) 2018 Elsevier Ltd. All rights reserved.