Heat transfer enhancement in tubular heat exchanger with double V-ribbed twisted-tapes

An experimental work on heat transfer enhancement in a round tube by insertion of double twisted tapes in common with 30 V-shaped ribs has been conducted. Air as the test fluid flowed through the test tube having a constant wall heat-flux with Reynolds number (Re) from 5300 to 24,000. The combined vortex generators (called V-ribbed twisted tape) were obtained by incorporating V-shaped ribs into the edges of double co-twisted tapes having a similar twist ratio of 4. The effect of pertinent V-rib parameters such as four relative rib heights, (called blockage ratio, BR=b/D= 0.07, 0.09, 0.14 and 0.19) and a relative rib pitch, (P-R=P/D=1.9) at an attack angle of rib, alpha=30 degrees on thermal characteristics was investigated. The experimental results reveal that the heat transfer and pressure drop in terms of the respective Nusselt number and friction factor for the V-ribbed twisted tapes show the increasing trend with the rise of Re and BR. The V-ribbed twisted tape with BR=0.19 yields the highest heat transfer and friction factor. However, the maximum thermal enhancement factor is about 1.4 for the V-ribbed twisted tape at B-R=0.09 but is around 1.09 for the twisted tape with no rib. (C) 2016 The Authors. Published by Elsevier Ltd.