CONJUGATE HEAT-TRANSFER OF A FINNED TUBE .B. HEAT-TRANSFER AUGMENTATION AND AVOIDANCE OF HEAT-TRANSFER REVERSAL BY LONGITUDINAL VORTEX GENERATORS

Numerical investigations of three-dimensional flow and heat transfer in a finned tube with punched longitudinal vortex generators (LVG's) are carried out for Reynolds numbers of 250 and 300. Air with a Prandtl number of 0.7 is used as the fluid. The flow is both thermally and hydrodynamically developing. The LVG is a delta winglet pair (DWP) punched out of the fin and is located directly behind the tube, symmetrically separated by one tube diameter. The DWP generates longitudinal vortices in the wake of the tube, defers flow separation on the tube, deflects the main stream into the tube wake, and strongly reduces the ''dead water zone.'' Heat transfer reversal is avoided by the DWP. Comparison of the span-averaged Nusselt numbers for the fin with and without DWP shows significant local heat transfer enhancement of several hundred percent in the tube wake. For Re = 300 and Fi = 200 the global heat transfer augmentation by a DWP, which amounts to only 2.5% of the fin area, is 31%.