INVERTED DELTA WING VORTEX GENERATORS FOR LOCAL HEAT TRANSFER ENHANCEMENT IN HIGH-ASPECT-RATIO CHANNELS IN LAMINAR FLOWS

The paper reports the effectiveness of an inverted delta wing vortex generator (VGs) for local enhancement in heat transfer in high-aspect-ratio ducts under laminar flow. The delta wing is attached by the tip to the surface opposite to the target surface thus resulting in an "inverted" orientation leaving a small clearance between the target wall and the base of the delta wing. Experimental studies reported the occurrence of a "double peak" denoting two distinct regions of enhanced heat transfer, a novel finding corroborated by the numerical results. The first peak was observed due to flow acceleration in the gap between the delta wing base and the heated surface while the second peak, having a larger zone of impact, was observed further downstream caused by vortex shedding. The average enhancement in heat transfer was numerically predicted to be 2.8 for the optimal configuration w.r.t. the channel without VG. The optimal configuration corresponded to a nondimensional clearance (<i>e/H</i>) of 0.1 and an angle of attack (&alpha;) of 30&deg;. The overall performance of the VG designs was captured through a figure of merit that incorporates the effects of both heat transfer enhancement as well as pressure drop penalty. Finally, investigations on the effect of two axially-spaced inverted delta wings showed an interesting "triple peak phenomenon" for optimal spacing of <i>S/H</i> &#61; 2 and four peaks in Nusselt enhancement for <i>S/H</i> &ge; 5 mm thus suggesting means of cooling multiple hot spots or expanding the zone of impact.