Mechanism of Unsteady Flow Heat Transfer and Its Modelling

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Experimental study was made for an enlarged model of offset fin array to be used in heat exchagers and it reveals that, under self-sustained oscillation flow regime, one-point second-order velocity cross-correlation -uv^^- takes positive value in the wakes of fins and that its sign becomes negative around the downstream fin surface. This means that unstable flow induced in the wake is stabilized around the downstream fin and that momentum transfer is suppressed there in contranst with the heat transfer enchancement occuring at the same positions. Therefore, dissimilarity between momentum transfer and heat transfer is generated. Two-dimensional unsteady numerical computation was applied to the same type of flow of and was used to analyze the mechanism of the enchancement of fin heat transfer and of the dissimilarity between momentum and heat transfer processes. Each of counter-rotating vortices appearing in the wake entrains fresh fluid from main stream into the wake and pumps out the hot and low-speed fluid from the central part of the wake to the outside. These are effective to accelerate the recovery of the velocity defect and temperature excess int the wake. Each vortex hits the downstream fin and exerts "washing action" to the fin surface. These result in the enhancement of fin heat transfer. The washing action of each vortex produces the dissimilarity. The same numerical computation was applied to other types of flows of different geometries. In the case of channel flow obstructed by an inserted square rod, crisscross motion of Karman vortex was found. This was confirmed to actually occur through flow visualization. Unsteady flow incurred downstream of the rod enhances the wall heat transfer. The mechanism of this heat transfer enhancement is also related to the washing action of an isolated vortex produced near the wall. All these studies suggest that new idea of implementing the intermittent, washing action of vortex is important in the modelling of un