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Show Spherical dimples as vortex enhancers of heat transfer have attracted a considerable attention of investigators for the past 10 years [6]. Experimental results that have been previously reported [6,7] show that spherical dimples significantly (up to 5 times) enhance the total heat transfer from the surface whereas the friction losses increase insignificantly. As this takes place, the heat transfer enhancement increases with the increasing a dimple depth and a density dimple distribution on the surface, as well as with the increase of a channel constriction. So, if the relative depth of the dimple is h/d=0.3, the heat transfer may be increased twice than that of the smooth surface. In this case the hydraulic resistance of the channel increases only twice. Investigations also show that the total resistance of the surface increases at the dimple depth increasing. Spherical dimples on aerodynamic bodies show astonishing, unpredictable by theory, hydrodynamic effects. So, for the same impact force, a golf ball with spherical dimples on its surface flies 5 times longer distance than the same smooth-surface ball [3]. Dimples on a cylinder surface result in considerable, up to 1.5 times, decrease of the total resistance of the cylinder [4]. Moreover, the effect of shallow dimples is more essential than that of the deep ones. In order to look insight the physics of the heat transfer enhancement and essential decrease of the resistance the surface and bodies with the spherical dimples, many investigators attempted to study in flow structure and heat transfer in dimples in more details. Following conclusions can be drawn from the investigations pursued. When fluid or gas flow streamlines the surface of deep dimples (h/d>0.2), self-organization of large scale vortex structures takes place [6,8]. Vortex formation epicentres are inside the dimple, in the upstream part of the dimple (Figure 1). There are 2 such epicentres. The vortex structures are alternately ejected from the epicentres onto the free stream. It is because of these vortices that an intensive heat and mass transfer from the dimple occurs. Figure 1. F L O W P A T T E R N IN HEMISPHERICAL DIMPLE (ACCORDING T O T H E E X P E R I M E N T A L D A T A [7]) As shown in paper [8], the secondary reverse flows are formed in dimples: that is the air volume entering the dimple is transported from the rear edge of the dimple upward the free stream direction. Then, in the front part of the dimple, the air is swirled and transported from it. The air ejected from the dimple is partially captured into it again forming a closed recirculating contour. Maximal velocity of the reverse flow comprises 0.4 of the velocity of the free stream. 2 |