Secondary Flow Behavior

Figure 1.4 shows a conceptualization of the secondary flow patterns over a dimpled surface.

As the flow approaches the upstream portion of the dimple, flow separation occurs, and a recirculation zone appears in the upstream portion of the dimple resulting in mitigation of the heat transfer. As the flow reattaches at the downstream half of the dimple, an increase in the heat transfer enhancement occurs. Continuing in the streamwise direction, it has been shown that a large upwash region is produced by the dimple. This upward directed flow mixes to some degree with the cold core mainstream flow. Finally, pairs of vortices are shed along the dimple diagonals, enhancing the heat transfer on the flat portion of the surface. Considering the dimple induced secondary flows and superimposing the rotation induced secondary flow upon it, it is apparent that there is no primarily constructive combination of the two, as was shown in the case of a 45° angled-rib rotating channel investigated by Griffith et al. [8]. This complex combination of the dimple induced vortices in various directions with the rotation induced secondary flow does not generate any vision of a primary coherent flow structure, however the heat transfer enhancement is still increased at the trailing surface due to the thinning of the boundary layer under rotation.

Figure 1.5 shows a conceptualization of the secondary flow induced by rotating a smooth, rectangular channel. The Coriolis force induces two counter rotating vortices, which serve to push the colder fluid closer to the trailing surface. When the channel is twisted such that 3=135°, the linear distance of the Coriolis force main vector increases from a relatively small distance (as in the case of3=90°) to a much longer distance. The Coriolis vector now traverses the diagonal from the leading most corner to the trailing most corner of the twisted channel. This shifting of the rotation induced vortices serves to mix the flow more effectively.

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