The boundaries of the domains of existence of flow regimes past single dimples made as spherical segments on a flat plate are determined with the use of available experimental results. Regimes of a diffuser-confuser flow, a horseshoe vortex, and a tornado-like vortex in the dimple are considered. Neither a horseshoe vortex nor a tornado-like vortex is observed in dimples with the relative depth smaller than 0.1. Transformations from the diffuser-confuser flow regime to the horseshoe vortex regime and from the horseshoe vortex flow to the tornado-like vortex flow are found to depend not only on the Reynolds number, but also on the relative depth of the spherical segment. Dependences for determining the boundaries of the regime existence domains are proposed, and parameters at which the experimental results can be generalized are given.Key words: dimple, vortex structures, horseshoe and tornado-like vortices, enhancement of heat transfer.
Introduction.A large amount of experimental data on specific features of the flow structure and heat transfer in the flow past surfaces with spherical cavities (dimples) has been accumulated. There are some summary papers where numerous aspects of vortex formation in dimples under the action of a large number of factors were considered [1-6]. It was found in the first experiments that the increase in heat transfer in the flow past a surface with dimples made as spherical segments is accompanied by an insignificant relative increase in hydraulic resistance [7]. Large-scale dynamic vortex structures observed in a wide range of flow regimes are formed inside the dimples and in their vicinity. Despite a large number of publications dealing with using surfaces with dimples, however, some disputable issues have not been resolved (hypothesis of the tornado-like mechanism of heat-transfer enhancement [7,8], role of dimples in flows with high turbulence levels [9, 10], and determining the boundary between the domains of existence of separated and non-separated flows past spherical cavities [11,12]). This is explained by the complexity of the phenomenon, which is affected not only by the flow velocity, but also by the dimple size, shapes of its edges, character and thickness of the boundary layer at the dimple entrance, and channel height.Here, we make an attempt to determine the boundaries of the domains of existence of flow regimes in the dimple, depending on the external flow velocity and on the geometric characteristics of the dimple, by means of analyzing available experimental data. Similar studies were performed previously (see, e.g., [11,13]), but the previous results can now be refined because a large amount of new data appeared.Regimes of the Flow Past Dimples. The following flow regimes past a surface with a dimple can be identified. As low velocities, the flow has a diffuser-confuser character. The streamlines passing close to the dimple become curved. As the flow velocity is increased, the flow becomes separated near the leading edge of the dimple, forming a recirculation zone,...
