Dimpled Channel Results

The data plots for the dimpled channel cases are presented in Figures 1.9-1.11. Figure 1.9

shows the stationary dimpled channel results. An enhancement of approximately 2.0 is produced by the dimpled surface. This is in close agreement with the results of Moon et al. [10]. The smooth side surfaces (inner and outer) appear to only benefit marginally from the mixing induced by the dimpled surfaces. It is shown that for Reynolds numbers 10000, 20000, and

40000, there appears to be almost no dependence upon Reynolds number. This observation was also made in the past by Moon et al. [10]. However, when the data for Re=5000 is considered, we see the emergence of Reynolds number dependence, with the enhancement decreasing with decreasing Reynolds number, but only at the lowest Re value. Perhaps this is because the Reynolds number is much closer to the laminar-to-turbulent flow transition region of [email protected] Whatever the reason, such a low Reynolds number is not encountered in gas turbines, therefore consideration of this Reynolds number effect is only necessary for those wishing to consider the use of dimples for some other application outside of gas turbine heat transfer.

Figure 1.10 presents the data for the dimpled channel under rotation, orthogonal to the plane of rotation (J = 90°). It is apparent that there is a definite enhancement due to rotation for the trailing surfaces, which increases with increasing rotation number. Symmetry is achieved relatively well between the two spanwise segments of each dimpled surface, and symmetry between the two side surfaces. Also, the side surfaces experience enhancement equal to the leading surface. This is completely different than the smooth case, where the side surfaces were more equal to the trailing surface.

Figure 1.11 shows the results for the dimpled channel under rotation, twisted with respect to the plane of rotation (J = 135°). The trailing-outer surface is enhanced more than the other surfaces, as it benefits from both the shifting of the cold flow toward the outer half of the channel, as well as the local mixing induced by the vortices shed by the dimples. In addition, the trailing-inner and leading-outer surfaces are enhanced (although to a lesser degree) by rotation. This occurs as the Coriolis vortex also serves to distribute some of the cold fluid in the core of the mainstream flow to the other surfaces, allowing the smaller scale dimple induced vortices to capture some of this cold fluid and pull it near to the wall. The outer surface is enhanced more than the inner surface due to the shifting of the majority of the colder flow toward the outer half of the twisted channel under rotation. This behavior was also seen in the smooth duct.

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