Measurement of flow structures and heat transfer behind a wall-proximity square rib using TSP, PIV and split-fiber film

Abstract

In the present study, complementary measurement techniques—temperature sensitive paint (TSP), planar particle image velocimetry (planar PIV) and a split-fiber film probe—were used to investigate the effects of a “wall-proximity square rib” on flow structure and surface heat transfer augmentation. TSP was used to measure the time-averaged wall temperature field at three different Reynolds numbers ($Re_{d}$ = 3800, 7600 and 11,400) based on the rib height d and the mainstream velocity $U_{o}$, and wall-proximity configurations with four different gap ratios (gap size $G$ over rib height d), G/d = 0, 0.25, 0.50 and 0.75. The two-dimensional distribution of the normalized Nusselt number convincingly demonstrated the existence of a hot spot immediately behind the rib in the attached rib configuration (G/d = 0) and surface heat transfer augmentation in the reattachment zone. Among the three wall-proximity configurations, G/d = 0.25 resulted in maximum heat transfer augmentation immediately behind the rib and overall improvement in surface heat removal. However, no distinctly different spatial patterns of the normalized Nusselt number distribution were found at the three different Reynolds numbers. A subsequent experiment examined the flow pattern and flow structures at $Re_{d}$ = 7600 and three wall-proximity configurations (G/d = 0, 0.25 and 0.50). Velocity field measurements using PIV, along with complementary measurements using split-fiber film, gave a clear view of the flow pattern behind the rib; a very slender separation bubble with highly unsteady flow reversal was found close to the surface when G/d = 0.25. For configurations with G/d = 0.25 and 0.50, the high-speed jet issuing from the gap had a complicated influence on the interaction between the upper free shear layer and lower strong shear layer, resulting in slanted movement of the coupled wake flow. Proper orthogonal decomposition was used to identify the spatial characteristics of the superimposed flow structures. In the configuration G/d = 0.25, coherent structures were found throughout most of the wake region, beginning roughly 1.2d behind the rib. Fluid flow in the region near the wall was strongly influenced by the dominant coherent structures, which were evidently the main mechanism for surface heat removal. In the configuration with G/d = 0.50, the area with large velocity fluctuation intensity shifted away from the wall toward the mainstream due to rapid expansion of the wall jet. This, in combination with large-scale coherent structures from the surface, meant a much reduced influence on the fluid near the wall and a corresponding deterioration of surface heat transfer beyond the station x/d = 4 (x being the downstream distance from the rib’s trailing edge).