Summary And Conclusions

Spatially-resolved Nusselt numbers and flow structure are presented for a stationary channel with an aspect ratio of 4 and angled rib turbulators inclined at 45 ° with perpendicular orientations on two opposite surfaces. The flow structure results include time-averaged distributions of local streamwise vorticity, local streamwise velocity, local total pressure, and local static pressure, surveyed over flow cross-sectional planes, as well as flow visualization images and friction factors. Results are given at different Reynolds numbers based on channel height from 270 to 90,000, and ratios of air inlet stagnation temperature to surface temperature ranging from 0.93 to 0.95. The ratio of rib height to hydraulic diameter is .078, the rib pitch-to-height ratio is 10, and the blockage provided by the ribs is 25 percent of the channel cross-sectional area.

Spatially-resolved local Nusselt numbers are highest on tops of the rib turbulators, with lower magnitudes on flat surfaces between the ribs, where regions of flow separation and shear layer re-attachment have pronounced influences on local surface heat transfer behavior. Also important are intense, highly unsteady secondary flows and vortex pairs (evident in flow visualizations and time-averaged surveys of local flow structural characteristics), which act to increase secondary advection and turbulent transport over the entire channel cross-section.

Because of these phenomena, local and spatially-averaged rib turbulator Nusselt numbers, normalized by values measured in a smooth channel, generally increase on the rib tops as Reynolds number increases. These Nusselt number ratios then decrease on the flat regions away from the ribs, especially at locations just downstream of the ribs, as Reynolds number increases. Because of the normalization employed, this means that the observed Nusselt number increases with Reynolds number are slower than smooth-surface baseline Nusselt number increases with Reynolds number on flat regions, and more rapid on the ribs themselves. Such changes are partially due to increases in the size and strength of the flow re-circulation region, and the shear layer associated with it, as the Reynolds number increases. In both cases, local and spatially-averaged Nusselt number ratios are generally much higher than 1.0 on most of the test surface, including the flat regions between the ribs, irrespective of the value of Reynolds number employed.

Globally-averaged Nusselt number ratios vary from 3.36 to 2.82 as Reynolds number increases from 10,000 to 90,000. Thermal performance parameters also decrease somewhat as Reynolds number increases over this range, with values in approximate agreement with, or slightly higher than 60 ° continuous rib data measured by Han et al. [4] in a square channel. Such performance parameter magnitudes and Nusselt number variations are due augmented three-dimensional turbulence transport and increased secondary flow advection, as well as the pressure losses and friction factors produced by the form drag which develops around the rib turbulators

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