Evaluation of Swirl and Tabs in Short Annular Diffusers
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Short annular diffusers were essential components for turbomachines that have been used to expand the air entering the compressor, as interstage ducts between gas generators and power turbines, and on the exhaust gases exiting the turbine. The industrial community was interested and invested in improving diffuser design that was challenging owing to the unfavourable fluid flow effects. Efficient design of fluid flow devices was possible through the complementary use of experimental testing and computational fluid dynamics (CFD). A numerical shape optimization study was undertaken to determine preferential annular diffuser configurations. Experimental data were compared against CFD that simulated the steady-state Reynolds-averaged Navier-Stokes equations with two-equation turbulence models. This investigation reached equivalent conclusions with respect to the influences associated with diffuser geometry and swirl. Vorticity effects caused by square tabs, that were not as well understood, were investigated. The tabs were effective in reducing the central toroidal recirculation zone created by a swirling flow, but at a static pressure penalty for the area ratio, AR<2.73, diffusers tested. Results identified several shortcomings in the CFD that typically over-estimated pressure recovery and outlet velocity uniformity; however, properly qualitatively predicted wall pressure distributions and outlet velocity profiles. The use of CFD on modest grids, with preference given to the realizable k-epsilon turbulence model, for annular diffusers that have length to inlet height ratio of 12 and at least AR=2.73 with up to 20-degrees inlet swirl was encouraged as a design tool.