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Short annular diffuser systems consisting of a conical expansion section with negative wall angles and a solid diffusing section were tested experimentally and numerically. Three centre bodies with different wall angles and three outer walls with area ratios with respect to the annular diffuser inlet of 1.65, 1.91, and 2.74 were manufactured. The designs were selected from a set of ideal solutions determined by a numerical multiobjective optimization study completed by Cerantola and Birk (2012). Results presented in this paper are Reynolds number independent and are based on tests completed with an inlet Reynolds number of Ret ≈ 1.4 × 105 and Mach number Mt ≈ 0.16. Through considering the various centre body and outer wall configurations, an initial flow angle of 14° provides the best performance and the larger diameter outer wall generates more static pressure recovery at the expense of a reduction in outlet velocity uniformity and greater total pressure loss. The three selected designs were found to be in agreement with their optimum design qualities. When compared to computational solutions, the realizable k-ε turbulence model on coarse grids delivered the best relative comparison whereas the SST turbulence model predicted the best absolute agreement.

A genetic algorithm was implemented to determine preferential solutions of a short annular diffuser exhaust system of length 1.5Do (outer annulus diameters). Five free variables defined the centre body shape and two variables determined the outer wall profile. Diffuser performance was evaluated using three objectives—(i) diffuser pressure recovery, (ii) outlet velocity uniformity, and (iii) total pressure loss—that were calculated from steady state solutions obtained using the computational fluid dynamics software FLUENT 13.0 with the realizable k-ε turbulence model and enhanced wall treatment. Inlet conditions were ReDh = 8.5 × 104 and M = 0.23. After thirty-five generations, a paraboloid-shaped centre body with length 0.74Do and initial annular expansion of approximately 14° produced preferential solutions. A configuration with a converging outer wall above the centre body developed greater outlet flow uniformity and lower total pressure loss whereas a straight outer wall followed by the solid diffuser generated more static pressure recovery.

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