Results from un-forced experiments in flows ensuing from circular and equivalent square coaxial nozzles with parallel sides are presented in this paper. The nozzles are contoured and are designed so that the hydraulic diameters of the internal flow passages are identical for both geometries. The flow experiments were conducted at a co-flow-jet Reynolds number of Re = 16,000 and inner-to-outer jet nominal velocity ratios of λ = 0, 0.5, 1.5. Axis switching, a phenomenon readily observed in single non-axisymmetric nozzles, is shown for the first time to occur in the square coaxial nozzles as well. Comparisons of the mixing regions of the flows from both geometries are made to examine mixing advantages when using square nozzle configurations. Comparisons of stream wise mean velocity fields measured on a center plane parallel to the square nozzle sides, on a diagonal plane of the square nozzle and the center plane of the corresponding circular nozzle, are presented and discussed. Axis switching is shown to be evident in the near-field shear regions for all velocity ratios, resulting in considerable mixing advantages. The spreading rates (and therefore mixing rates) of the outer mixing region of the square nozzles clearly exceed the spreading rate observed in the circular case on the central plane. Axis switching and improved mixing is also observed in the inner mixing region of the square nozzle. This work is relevant to coaxial nozzles for gas turbine combustor applications, although the study has been carried out in a scaled up geometry with respect to this application.

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