Mixing of multiple jets with a confined subsonic crossflow

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This paper summarizes experimental and computational results on the mixing of single, double, and opposed rows of jets with an isothermal or variable temperature mainstream in a confined subsonic crossflow. The studies from which these results came were performed to investigate flow and geometric variations typical of the complex three-dimensional flowfield in the dilution zone of combustion chambers in gas turbine engines. The principal observations from the experiments were that the momentum-flux ratio was the most significant flow variable, and that temperature distributions were similar, independent of orifice diameter, when the orifice spacing and the square-root of the momentum-flux ratio were inversely proportional. The experiments and empirical model for the mixing of a single row of jets from round holes were extended to include several variations typical of gas turbine combustors, namely variable temperature mainstream, flow area convergence, noncircular orifices, and double and opposed rows of jets, both inline and staggered. All except the last of these were appropriately modeled with superposition or patches to the basic empirical model. Combinations of flow and geometry that gave optimum mixing were identified from the experimental and computational results. Based on the results of calculations made with a three-dimensional numerical model, the empirical model was further extended to model the effects of curvature and convergence. The principal conclusions from this study were that the orifice spacing and momentum-flux relationships were the same as observed previously in a straight duct, but the jet structure was significantly different for jets injected from the inner wall of a turn than for those injected from the outer wall. Also, curvature in the axial direction caused a drift of the jet trajectories toward the inner wall, but the mixing in a turning and converging channel did not seem to be inhibited by the convergence, independent of whether the contraction was radial or circumferential. The calculated jet penetration and mixing in an annulus were similar to those in a rectangular duct when the orifice spacing was specified at the radius dividing the annulus into equal areas.