Radial Swirlers and Mixing Tube Assembly Aerodynamics and Performance Parameters Evaluation: Part 2 - LDV Measurements at the Mixing Tube Outlet

Present work represents the second part of a two parts paper. The complex aerodynamic phenomena, which take place both inside and downstream the swirler - mixing tube assembly, have been studied with different and complementary techniques in order to have a complete characterization of the effect of the geometrical parameters variation on many aspects of the flow. The first part was focused on the aerodynamic analysis of the flow in the swirler exit plane, while this second part is focused on the experimental investigation on the flow inside and downstream the mixing tube. Measurements have been performed on three axial stations. One is placed inside the mixing tube, in order to characterize the flow arriving from the swirler. The other two are placed downstream the mixing tube discharge section in order to identify the main flow features, such as the presence of flow recirculation and consequently the radial extension of the separation bubble. A four beams two colours Laser Doppler Velocimeter (Dantec Fiber Flow), in backward scatter configuration mounted on a three-axis computer-controlled traversing mechanism, was employed for the present study. Results indicate a strong influence of the geometrical parameters on the flowfield in terms of velocity component distributions, turbulence intensity and separation bubble extension. Moreover results from the Laser Doppler Velocimeter measuring campaign allow basically to evaluate three main performance parameters which are the friction loss coefficient, the semi-theoretical swirl number evaluated from the assembly geometry and the measured swirl number, which is calculated in the mixing tube outlet section. The first one is indicative of the angular momentum dissipation, while the other two represent the main parameters which are involved in the vortex breakdown onset and evolution, that in one case is derived from measurements and in the other is partially theoretical.