Experimental Drag and Mixing From Lobed Nozzles Under High Swirl Conditions From a Ducted Fan

Abstract

Abstract The availability of aerodynamic performance and vorticity production data from mixers under swirl was a challenge for future full-scale design and CFD validation. This paper presents an experimental comparison of drag and mixing performance of a circular trialing edge, lobed nozzle and scalloped nozzle under high swirl conditions as produced by a ducted fan in a subsonic wind tunnel. The design methodology is shared in detail allowing for geometry reproduction. Swirl angles produced by the fan naturally varied between 12° up to 45° according to a free-vortex profile. Performance is compared in terms of net thrust, uniformity factor and vorticity production as measured by 6-component loadcells and a seven-hole pressure probe traverse. The goal of this work was threefold: to study the axial and normal vorticity production from mixers produced by the design methodology, a preliminary investigation into lobed mixers potential in engine plume cooling and to provide a data set for RANS-CFD validation. A better understanding of lobed mixer mixing mechanisms relative to performance is offered. It is shown that the change in minimum throttle to achieve forward thrust varied between devices as did the twist load due to angular forces. A 4% reduction in required fan power to achieve forward thrust was achieved with the lobed mixer. Furthermore, maximum net thrust increased up to 20% with the mixing nozzles compared to the standard round nozzle suggesting flow straightening can lead to thrust gain in high swirling jet flows to a level that counters increases in drag. Axial and normal vorticity were clearly identified. Co-rotating vortex pairs were produced by the mixers of physical size proportional to the lobe height and wavelength. Axial vorticity levels integrated up to 110% of the round nozzle and occupied 5 times the area. Similarly, integrated normal vorticity increased up to 80% over an area 120% larger. Uniformity factor was best for the scalloped mixer due to enhanced mass flow entrainment through the notch and not the vortex production itself.