Influence of Inlet Swirl on the Aerodynamics of a Model Turbofan Lobed Mixer

This paper presents a detailed experimental investigation of the influence of core flow inlet swirl on the mixing and performance of a 12-lobe un-scalloped turbofan mixer. Measurements were made downstream of the mixer in a co-annular wind tunnel. The core-to-bypass velocity ratio was set to 2:1, temperature ratio to 1.0, and pressure ratio to 1.03, giving a Reynolds number of 5.2×10⁵, based on the core flow inlet velocity and equivalent hydraulic diameter. In the core flow, the background turbulence intensity was raised to 5% and the swirl angle was varied using five vane geometries, with nominally uniform swirl angles of 0°, 5°, 10°, 20° and 30°. Flow measurements captured flow structures involved in the mixing process. Most of mixing took place immediately downstream of the exit nozzle. The vane wake slightly enhanced large scale mixing of streamwise vortices. At low swirl angles, mixing was found to be mainly due to the interaction between streamwise vortices and normal vortices. At high swirl angles, the lobed mixer acted similar to a guide vane and removed most of the inlet swirl between the crest and trough of the mixer. However, the upstream swirling flow persisted in the core region between the center-body and lobed mixer trough, causing a reverse flow zone downstream of the centre-body. As the reversed flow became larger with increasing swirl, the swirling flow in the core region moved radially outwards and further interacted with the outer region flow. The stronger interaction of streamwise vortices with normal vortex improved mixing from the trough to the crest of the lobed mixer. The balance between enhanced mixing and increased reversed flow downstream of the centre-body, resulted in increased overall total pressure losses with increasing inlet swirl angles.