Three-dimensional organization of the flow structure in a non-reactive model aero engine lean burn injection system

• Tomo-PIV investigation of the flow field of a non-reactive lean burn injector system.
• The confinement increases the Swirl number (free: 0.90; confined: 1.27).
• Spreading angle of confined jet is twice larger than for free jet.
• The precessing vortex core is characterized by azimuthal waves before its breakdown.
• The most energetic POD modes spotlight the 3D helical vortices in shear layers.

An investigation of the three-dimensional flow field of a turbulent swirling jet at Re = 50 × 103 generated by a non-reactive model aero engine lean burn injector is carried out in a water facility with tomographic Particle Image Velocimetry. This work is focused on the organization of the coherent structures arising within the near field of the swirling jet both in free and confined configurations. The confinement causes an increase of the Swirl number: the measured values are equal to 0.90 and 1.27, respectively for free and confined swirling jets. The effects of the confinement induce a larger spreading of the swirling jet promoting the enhancement of turbulence at the nozzle exit, but the expected upstream displacement of the reverse flow region is not observed. The instantaneous flow field is characterized by the presence of the precessing vortex core (PVC), of the outer helical vortex and of smaller turbulent structures perturbing the structure of the PVC. A three dimensional modal analysis of the velocity field using the Proper Orthogonal Decomposition (POD) highlights that the flow is dominated by the precessing vortex core. Using the first two POD modes a low order reconstruction of the velocity field is calculated. It is found that the small-scale structures shown in the instantaneous velocity field are not captured in the low order reconstruction due to smoothing effects, but the precessing vortex core and the outer helical vortex are properly represented.