We report on the experimental study of turbulent transport in the near field of swirling jets at Re = 5000 by using simultaneously the stereoscopic particle image velocimetry and planar laser-induced fluorescence methods. The present focus is on the analysis of terms of the mean (axial) momentum and scalar (mass) transport equations, estimated from the mean velocity and concentration, the Reynolds stress and turbulent flux components, measured in the central axial plane of the jets. A proper orthogonal decomposition has also been applied to the velocity data sets to analyse contribution of the coherent velocity and scalar fluctuations to the turbulent momentum and mass fluxes for different swirl rates.

We report on a study of the impact of coherent helical vortex structures on the shape of the reaction zone and heat release in swirling methane/air flames in regimes with a vortex breakdown. Three kinds of atmospheric flames are considered, viz., fuel-lean and fuel-rich premixed flames and a partially premixed fuel-rich lifted flame. Based on the measurements of the velocity fields by a stereo PIV in combination with the OH PLIF and HCHO PLIF, the impact of the coherent flow structures on large-scale corrugations of the reaction zone is evaluated. Helical vortex structures, detected in both the non-reacting and reacting high-swirl flows by using proper orthogonal decomposition, are found to promote combustion both in the lean premixed and fuel-rich partially premixed flames. In the first case, based on the phase-averaged intensity of the HCHO×OH signal and the location of the helical vortex structure in the inner mixing layer, it is concluded that the vortex locally increases the heat release rate by enlarging the flame front and enhancing the mass exchange between the combustion products inside the recirculation zone and the fresh gases. The events of the local flame extinctions are detected in the instantaneous PLIF snapshots for the lean mixture, but they do not cause extinction of the entire flame or a blow-off. In case of the lifted flame, the outer helical vortex structure promotes combustion by locally intensifying the mass exchange between the fuel-rich jet with the surrounding air.

The paper reports on a parallel study of the dynamics of large-scale vortical structures in non-swirling, low-swirl and high-swirl turbulent jets by means of the time-resolved tomographic PIV technique. 3D realizations of the instantaneous velocity and velocity gradients, local strain and rotation rates, helicity and pressure fluctuations are analyzed. Deformation of the large-scale vortices is studied by evaluating the local stretching/compression rates and local values of the Lamb vector divergence. Linear stability analysis is also applied to the mean velocity profiles in order to identify the most unstable azimuthal modes.

We report on a parallel study on properties of large-scale vortical structures in low- and high-swirl turbulent jets by means of the time-resolved tomographic particle image velocimetry technique. The high-swirl jet flow is featured by a well-established bubble-type vortex breakdown with a central recirculation zone. In the low-swirl flow, the mean axial velocity, while intermittently acquiring negative values, remains positive in the mean but with a local velocity defect immediately downstream from the nozzle exit, followed by a spiralling vortex core system and its eventual breakdown. Measurements of the 3D velocity fields allowed direct analysis of the azimuthal/helical modes via Fourier transform over the azimuthal angle and proper orthogonal decomposition (POD) analysis in the Fourier space. A precessing vortex core is detected for both swirl cases, whereas the POD analysis showed that the one originating in the bubble-type vortex breakdown is much more energetic and easier to detect.