Particle image velocimetry and planar laser-induced fluorescence measurements on lobed jet mixing flows

An experimental investigation of the vortical and turbulent structures in lobed jet mixing flows was conducted. The techniques of planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) were used to accomplish flow visualisation and velocity filed measurements of the lobed jet mixing flows. Compared with a conventional circular jet flow, the lobed jet mixing flows were found to have a shorter laminar region, a smaller scale of spanwise Kelvin-Helmholtz vortices, quicker transition to turbulence and earlier appearance of small-scale vortical and turbulent structures. The intensive mixing of the Corp jet flow with ambient flow was found to concentrate within the first two nozzle diameters in the lobed jet mixing flow. More rapid growth of the shear layer at the near field and quicker decay of the central line velocity were also found in the lobed jet mixing flow. All these indicated a better mixing enhancement performance of the lobed nozzle compared with the conventional circular nozzle in the near-field region. Based on the PLIF and PIV results, two aspects of the mechanism of mixing enhancement in a lobed jet mixing flow were suggested. One is that a lobed nozzle can cause big azimuthal perturbations in the jet flow due to its special geometry, and the streamwise vortices produced by the lobed nozzle can enhanced the azimuthal perturbations. The "cut and connect" process of the large-scale spanwise Kelvin-Helmholtz vortex rings was accelerated. This is responsible for the avalanche of three-dimensional and smaller-scale motions and the generation of high turbulence. Another is that the "stretch effect" of streamwise vortices generated by the lobed nozzle on the spanwise Kelvin-Helmholtz vortical rings reduced the scale of the spanwise Kelvin-Helmholtz vortices, which also results in the creation of much small-scale intense turbulence and enhances the mixing of the core jet flow with the ambient flow.