Near nozzle PIV measurements and droplets size pattern of a GDI swirled spray

The aim of the present work is to analyze the effect of the injection pressure on the flow pattern and droplets size in a gasoline hollow-cone spray generated by a swirled injector, applying Particle Image Velocimetry (PIV) and Phase Doppler Anemometry (PDA). Experiments were carried out in the range of injection pressures between 6 and 10 Mpa using a common rail injection system, a commercial swirled type injector with a nozzle diameter of 0.50 mm and a cone angle of 67°. The investigation was carried out injecting the fuel in a chamber at ambient temperature and pressure. A 2-D imaging technique was used to follow the global evolution of the spray as function of the injection time in order to estimate the jet development, the morphology of the spray, and the instantaneous velocity field of fuel droplets. PIV images were captured, firstly, aligning the light sheet to the vertical axis of the spray; then, experiments were also taken with the light sheet placed through the cross section of the spray in order to explore the structure and velocity field, at different distances from the nozzle. A PDA system was used to acquire, simultaneously, the droplets velocity as well the droplets size. The system, equipped with an argon-ion laser, was set in forward scattering mode at an off-axis of 30°. Measurements were performed at the same operative conditions as for the visualization ones, choosing different distances from the nozzle. Droplets velocity and size data set provided a minimum of 40,000 in-cycle resolved valid data that were analyzed using the ensemble averaging technique. Results provided detailed information on the spatial velocity distribution and size of droplets close to the nozzle region. Ensemble averaged velocity field of fuel droplets obtained on the vertical plane through the injector axis showed high velocity values up to 100 m/s in the early stage of injection, whereas during the transition to completely developed main spray, droplets travel at lower velocities. The velocity field obtained on the cross section of the spray, at locations close to the nozzle (5 and 10 mm) and in the early stage of injection (up to 0.6 ms), showed a symmetric radial velocity field around the spray axis. Size measurements performed near the nozzle showed a D_<10> profile constant along the stationary period of injection and not affected by the injection pressure. Results also gave a global increasing trend of the temporally averaged mean diameter along the spray axis.