Droplets size and velocity in a GDI spray by PDA and laser light extinction techniques

In this paper, tests on a GDI spray emerging from an electronically controlled swirled-type injector, have been carried out at two injection pressures, 7.0 and 9.0 MPa, respectively, under atmospheric pressure and ambient temperature in an optically accessible chamber. The injector, having two swirled fuel adduction ducts and a nozzle diameter of 0.55 mm, produces a hollow-cone-type spray with a nominal cone-angle of 80\mD. A gasoline-like non-volatile calibration fluid in non-evaporative conditions, EXXSOL D40 ( = 762 kg/m\u3, = 1.05 cSt at T=40\mDC) has been used. A single channel phase doppler analyzer (PDA) has been used to acquire time- resolved droplets size and velocity at different locations within the spray along and off-axis. The system has allowed the reconstruction of the size and velocity data along the injection timing with a time resolution of 100 \gms. Time-resolved size and axial and radial velocity components have been measured at different locations within the spray. The cycle-resolved data, buffered in a PC via a DMA interface, have been analyzed off-line by the ensemble-averaging procedure. The main results of the investigation have provided a velocity distribution representative of a hollow-cone structure in the first stage of the injection, followed by a droplets-refilling process of the central region at later time. A small-scale droplets recirculation, close to the nozzle tip, has been evidenced. Decreasing values of D10 have been found during the initial non- stationary injection phase, followed by a flat profile with diameters ranging from 2 to 5 \gmm. Estimation of the atomization degree in the near nozzle region has been performed by the laser light extinction technique. The technique has also provided a measurement of the tip penetration and nozzle discharge coefficient in the region up to 5 mm from the nozzle. A higher light extinction degree, at increasing injection pressure, has been found at the nozzle exit both along the spray axis and at different radial coordinates with no evidence of the breakup length.