Transient liquid sheets and their relationship to GDI sprays
Developments m Gasoline Direct Injection, GDI, technology have enhanced the viability of long tenn SI engine development. Many automotive manufacturers are developing and offer production cars with first generation GDI engines. GDI fuel injection strategies provide power and effi ciency improvements, due to superior fuel metering, incylinder mixture preparation and the ability to run throttle-less under di fferent combustion modes depending on engine load. Although significant improvements in perfonnance and economy have been demonstrated, work is still required to optimise the GDI strategies fo r varying engine loads and emissions. Matching liquid fuel sheet break up and atomisation timescales to those of the charge motion occurring in the engine cylinder is essential. Many fundamental studies have investigated the mechanisms of liquid sheet break up, however, most have concentrated on steady state low pressure conditions. It is felt that little can be applied from these studies to analyse high pressure GDI sprays which produce an initial liquid sheet annulus then a complex hollow cone spray, transient in nature due to the cyclic behaviour of an SI engine. This experimental study assesses the liquid fuel sheet break up mechanism of a GDI pressure-swirl injector in the pressure range 10-50 bar. The fundamental study simplifies the problems associated with a 3-dimensional spray by considering a 2-dimensional transient liquid sheet and characterising the sheet wave structure and break up process. A unique rotary valve has been specifically designed and manufactured to allow the break up of a transient flat liquid sheet to be studi ed under an injection pressure range of 10-50 bar. A precursor to liquid sheet break up is the appearance of perforations in the sheet. The onset of perforations in the fl at sheet were measured as a function of distance downstream from the nozzle for a range of sheet velocities 12-36m/s; i.e. Reynolds number range 800 - 2400. This highlighted a peak in the perforation onset length between 20 and 25 bar injection pressure; i.e. sheet velocity of approximately 25m/s. Subsequent increases of sheet velocity lead to a reduction in the perforation onset length, strongly indicating that above 25rn/s, aerodynamic forces dominated the sheet break up process. Spreading the liquid laterally, introduced sheet stretching, which affected the position of the perfo ration onset by as much as 30% at higher injection pressures. Estimations of sheet thickness at the perforation location were calculated to be in the range 0.05-0. llrrun. Particle Image Velocimetry, PIV, and Laser Doppler Anemometry, LDA, was used to assess the liquid sheet velocity flow field, which indicated the presence of large velocity gradients in both the axial direction and across the sheet respectively.