EXPERIMENTAL INVESTIGATIONS ON A PIEZO-ACTIVATED HOLLOW CONE INJECTOR - PART II: THE INFLUENCE OF NEEDLE LIFT ON DROPLET SIZE DISTRIBUTIONS AND VORTEX FORMATION

摘要

The outward-opening, piezo-driven pintle injector, designed for spray guided direct injection gasoline engines, produces a hollow cone spray formed by a dense array of distinct streaks of fuel systematically ordered around the circumference of the cone. The most significant advantage of the piezo drive is its ability for precision control of the injector needle. In this work, different aspects of the dynamics of the hollow cone spray produced by the injector have been studied for short injection durations as a function of the needle lift. The time-varying structure of the hollow cone and, in particular, the entrained flow field have been evaluated with planar Mie imaging. The Phase Doppler technique (PDA) has allowed the axial and radial droplet velocities and sizes to be quantified in a vertical plane containing a single fuel streak. During the fuel injection period the main droplet velocity field generally aligns with the imaged spray half cone angle. The PDA sample count has allowed estimates to be made of the location of the dense streak as a function of time. In the early phase of the injection, the axis of the streak is unstable, as it moves toward the outside of the cone until a stable position is reached. The droplet size distribution at a given distance from the nozzle is also influenced by the needle lift as the breakup length is also a function of the needle lift. As the main spray propagates, droplets in the shear layers transfer momentum to the entrained air and results in two counter-rotating vortices being generated, clockwise on the inside and counter-clockwise on the outside of the cone. The outer vortex, which dominates the axial flow field, has been tracked with time to determine the radial and axial displacements of its centre with respect to the injector nozzle.