The formation and breakup of diesel sprays was investigated
experimentally on a common rail diesel injector using a long range
microscope. The objectives were to further the fundamental
understanding of the processes involved in the initial stage of
diesel spray formation.
Tests were conducted at atmospheric conditions and on a rapid
compression machine with motored in-cylinder peak pressures up to 8
MPa, and injection pressures up to 160 MPa. The light source and
long range imaging optics were optimized to produce blur-free
shadowgraphic images of sprays with a resolution of 0.6 μm per
pixel, and a viewing region of 768x614 μm. Such fine spatial and
temporal resolutions allowed the observation of previously
unreported shearing instabilities and stagnation point on the tip
of diesel jets. The tip of the fuel jet was seen to take the shape
of an oblate spheroidal cap immediately after leaving the nozzle,
due to the combination of transverse expansion of the jet and the
physical properties of the fuel. The spheroidal cap was found to
consist of residual fuel trapped in the injector hole after the end
of the injection process. The formation of fuel ligaments close to
the orifice was also observed, ligaments which were subsequently
seen to breakup into droplets through hydrodynamic and capillary
instabilities.
An ultra-high-speed camera was then used to capture the dynamics
of the early spray formation and primary breakup with fine temporal
and spatial resolutions. The frame rate was up to 5 million images
per second and exposure time down to 20 ns, with a fixed resolution
of 1280x960 pixels covering a viewing region of 995x746 μm. A
vortex ring motion within the vaporized spheroidal cap was
identified, and resulted in a slipstream effect which led to a
central ligament being propelled ahead of the liquid jet.