High-speed microscopic imaging of the initial stage of diesel spray formation and primary breakup

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 optimised 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 vapourised spheroidal cap was identified, and resulted in a slipstream effect which led to a central ligament being propelled ahead of the liquid jet.