High-Speed Microscopic Imaging of the Initial Stage of Diesel Spray Formation and Primary Breakup

Abstract

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 768×614 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 1280×960 pixels covering a viewing region of 995×746 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.

Microscopic imaging of the initial stage of diesel spray formation

“…However, the energy output of high-power LEDs is still insufficient to enable blur-free microscopic imaging of high-speed sprays. The challenging lighting requirements for such experiments has limited their application to atmospheric conditions, with the exception of our preliminary studies [16][17][18][19], and the recent work of Manin et al [20]. Additional complications introduced in high pressure and temperature environments include the presence of turbulence, density and temperature gradients in the gas phase, which will result in refractive index fluctuations.…”

Modeling the atomization of high-pressure fuel spray by using a new breakup model

“…Several studies [21][22] have revealed thought that the TAB model can produce excessive droplet breakup that is not in agreement with experimental data. This is attributed to the fact that cavitation and turbulence phenomena inside the injector nozzle and liquid core, experimentally shown to be of great importance to atomization [23][24][25], as well as non-linear droplet distortion effects are not accounted for in the TAB model. In the WAVE model [26][27], derived from a linear stability analysis of liquid jets, the breakup time is determined by surface instability of droplets as a function of the wavelength and the frequency of the Kelvin-Helmholtz wave.…”

Investigation of Spray Angle Measurement Techniques

“…The injected high pressure diesel sprays go through different temporal stages that can be broadly classified into four regimes. The first regime is the start of injection (SOI), where a 'mushroom head' forms as fuel leaves the nozzle because of the residual fuel in the nozzle sac [3]. The transient regime follows, which covers from the SOI to the time instant at which the needle inside the nozzle reaches a fully open position.…”