Optical Characterisation of Diesel, RME and Kerosene Sprays by Microscopic Imaging

The spray formation and breakup of fossil diesel, biodiesel (RME) and kerosene was investigated experimentally on a common rail diesel injector using a long working distance microscope. The objectives were to further the fundamental understanding of the processes involved in the initial stage of diesel spray formation, and the influence of viscosity, lubricity and surface tension. Tests were conducted at atmospheric conditions and on a rapid compression machine. Analysis of the fuel sprays at the start of injection revealed jets for each fuel whose morphology was governed by their Reynolds and Weber numbers. As the fuel viscosity was one of the most varying properties this pro-duced jets that were dynamically different. The differences were also reflected in the injection timing of the fuel sprays where the friction between the fluid and the nozzle wall controlled the start of injection (SOI). Kerosene was found to advance the SOI when compared to the diesel and as such suspected to have an impact on the combustion phasing. The RME results fully corroborated with the conclusions drawn based on kerosene. Downstream from the nozzle the stabilising effect of viscosity can be seen to reduce the formation of surface waves, ligaments and droplets prior to the reaching the breakup length. It is speculated that the liquid core of the RME jet does not generally breakup directly into ligaments and droplets, but transitions into liquid sheets which then breakup or recombine into ligaments through surface tension.