Direct imaging of primary atomisation in the near-nozzle region of diesel sprays

The spray formation and breakup of n-dodecane 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 under engine-like operating conditions, i.e. high ambient pressure and temperature. Present measurements show that the end of injection is dependent on injection pressure for low injection pressure of 50 MPa and independent for 100-150 MPa pressure range. The end of injection was characterized by large ligaments and deformed droplets along with spherical droplets. It was noted that formation of large droplets during end of injection was not related to injection pressure. The large droplets were found to be in the range of up to 50 μm, which were moving with relatively low velocity. Typical velocity range for large droplets (30-50 μm) was between 1.5 to 5 m/s. The trajectory of individual droplets appeared to be random from injection to injection. It was particularly emphasized that the real fuel injector under engine-like operating conditions can produce a fuel spray, which can be a mix of liquid and vapour at the start of injection.
In this publication we report on progress made with ongoing experimental investigations of the atomisation of n-dodecane by using microscopic imaging and high-speed video using ECN ‘Spray A’ injector. A long-distance microscopy was used to study near-nozzle region (1.025x0.906 mmm). Our study focuses on the primary atomisation during the start, the steady-state and the end of the injection process.