A metrological view on in-nozzle flow and primary atomization of modern GDI injectors
Prof Michael Wensing (FAU Friedrich-Alexander Universität Erlangen-Nürnberg, GE)
Due to small scales, high velocities and high optical density and/or limited optical access the in-nozzle flow and the primary breakup of modern GDI sprays is still not fully understood. Comparable atomization to diesel injection processes is reached with significantly lower fuel pressures. This is possible by massive cavitation, which creates a very complex multiphase transient flow inside the injector – sac volume and nozzle holes. New optical measurement technologies and high speed cameras provide new insight to the flow and atomization processes. Three synchrotron X-ray techniques are applied at matching experimental conditions and for the same injectors to study the initial spray velocity field, the mass distribution and the spray structure in the primary spray region. Additionally, a newly developed Light Sheet Fluorescence Microscopic Imaging (LSFM) technique and injection mass rate measurements provide information on primary spray structures and the underlying breakup mechanisms in the first 2mm after nozzle exit. A completely different spray breakup is found for the tested GDI in comparison to diesel injectors. While diesel jets leave the nozzle as an intact jet and disintegrate then very fast from the surface, GDI spray leave the nozzle almost broken up due to the massive cavitation inside the nozzle. High speed films provide a very clear view for the audience into the processes in the primary atomization region for present injector designs. The findings are summarized in the degree of spray hole filling — around 65 % for the GDI injector (approximately 100 % for the diesel case) – and penetration velocities and velocity profiles across the nozzle-hole. Glass nozzle investigation with different measurements techniques show cavitation structures inside the nozzle hole and variations of these structures with important design and operating parameters. From a fundamental point of view the breakup of liquid fuel and spray initiation in automotive applications is driven by cavitation, turbulence, aerodynamic forces and relaxation of velocity profiles. High sophisticated optical diagnostics provide an insight into the dominating spray breakup mechanisms at different stages of gasoline direct injection processes.