High-speed X-ray imaging of in-nozzle cavitation and emerging jet flow of multi-hole GDI injector under practical operating conditions

For the last decade, a propagation-based X-ray imaging technique has been used for analysis of in- and near-nozzle flow characteristics of high-pressure fuel injectors. Despite remarkable progress has been made in the near-nozzle jet flow analysis, the structure and dynamics of in-nozzle flow have been difficult to be analyzed due to severe X-ray absorption from the steel nozzle enclosure. In this study, we investigate the structure and dynamics of in-nozzle and emerging jet flow of a multi-hole GDI injector using a single-shot high-speed X-ray imaging technique. The multi-hole GDI injector was equipped with the aluminum (Al) nozzles containing the holes with different geometries. The high X-ray transmittance of the Al nozzles enabled the single-shot high-speed imaging of the in-nozzle flow using an ultra-short X-ray pulse with a 150 ps duration. At first, the transient characteristics of the in-nozzle flows were discussed using the X-ray images. Liquid fuel and bubbles coexisted inside the nozzle before the start of injection. As the needle lifts up, the geometric cavitation was formed inside the nozzle hole that reached nozzle outside, like a hydraulic flip. After the needle close, the bubbles were formed inside the sac, and they survived until the next injection event. Second, we characterized the effects of nozzle hole geometry on the structure and dynamics of in-nozzle and emerging jet flow at the steady-state of fuel injection. The geometric cavitations were usually formed at which the nozzle hole inlet angle is less than 90^o. Two main flows were associated with the formation of geometric cavitation inside the nozzle hole, one directly approaching from needle upstream and the other approaching from the sac that caused flow branching outside the nozzle. The flow branching was promoted as the nozzle hole inclination angle increased which resulted in an increase in the flow dispersion angle outside the nozzle. The initial jet flow characteristics of the multi-hole GDI injector seemed predominated by the bulk flow motion inside the nozzle rather than the formation of geometric cavitation inside the nozzle hole.