Hydraulic flip in a gasoline direct injection injector and its effect on injected spray

Abstract

Hydraulic flip is a common phenomenon that occurs in a short injector nozzle with a contraction flow which results in a separation flow without a wall reattachment. This phenomenon usually occurs in a gasoline direct injection (GDI) injector because the nozzle’s length-to-diameter ratio is low with high injection pressure. The efforts to investigate the hydraulic flip in a fuel injector nozzle have been paid in several studies. However, the investigation on the phenomenon in an actual GDI injector has not been well clarified yet since it is complicated to be quantitatively investigated due to its microscale and the flow location inside a solid non-transparent material. Besides, the hydraulic flip also affects the near-nozzle spray characteristics, but the previous studies are insufficient to explain the whole mechanism and characteristic despite their importance on the engine combustion quality. In this study, the hydraulic flip characteristic is investigated by varying the hydraulic flip thickness. The nozzle with a thinner hydraulic flip resulted from the application of hydro-grinding machining on the nozzle inlet so it forms a nozzle with a round inlet edge and less flow contraction. The characterization of internal flow and its correlation with the injected spray was done by investigating the effect of hydraulic flip thickness. The hydraulic flip phenomenon and the injected spray were captured by a high-speed visualization using a synchrotron X-ray to obtain a phase-contrast image. As a result, the hydraulic flip phenomenon was observed clearly in the tested actual GDI injector nozzles. The hydraulic flip thickness difference and how it affects the spray characteristics, i.e., velocity distribution, flow coefficients, turbulence intensity, spray dispersion, and SMD, were clarified. The results show that the thinner hydraulic flip leads to a higher discharge coefficient, lower turbulence intensity which results in slightly worse atomization, and a slightly narrower spray angle with more uniform spray velocity distribution.