Initial Atomization of an Intermittent Spray from a Diesel Nozzle

The purpose of this study is to clarify the atomization mechanisms of an intermittent fuel spray in its very early stage of injection. A single hole diesel nozzle was used in rather wide range of valve opening pressure from 5 to 70 MPa. The measurement of injection pressure inside the nozzle-sac, that is just at the upstream of the orifice, was attempted by using a micro-strain gauge. Two observation apparatuses were used; an image-converting high-speed camera with the framing rate of 10⁵ f.p.s. and an image-intensified CCD camera with high-speed gating. At the very beginning of injection, an emergence of a liquid column was observed, which had almost the same diameter as the orifice. Then the liquid column spread radially, forming a spray angle in the vicinity of the nozzle exit. The timing of the spray angle formation was shown to become earlier with increasing the in valve opening pressure. Furthermore the pressure inside the nozzle-sac at this moment did not vary. This pressure approximately corresponds to that of the transition from a wavy jet to a developed spray in the atomization regime for a steady jet. Thus, it is suggested that the atomization mechanism of a steady jet could be applicable to the unsteady spray formation process. Under several injection conditions, it was observed the formation of a thin thread at the tip of the original spray. It appeared at the very beginning of injection, and developed to the length of 2 or 3 mm. The tip gradually broke up, decreased the velocity, and was finally caught up by the original spray.