The structure of a break-up zone in the transient diesel spray of a valve-covered orifice nozzle

Abstract

A break-up zone in the diesel spray of a valve-covered orifice nozzle was investigated to observe the effect of the injection rate on the spray structure and to obtain physical insight into the development of a transient diesel spray. The surface shape and internal structure of the diesel spray from a common-rail injection system were visualized with high spatial and temporal resolution under atmospheric ambient conditions. During the injection period, common-rail pressures of 39.5 MPa and 112 MPa were used to obtain highly magnified spray images from the nozzle exit to about 260 nozzle diameters downstream. Before the atomization break-up regime appeared, a short transition period was observed, while ligaments were formed on the disturbed liquid column surface. During atomization, the spray was surrounded by these short fine ligaments, which were arranged and bent towards the direction of the spray penetration. The internal structure of the break-up zone consisted of complex entangled ligaments and dispersed liquid drops. The break-up process occurred simultaneously at the spray surface and the core. The entrained ambient air that penetrated through the crevices of the densely packed ligaments seemed to stretch the coherent structure and to carry the small droplets from the surface of the spray. The collapse of a cavitation bubble might have caused the ligaments to form near the nozzle exit at the core of the spray, although details of the process remain to be identified. Using image-processing techniques, the sizes of the liquid drops downstream from the spray were measured. The average size of the drops decreased as the injection rate decreased and, as expected, a higher common-rail pressure produced smaller droplets.