Analysis of Diesel Sprays Through Two-Dimensional Laser Light Scattering

wo-dimensional laser light scattering has been used in the study of atomization and dispersion processes of unsteady diesel sprays. Main features, calibrations and digital procedures of the technique have been presented and its diagnostics limits for dense sprays have been discussed in relation to test experiments performed on single droplets and dilute sprays. It has been preliminarily shown that the technique, taking advantage of its quantitative recording of calibrated scattering intensity, yields the evaluation or the signature of total surface area of the liquid according as the spray is dilute or dense. Furthermore it has been shown that the visual examination of the scattering patterns from sections perpendicular to spray could be synthetized by two parameters, describing atomization and dispersion level, which are obtained by digital image processing. The synoptic analysis of these two parameters, determined for a single-hole diesel spray injected into a quiescent atmospheric environment, have evidenced different atomization regimes in different space-time positions. Two of these are clearly related to the needle displacement. During the needle lift the fuel is fragmented at the nozzle outlet, whereas during the needle shut off the atomization is a continuous process which evolves in a large space range. The main part of the liquid is fragmented, also in the inner region of the jet, at a distance from the nozzle (break-up length) shorter than that measured on stationary sprays. It is shown that unsteady fluid-dynamics of the liquid jet inside and outside the nozzle is essential in giving tentative mechanisms of atomization regimes and that time-varying injection effects are relevant to the knowledge of diesel combustion processes.