Air-fuel mixture formation is one of the most decisive processes on Diesel engine performance and pollutant emissions. With the aim of improving the knowledge about this phenomenon numerous studies, both theoretical and experimental, in addition to computational fluid dynamics tools, have been developed over the last decades. Nevertheless, the study of internal flow characteristics of injection nozzles, as well as their effect on fuel atomization and spray behavior in its first stages, has still significant uncertainties, appearing as an important challenge for research in this field.

In the current Thesis, a high-resolution visualization technique for the characterization of Diesel sprays in the near-nozzle field has been developed. Thus, the relationship between internal nozzle flow characteristics and spray behavior can be studied more directly. Furthermore, the acquisition of high-resolution images allows to characterize spray structure more precisely than using other visualization techniques.

In this sense, firstly the influence of cavitation appearance inside the nozzle orifices on spray formation has been analyzed. For this purpose, a collection of simplified nozzles consisting in drilled steel plates, as well as a single-hole cylindrical nozzle, have been used. As a result of this study it can be seen that cavitation leads to a significant improvement of spray cone angle, together with an increment of spray contour irregulatiries, which enhances atomization process.

Once the influence of cavitation on spray behavior has been studied, a group of three tapered nozzles has been selected in order to carry out a study under non-cavitating conditions. Thus, statistical correlations between spray characteristics and parameters such as nozzle outlet diameter, injection velocity or chamber density have been obtained. Finally, a one-dimensional model which predicts the axial behavior of spray microscopic characteristics has been used in order to analyze some of the results obtained from spray visualization.