No abstract

A novel algorithm is proposed for the automatic segmentation of diesel spray images and the calculation of their macroscopic parameters. The algorithm automatically detects each spray present in an image, and therefore it is able to work with diesel injectors with a different number of nozzle holes without any modification. The main characteristic of the algorithm is that it splits each spray into three different regions and then segments each one with an individually calculated binarization threshold. Each threshold level is calculated from the analysis of a representative luminosity profile of each region. This approach makes it robust to irregular light distribution along a single spray and between different sprays of an image. Once the sprays are segmented, the macroscopic parameters of each one are calculated. The algorithm is tested with two sets of diesel spray images taken under normal and irregular illumination setups.

A methodology to characterise the pressure losses in quasi-steady conditions (i.e. at full needle lift) of common rail diesel injectors was developed. The aim was to quantify the error when experimental results of nozzle internal flow are compared with CFD results, where pressure losses are usually neglected. The proposed methodology is based mainly on experimental tests that are complemented with some approximate calculations, based on the physics of the phenomenon, to take into account the effect of the needle deformation.The results obtained in the work lead to two important conclusions: on the one hand, that it is dangerous to extrapolate results relative to the injection (internal flow, spray atomization, spray penetration..) and combustion processes from low permeability nozzles (e.g. single-hole nozzles) to high permeability nozzles (e.g. multi-hole nozzles), and, on the other hand, that the comparison of these results between experiments and CFD simulations should be carried out carefully, because the pressure losses in the injector can be high under certain conditions. Finally, people working on the study of the injection and/or combustion processes, through experiments or simulations, will find here some interesting information to better know the actual injection pressure to be used in their analysis and/or simulations.

A study to experimentally analyze the effect of cavitation on the mixing process in diesel nozzles was carried out. The mixing process was studied through the spray cone angle. It was characterized in two different scenarios: with the liquid length (nearly realistic conditions, i.e. evaporative but non-reactive spray) and the heat release fraction (fully realistic conditions, i.e. evaporative and reactive spray). In both studied scenarios the increase of spray cone angle caused by the cavitation phenomenon, which leads to a better mixing process, has been confirmed. Nevertheless, when the variations of the effective injection velocity and the spray cone angle obtained by comparing a cylindrical nozzle (i.e. a nozzle that promotes the cavitation phenomenon) with a conical nozzle (i.e. a nozzle that inhibits this phenomenon) were analyzed togother, it was found that, for the cases studied here, the mixing process worsens with the cylindrical nozzle.

Arregle, JJP. (2012). A comprehensive study on the effect of cavitation on injection velocity in diesel nozzles. Energy Conversion and Management. 64:415-423. doi:10.1016/j.enconman.2012.03.032. A comprehensive study on the effect of cavitation on injection velocity in diesel nozzles AbstractResults when testing cavitating injection nozzles show a strong reduction in mass flow rate when cavitation appears (the flow is choked), while the momentum flux is reduced to a lesser extent, resulting in an increase in effective injection velocity. So as to better understand the origin of this increase in effective injection velocity, the basic equations for mass and momentum conservation were applied to an injection nozzle in simplified conditions. The study demonstrated that the increase in injection velocity provoked by cavitation is not a direct effect of the latter, but an indirect effect. In fact, the vapor appearance inside the injection hole produces a decrease in the viscosity of the fluid near the wall. This leads to lower momentum flux losses and to a change in the velocity profile, transforming it into a more "top hat" profile type. This change in the profile shape allows explaining why the momentum flux reduction is not so important compared to that of the mass flow rate, thus explaining why the effective injection velocity increases.