Influence of cavitation phenomenon on primary break-up and spray behavior at stationary conditions

In this paper, a special technique for visualizing the first 1.5 mm of the spray has been applied to examine the link between cavitation phenomenon inside the nozzle and spray behavior in the near-nozzle field. For this purpose, a Diesel axi-symmetric nozzle has been analyzed. Firstly, the nozzle has been geometrically and hydraulically characterized. Mass flow measurements at stationary conditions have allowed the detection of the pressure conditions for mass flow choking, usually related with cavitation inception in the literature. Nevertheless, with the objective to get a deeper knowledge of cavitation phenomenon, near-nozzle field visualization technique has been used to detect cavitation bubbles injected in a chamber pressurized with liquid fuel. Using backlight illumination, the differences in terms of density and refractive index have allowed the distinction between vapour and liquid fuel phases. From these visualization results, two important conclusions can be established: on the one hand, cavitation bubbles have been detected at the nozzle exit for pressure drop conditions at which mass flow was not choked yet. On the other hand, it could be seen that the jet formed by cavitation bubbles spread as pressure drop conditions became stronger. Finally, spray visualization in a nitrogen pressurized chamber has been developed at stationary conditions. In order to analyze cavitation influence on spray characteristics, pressure drop has been modified near the values at which cavitation bubbles have been detected out of the nozzle. Two different test strategies have been used for this purpose: fixing injection pressure, which implied a change in chamber density for each test point, or fixing chamber pressure. Both kinds of measurements revealed a noticeable increment of spray cone angle and spray contour irregularities related with the presence of cavitation bubbles at the orifice outlet. This fact can be assumed as an indicator of atomization improvement induced by the collapse of cavitation bubbles at the nozzle exit.