Cavitation effects on spray characteristics in the near-nozzle field 2009-24-0037
In this paper, a special technique for visualizing the first 1.5 millimetres of the spray has been applied to examine the link between cavitation phenomenon inside the nozzle and spray behaviour in the near nozzle field.
For this purpose, a real 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 pressurized chamber filled with Diesel fuel. Using backlight illumination, the differences in terms of density and refractive index allowed the distinction between vapour and liquid fuel phases. From these visualization results, two important conclusions can be established: on the one hand, supercavitation is detected for pressure drop conditions at which mass flow choking has not appeared yet. On the other hand, it can be seen that the spray formed by cavitation bubbles spreads as pressure drop conditions get stronger.
Finally, spray visualization in a nitrogen pressurized chamber has been developed for stationary conditions. In order to analyze cavitation influence on spray characteristics, pressure drop has been modified near the values at which cavitation bubbles had been detected out of the nozzle. Two different test strategies will be used for this purpose: fixing injection pressure, which implies changing chamber density for each test point, or fixing chamber pressure. Both kinds of measurements revealed a noticeable increment of spray cone angle when supercavitation appears. This fact can be assumed as an indicator of atomization improvement induced by the collapse of cavitation bubbles at the nozzle exit.