Effects of Cavitation in Common-Rail Diesel Nozzles on the Soot Formation Process

A fundamental study to experimentally analyze the effect of cavitation in common-rail diesel nozzles on the soot formation process was carried out. The soot content was characterized by measuring the soot radiation, and an original methodology was developed to suitably characterize the soot formation process from this soot content. After a significant effort to overcome the different difficulties when analyzing the experimental data, the results seem to show a promising conclusion: cavitation reduces the soot formation rate. This reduction is explained, on the one hand, because it leads to a reduction in the effective diameter, thus diminishing the equivalence fuel/air ratio at the lift-off length; and, on the other hand, because it provokes an increase in effective velocity, thus increasing the lift-off length and reducing the corresponding equivalence fuel/air ratio. Concerning this last aspect, the experimental results seem to show that the lift-off length is significantly higher than what may be expected taking into account the increase in effective velocity and the decrease in effective diameter, thus giving evidence that an additional effect must exist to fully explain the increased lift-off length. A possible explanation for this additional increase in the lift-off length could be the higher level of turbulence of the flow exiting the injector, which might lead to a stabilization of the lift-off further downstream. Based on the results obtained in this study, the cylindrical nozzles under cavitating conditions may offer a potential to reduce soot emissions in some specific applications.