Cavitation Inside Multi-hole Injectors for Large Diesel Engines and Its Effect on the Near-nozzle Spray Structure

The cavitating flow characteristics inside multi-hole injectors for large Diesel engines have been investigated. Flow imaging obtained inside transparent nozzle replicas using simultaneously two high speed cameras and various illumination light sources has shown that cavitation is formed not only at the hole entrance due to the local pressure drop induced by nozzle inlet geometry, but also inside the volume of nozzle tip just upstream of the injection holes. CFD calculations of the internal nozzle flow have shown that cavitation strings are formed in the areas where large vortical structures are present. Processing of the acquired images has allowed estimation of the mean location and probability of appearance of the cavitating strings in the 3-D space. Parametric studies have also revealed the effect of the needle lift, cavitation number and Reynolds number. Simultaneous imaging of the flow inside the nozzle, the injection holes and the near-nozzle spray structure has revealed the effect of string cavitation on the hole flow structure and the consequent effect on spray cone and deflection angles. Those have been found to vary considerably and in a transient mode in the presence of a string inside inside the hole from which they are injected.