In-Nozzle Flow Visualization of Marine Diesel Injector Nozzles with Different Inlet Radii

Injector geometries of large marine two-stroke Diesel engines differ extensively from configurations typically used in diesel engines. The multi-hole injector orifices are generally arranged asymmetrically as all the bores face a similar direction. Due to this geometric setup, the orifices are also distributed eccentrically with respect to the central bore of the injector nozzle. Experiments have shown that spray propagation from such orifices is not symmetric with respect to the nominal axis of the orifice. These spray deviations can lead to wall wetting which increases fuel consumption, emissions, and component temperatures as well as loss of lubrication film.

To further investigate the in-nozzle flow and how it affects the spray morphology for large marine two-stroke Diesel engines, experiments were performed using a transparent nozzle holder for single-hole, transparent nozzles made of PMMA. The transparent nozzles were used with Diesel fuel under real injection pressure conditions of 50 MPa. PMMA was chosen to reduce optical distortion due to refraction index differences between the fluid and transparent nozzle material. The transparent nozzles used were orthogonally arranged 0.75 mm diameter mono-hole designs with different levels of hydro-erosive grinding and therefore different inlet radii between main bore and orifice of the nozzle. These transparent nozzle designs were chosen to match those of large marine two-stroke diesel engines.

Shadowgraphy using a CCD camera, a far-field microscope and a pulsed Nd:YAG laser together with a diffusive optics were applied to visualize the cavitation in the nozzle during the complete quasi-steady-state injection process with a duration of 8 ms.

The observation of cavitation and flow behavior in the nozzles allows qualitative validation of CFD cavitation models for specific cavitation conditions relevant for marine fuel injection.