Cavitation Flow Visualization in Marine Diesel Injectors

Abstract

Injector geometries of large marine two-stroke diesel engines differ extensively from configurations typically used in diesel engines for heavy duty and automotive applications. Fuel is usually injected into the cylinder using multiple injectors that are arranged around the cylinder wall. As a result, the injector orifices are asymmetrically arranged as all the bores face a similar direction. Due to this geometric setup, the orifices are also distributed eccentric with respect to the central bore of the injector. Experiments have shown that sprays from such orifices propagate nonsymmetrical to the nominal axis of the orifice. Those spray deviations can lead to wall wetting which increases fuel consumption, emissions, component temperatures and loss of lubrication film on the cylinder wall. In addition, preliminary work using CFD simulations with an advanced cavitation model indicates that the two-phase, highly compressible flow inside the nozzle may strongly influence the spray propagation. To further investigate the in-nozzle flow and how it affects the spray morphology for large marine two-stroke diesel engine injectors, experiments were performed using transparent nozzles made of PMMA. The transparent nozzles were mounted on a marine diesel injector which was operated under ambient conditions and run with diesel at a pressure of 50 MPa. The nozzle used was an orthogonally arranged 0.75 mm diameter, one-hole setup that matches large marine two-stroke diesel engines injector nozzle diameters. High-speed shadowgraphy using a far-field microscope was applied to visualize the cavitation in the nozzle during the quasi-steady state injection process. These imaging results are used to compute statistical evaluations of cavitation in the nozzle over a range of conditions.