Numerical analysis and statistical description of the primary breakup in fuel nozzles of large two stroke engines for the application in CFD engine simulations

Abstract

In this work, the in-nozzle flow and primary breakup of fuel nozzles applied in large two stroke engines was analyzed by combined RANS and LES simulations. Intensive investigations have been performed to describe the influence of geometrical design features and flow conditions inside the nozzle on the droplet diameters, velocities and locations of origin. It is shown that the spray and droplet formation in large two stroke engines is highly unsymmetrical and depends on the nozzle geometry, location of the orifice and also on the upstream flow conditions. Disturbances and a sharp redirection of the flow into the nozzle orifice lead to a deflection of the dense fuel core, while an eccentricity of the nozzle bore induces a wrinkling up and rotation of the dense core around its central axis. Both phenomena lead to an asymmetrical disintegration of the liquid core. To describe the primary breakup and the aforementioned phenomena in CFD in-cylinder simulations, prior in-nozzle flow LES simulations are realistically not manageable and inefficient, as they require extensive computational effort. Hence, a statistical model to describe the primary breakup in Lagrangian spray simulations is introduced. Depending on thermodynamic conditions and nozzle geometry, droplet parcels are generated around the dense core by using probability density functions (PDF) for the droplet diameter, velocity and location. The PDFs as well as correlations for the dense core length, deflection of the spray and mean droplet diameter were derived from LES simulations.