Influence of a swirling air flow on an evaporating Diesel spray from a common rail injection system under realistic engine conditions

The aim of the present paper is to provide an insight into the fluid dynamic processes that occur during the air/fuel mixture formation period in direct injection diesel engines. An experimental and numerical investigation has been performed to analyze the mixing process between an evaporating diesel spray and a swirl air flow under realistic engine conditions. Experimental tests have been carried out spraying the fuel within an optically accessible prototype 2-stroke diesel engine equipped with an external combustion chamber having cylindrical shape. The intake air flow, coming from the engine cylinder, is forced within the combustion chamber by means of a tangential duct generating a well-structured swirl flow similar to that developing in a real light-duty diesel engine with a high swirl ratio. A micro-sac, 5-hole, 0.13 mm diameter, 150\mD spray angle electro-hydraulic injector supplies the fuel by a common rail injection system able to manage multiple injection strategies. The air/fuel spray interaction for two injection strategies under controlled air swirl levels as well air density and gas temperature has been investigated. The Mie-scattering-based, 2D-imaging technique has provided global information on the spray evolution in terms of liquid spray morphology and tip penetration. The Particle Image Velocimetry (PIV) technique has also been applied to estimate the velocity vector distribution of the liquid fuel droplets. CFD transient analysis has been carried out by the 3-D Star_CD code. The k-\ge turbulence model, Huh-Gosman atomization model and Reitz-Diwaker secondary breakup model have been adopted to predict the fuel spray evolution and its interaction with the swirl flow under the same experimental conditions. The grid, reproducing the geometry of the combustion chamber, has been made using Star_CD tools setting the boundary conditions the same as the experimental ones.