Limitations of sector mesh geometry and initial conditions
to model flow and mixture formation in direct-injection diesel engines 2019-01-0204
The frontier of compression ignition engine technologies blends classifies combustion strategies in over a range of reactivity gradients, from homogeneous-charge – HCCI, increasing to reactivity-controlled – RCCI, diesel-pilot DPI, partially-premixed – PCCI, to conventional diesel combustion – CDC. However, sector mesh modeling is still the dominant computational tool to design their usage across engine loads and speeds. The aim of this work is to quantify the errors descending from the sector mesh approach through three geometric modeling approaches to an optical diesel engine. A full engine geometry mesh is created, including valves and intake and exhaust ports and runners, and a full-cycle flow simulation is performed until fired TDC. Next, an axisymmetric sector cylinder mesh is initialized with homogeneous bulk in-cylinder initial conditions initialized from the full-cycle simulation. Finally, a 360-degree azimuthal mesh of the cylinder is initialized with flow and thermodynamics fields at IVC mapped from the full engine geometry using a conservative interpolation approach. A study of the in-cylinder flow features until TDC showed that the flow complexity is mostly modified by the geometric features on the cylinder head (valve tilt and protrusion into the combustion chamber, valve recesses). As a result, errors in near-TDC swirl ratio, vortex structure and turbulence availability were seen when employing sector meshing, even if a 360-degree sector, with direct IVC flow mapping, was used. FurthermoreDuring injection, lack of geometric details on the head led to the inability to predict the formation of an upper recirculation region on the tumbling plane, above the piston step, which has been associated with thermal efficiency benefits with the stepped-lip bowl. Initialization of the flow anisotropies in the cylinder resulting from the intake process at IVC were instead seen to have a smaller effect. The results also showed that tuning IVC quantities in a sector mesh cannot effectively compensate for its missing geometric and flow details.
Federico Perini, Stephen Busch, Eric Kurtz, Alok Warey, Richard C. Peterson, Rolf Reitz
University of Wisconsin-Madison, Sandia National Laboratories, Ford Motor Company, General Motors