Keywords: diesel engine, swirl, CFD, moving mesh.

Modern direct-injecting Diesel engines apply swirl to support the combustion process. Design of such combustion systems is usually based on swirl numbers defined from stationary flow through the cylinder head. In fact, a clear dependency of soot- and NOx-emissions from such swirl number can usually be measured. However, identical swirl numbers can be generated by completely different flow patterns. Among others, this has been shown by [1], who performed transient measurements of velocities in a cylinder. They showed that cylinder heads of different type but with similar swirl numbers can lead to quite different emissions.

The integrated angular momentum finally occurring in the piston bowl at Top Dead Center is describing the conditions for injection and combustion better than stationary swirl numbers. Since the angular momentum can be measured only cost- and labor intensive with e.g. optical devices, transient CFD simulations with moving/deforming meshes are useful to obtain this information. Furthermore they provide important information on the destruction of angular momentum by the squish-flow into the bowl.

Most of the conventional moving mesh approaches are using multi-block structured grids for the moving parts. This is going to become a challenging task in case of complex geometries. The present work applies an alternative approach for moving and deforming meshes using a hybrid grid technology. The simulations are executed using the CFD-Code FLUENT Version 6 offering an advanced in-cylinder mesh motion capability following a new approach. It offers three different schemes to describe the mesh motion. This is a spring analogy, a local re-meshing procedure and a dynamic layering approach.

In this paper, after an introduction to swirling flow in Diesel engines, swirl numbers and the respective stationary measuring methods, the results of CFD-simulations are compared with experimental data. Comparable swirl numbers and flow coefficients were obtained with a conventional cylinder head test rig, and the PDV-technique of the German Aerospace Centre DLR was used to measure velocities. The functionalities of moving and deforming meshes applied for transient CFD-simulations are described and illustrated by an academic 2D engine cycle case. Finally the results of a three dimensional and transient simulation of the intake flow in a Deutz Diesel engine are presented and discussed.

1

L. Barthelmä, W. Spindler, G. Woschni, "Messung der örtlichen Luftbewegung im Brennraum eines direkteinspritzenden Dieselmotors", MTZ Motorentechnische Zeitschrift 44 (1983) 2

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