Comparison of measured and simulated in-cylinder flow fields in an optical diesel engine

CFD models are used to study combustion processes both in academic research and in industry. In academia, the focus is on fundamentals and much effort is put into choosing and developing submodels. In industry the models are used as development tools and the focus is reliability and turn-around time. It is therefore common in industry to use commercial CFD codes with pre-implemented submodels. Simplified simulation methods are often used to decrease turn-around times. To facilitate interpretation of the results by engineers, standardized procedures and cases are employed. Such approaches are justified by their capability to predict qualitative trends, which is often good enough for development purposes. This paper investigates the validity of such a simplified CFD approach. Measured flow fields from an optical light-duty diesel engine are compared with CFD simulations of the corresponding cases using a commercial code (Star CD). The analysis focuses on the model's capability of reproducing the measured bulk flow structures. These structures are of major importance to light duty diesel engines. In-cylinder flow patterns were measured in an optical diesel engine using particle imaging velocimetry (PIV). The optical piston used maintained the major features of a typical, re-entrant piston bowl, and provided unique optical access to the combustion chamber. The simulations were set up in a sector of the combustion chamber containing one spray. Periodic boundary conditions were used. Inlet ports and valves were omitted. The initial flow field was represented by a rigid body rotation having the same swirl ratio as the engine. Standard spray, combustion, and turbulence models were used and no tuning was made. Previous experience has shown that this approach is capable of predicting emissions trends qualitatively. The aim here was to investigate if the simulated flow fields captured important characteristics of the measured ones.