Numerical and Experimental Investigation of Turbulent Flows in a Diesel Engine

This paper presents a study of the turbulence field in an optical diesel engine operated under motored conditions using both large eddy simulation (LES) and Particle Image Velocimetry (PIV). The study was performed in a laboratory optical diesel engine based on a recent production engine from VOLVO Car. PIV is used to study the flow field in the cylinder, particularly inside the piston bowl that is also optical accessible. LES is used to investigate in detail the structure of the turbulence, the vortex cores, and the temperature field in the entire engine, all within a single engine cycle. The LES results are compared with the PIV measurements in a 40 × 28 mm domain ranging from the nozzle tip to the cylinder wall. The LES grid consists of 128³ cells. The grid dynamically adjusts itself as the piston moves in the cylinder so that the engine cylinder, including the piston bowl, is described by the grid. In the intake phase the large-scale swirling and tumbling flow streams are shown to be responsible for the generation of large-scale vortex pipes which break down to small-scale turbulent eddies. In the later phase of compression turbulence is mainly produced in the engine bowl. The bore wall and the piston bowl wall heat the fluid near the walls. Turbulence and the large-scale coherent vortex shedding due to the Kelvin-Helmholtz instability are responsible for the enhanced heat transfer between the bulk flow and the walls. A temperature inhomogeneity of about 50 - 60 K can be generated in the cylinder.