Numerical Study of the Scavenging Process in a Large Two-Stroke Marine Engine Using URANS and LES Turbulence Models

A computational fluid dynamics study of the scavenging process in a large two-stroke marine engine is presented in this work. Scavenging which is one of the key processes in the two-stroke marine engines, has a direct effect on fuel economy and emissions. This process is responsible for fresh air delivery, removing the combustion products from the cylinder, cooling the combustion chamber surfaces and providing a swirling flow for better air-fuel mixing. Therefore, having a better understanding of this process and the associated flow pattern is crucial. This is not achievable solely by experimental tests for large engines during engine operation due to the difficulties of measuring the flow field inside the cylinder. In this study, the axial and tangential velocities are compared and validated with the experimental results obtained from Particle Image Velocimetry (PIV) tests [1]. The simulations are conducted using both Unsteady Reynolds Averaged Navier Stokes (URANS) and Large Eddy Simulation (LES) turbulence models. We observe in general, there is a good agreement between the numerical and experimental results. The flow inside the cylinder is studied in different locations related to the bottom of the scavenging ports during the period with open exhaust valve. Moreover, the replacement of combustion products with fresh scavenge air is analysed. The effective flow angle is calculated for the air flow through the scavenging ports. It is found that the effective flow angle is different from the geometrical angle of the ports (20°). Results illustrate better performance of LES, especially in the prediction of the tangential velocity which is crucial for the simulation of an accurate swirl and air-fuel mixing inside the marine engines. LES predicts a uniform profile for the tangential velocity at the top of cylinder which is consistent with the experimental results while URANS predicts a solid body rotation.