CFD modelling and numerical investigation of a large marine two-stroke dual fuel direct injection engine

This study aims at the development of a CFD modelling approach for the simulation of a large marine two-stroke dual fuel engine with gaseous fuel direct injection at high pressure. The developed model employs various sub-models to represent the injection, mixing and combustion processes of the pilot liquid and main gaseous fuels, which following their customisation are integrated with the ANSYS Fluent software. The shock tube theory and the pseudo-diameter concept are employed to sufficiently represent the gaseous fuel injection, jet penetration and air entrainment processes. The steady diffusion flamelet model is employed to represent the gaseous fuel non-premixed combustion process, whereas the pilot fuel combustion is employed to estimate an ignition kernel. The conserved-equation sources approach is employed appropriately estimating the sources representing the pilot and gaseous fuels injection and combustion processes. The developed CFD model is first validated based on the previous published results obtained in a rapid compression and expansion machine. Subsequently, the closed cycle of the large marine two-stroke dual fuel engine is simulated for both the gas and diesel operating modes considering the engine operation at 75% load. The derived in-cylinder pressure variations were validated against respective experimental data, whilst a number of performance and emission parameters for both operating modes are comparatively assessed to delineate the involved phenomena. The derived results demonstrate that the model exhibits adequate accuracy. At the gas mode, the combustion takes place in lower maximum temperature and leaner conditions compared to the diesel mode, resulting in lower NOx emissions. This study contributes to the better understanding of the phenomena and physical processes taking place in large marine two stroke engines with gaseous fuel direct injection and is expected to benefit the development of future engine designs and the engine settings optimisation targeting to reduce emissions and increase efficiency.