A novel technique for detailed and time-efficient combustion modeling of fumigated dual-fuel internal combustion engines

The present work details a study of the 3D-modeling of dual-fuel engines, with a specific focus on medium speed marine engines. These operate under the fumigated approach, which means that the second fuel, which replaces most of the diesel fuel, is added in the intake duct. This type of engine requires the least amount of modifications to the current medium speed diesel engine and is therefore an ideal retrofit solution to tackle global warming and local air quality issues. In this work, the operation of such an engine has been modeled, with special care for the combustion modeling. Namely, it comprises both auto-ignition of the diesel pilot, and flame propagation in the premixed fuel-air mixture. A tabulated kinetics approach, currently being used to model diesel operation, has been extended to include this flame propagation. Both a Coherent Flame Model (CFM) and a Flame Surface Wrinkling Model (FSWM) have been implemented to handle this premixed combustion mode. Coupling a flame propagation model to the tabulated chemistry should provide a practical way of capturing both the complex dual-fuel combustion process as the chemistry information, while maintaining an acceptable computation time. Additionally, a tabulated laminar flame speed method has also been implemented. Experimental results were obtained on the operation of a medium-speed engine in diesel, natural gas/diesel dual-fuel and methanol/diesel dual-fuel mode, under varying operating loads and speeds. Results have been compared to the ones obtained from the simulations performed in OpenFOAM with the dedicated combustion modeling technique. It was found that while both FSWM and CFM capture some physical trends, they are currently not able of capturing the dual-fuel combustion process in total. Improvements with regards to the ignition and mixing-controlled combustion modeling are necessary, since the current approach is not able of fully capturing the dual-fuel phenomena. This work however provides insight in the complex combustion process and serves as a basis for further developments. It can also be used as an initial engine development tool for fast calculation and optimization of dual-fuel operation.