Ducted Fuel Injection (DFI) is an innovative hardware-related concept which has been experimentally shown to eliminate, or at least drastically reduce, the soot formation in Diesel mixing-controlled combustion, maintaining all the associated notorious advantages, in terms of efficiency and controllability. Nonetheless, the physical phenomena leading to this remarkable results are today not completely understood, due to the non-intrusiveness related experimental limit in the local analysis inside the duct. Therefore, the duct geometry optimization becomes a complex engineering problem, hard to manage due to the huge amount of variables, whose influence is not known. A 3D Computational Fluid Dynamics (CFD) simulation approach is proposed in this work, with the purpose to reach a deeper comprehension of the physical processes occurring in presence of DFI.