Ducted Fuel Injection: a Computational Fluid Dynamics analysis of soot formation mitigation mechanisms

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. In fact, the virtuality of the results allows to examine in detail every cell in the computational domain, including those inside the duct, and to focus on several physical variables which could have an effect on the final soot outcome. The simulation activity, carried out on the software \textit{Converge CFD V2.4}, is the sum of the following tasks: a previous non-reacting spray experimental imaging analysis in a Constant Volume Vessel; experiments-based calibration of a non-reacting spray model; validation of the spray model on a large set of experimental conditions and geometrical configurations; extension of the spray model to several combustion simulations. The above-mentioned process resulted in an effective soot abatement in presence of a duct-guided combusting spray with respect to a free spray, of the same order of magnitude of that presented in the scientific literature. Furthermore, it was possible to make additional considerations about the underlying reasons behind this drastic soot reduction: as an example, the pressure field, the turbulence and the momentum conservation play a major role in the final result. Moreover, the effects of two duct geometric variables, length and stand-off distance, were investigated and it has come to a preliminary duct geometry optimization. The stand-off distance has proved to have the greatest influence on the final output. The presented work does not pretend to be exhaustive about the physics involved in the DFI but it would like to be a first understanding of a very promising technology. Further and many more investigations have yet to be conducted to fully comprehend and optimize this concept, so as to achieve a sufficient industrial maturity for the application on Diesel engines.