Ducted Fuel Injection (DFI) is a recently developed concept to curtail soot
formation in diesel flames and based on fuel injection along the axis of a small
cylindrical pipe within the combustion chamber, enhancing mixture preparation
upstream the autoignition zone. Experimental observations have shown a
remarkable DFI effectiveness in soot mitigation; however, the mechanisms enabled
by duct adoption are not yet fully clear, especially when different duct
geometries are considered.
This article proposes an experiment-simulation coupled approach for the analysis
of DFI in a constant volume vessel, operating in both non-reacting and reacting
conditions. In particular, a previously calibrated three-dimensional
computational fluid dynamics (3D-CFD) spray model was further
validated against experimental liquid penetration considering different duct
geometries, proving its reliability for testing duct geometrical variations.
Afterward, the validated spray model was employed to investigate the influence
of the main geometrical features (stand-off distance, duct length and diameter,
inlet and outlet shape) on the ducted spray characteristics and on the
combustion and emissions formation processes.
The reduction of both stand-off distance and duct length, up to the flow area
limit in which the air entrainment is almost zeroed, leads to the best soot
mitigation performance. Furthermore, a chamfer at the duct inlet enhances the
duct adoption benefits due to improved air entrainment, confirming previous
experimental observations. Thereby, it was possible to figure out an optimal
duct configuration in terms of soot emission minimization by evaluating air
entrainment and turbulent mixing at duct inlet and outlet, and flame lift-off
length, achieving a soot mass curtailing of more than an order of magnitude.
