Abstract
The aim of this numerical study is to investigate the soot formation processes in a ducted fuel injection (DFI) spray under engine-like conditions. Furthermore, the effect of the pre-injection dwell period on the spray, combustion, and soot characteristics are also investigated. The DFI configuration considered here is D3L14G2. Large eddy simulations coupled with a two-equation soot model and conjugated heat transfer are performed. Inert and reacting spray simulations conducted for the free spray (FS) and DFI-configurations are validated against measured data. Reasonable agreement to the measurements is achieved for the simulated ignition delay time (IDT) and lift-off length (LOL) under FS configuration. The dwelling period is shown to significantly affect the combustion characteristics in the DFI-configuration. A dwell time of 1seconds causes longer IDT and LOL, which leads to them being closer to the measured data. In contrast, the IDT and LOL predicted in the non-dwelled DFI case are significantly shorter and the ignition sites are shown to form inside the duct. The difference between dwelled and non-dwelled case is due to the presence of thermal boundary layer in the former case. In the dwelled DFI case, the increase in LOL leads to a higher overall air entrainment process and better air–fuel mixing. This in turn decreases the soot precursor C2H2-mass spike by two-fold and the soot mass spike by three-fold relative to that obtained in the FS case. Hence, the present study highlights the significance of the dwell period for this particular DFI configuration (D3L14G2). Omitting this crucial step in a DFI setup is demonstrated in this study to cause ignition to occur inside the duct, which consequently lead to a greater soot mass spike than that in the FS case.
Highlights
Diesel engines have been a solid choice in the transport and energy industries due to having high thermal efficiency and fuel economy
Omitting this crucial step in a ducted fuel injection (DFI) setup is demonstrated in this study to cause ignition to occur inside the duct, which lead to a greater soot mass spike than that in the free spray (FS) case
liquid penetration length (LPL) is defined as the maximum axial distance from the injector encompassing 95% of the liquid phase mass, while vapour penetration length (VPL) is defined as the maximum axial distance from the injector to 0.1% fuel mass fraction
Summary
Diesel engines have been a solid choice in the transport and energy industries due to having high thermal efficiency and fuel economy. In recent times there has been an increasing focus on the pollutants (e.g. soot) associated with this technology and their influence on the climate change and public health. This has led to ever more stringent emission regulations and standards. Chan et al [4] reported that the soot mass may increase due to more high-temperature fuel-rich regions despite EGR being able to reduce NOx due to the lower peak in-cylinder temperatures
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