Effects of inversed-delta injection rate shaping on diesel spray flame liquid length, lift-off length and soot onset

In this paper, effects of continuous injection rate ramp-down (i.e., “inversed-delta” injection rate shaping) on liquid length, lift-off length and soot onset of diesel spray flame are investigated for better understanding of its combustion characteristics. Time-sequence images of the diesel spray flame are acquired in a constant volume combustion vessel CVCC using a novel TAIZAC injector. One-dimensional modeling is performed to discuss the air entrainment and mixture distributions. During the injection period, no systematic trend is observed on the liquid and lift-off length for any injection profiles. Contrarily, the soot onset for the inversed-delta injection progressively recedes towards the nozzle while it appears constant for the conventional “rectangle” injection. After the end-of-injection EOI, the fuel vaporization and combustion recession occur at a faster rate in the rectangle injection proportional to its injection pressure, possibly due to stronger and faster entrainment waves than that of the inversed-delta injection. From 1D simulation, the liquid length and spray equivalence ratio distributions stratified “layer” seems to be continuously shifted upstream during the injection period in the inversed-delta injection. It is interesting to note that the centerline equivalence ratio value of ∅_cl = 2 correlate well with the soot onset observed experimentally, suggesting that the air entrainment per injected fuel could likely be promoted during injection period in the inversed-delta injection.