The set-off length (also referred to as the “lift-off length”) is reduced by the re-entrainment of the burned gas by the backward flow surrounding a diesel spray jet produced by a multi-hole nozzle. In the present study, to estimate the equivalence ratio at the set-off length, a means of estimating the amount of burned gas that is re-entrained into the near-nozzle region of the diesel spray jet was established. The results revealed that the suppression of soot formation in quasi-steady diesel spray flames produced by a multi-hole nozzle and a high injection pressure is not attained by reducing the equivalence ratio at the set-off length. Analysis of the amount of soot along the spray axis using a two-color method revealed that the maximum soot amount position appears in a quasi-steady spray flame, after the collapse of the head vortex in which a dense soot cloud is formed. The maximum soot amount position does not change even if the injection pressure varies. The suppression of soot formation by the application of a high injection pressure in an actual diesel engine was found to be caused by the shortening of the residence time in the soot formation region between the set-off length and the maximum soot amount position.
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