Evidence for supercritical mixing layers in the ECN Spray A

Prior work on an engine combustion network Diesel fuel injector led to the conclusion that under Diesel engine conditions (combustion chamber pressure and temperature) the jet was transitionally supercritical; meaning that the core of the jet would be condensed liquid while the edge of the jet would be supercritical. We report initial experiments aimed at observing the thickened turbulent mixing layer that would result if the jet were transitionally supercritical. We have applied ballistic imaging to the same Diesel fuel injector, under similar conditions, and we find that the images do indicate a structural change when going from subcritical to supercritical conditions. Under subcritical conditions we observe a well-defined liquid/gas interface, surface wave structure, and formation of ligaments and voids. Under supercritical conditions the interface transitions into a continuous, turbulent mixing layer. Images of this layer include the cellular structure characteristic of gas jets. These changes are consistent with experimental literature on cryogenic supercritical jets and with DNS modeling of supercritical mixing layers.