MORPHOLOGICAL EXPLORATION OF EMERGING JET FLOWS FROM MULTI-HOLE DIESEL INJECTORS AT DIFFERENT NEEDLE LIFTS

摘要

The current study takes a morphological approach to interpret the emerging jet flows from multi-hole diesel injectors. Several types of multi-hole injectors, a six-hole injector and two two-hole injectors with different needle control mechanisms, were used to investigate the emerging jet flows and related flow breakup at different needle lifts. A short X-ray pulse with 150 ps duration was used to visualize the near-field morphologies of the emerging jet flows using an ultrafast X-ray phase-contrast imaging technique. A few X-ray pulses with 68 ns periodicity were also used to analyze the dynamics of the emerging jet flows by tracking the movement of the structures inside the spray. At first, the effects of needle lift on emerging flow pattern and breakup were investigated using a six-hole injector under practical injection conditions. A highly expanding spray was observed at the low needle lifts. The degree of flow expansion was however suppressed with an increase in the needle lift. The higher degree of flow expansion at the low needle lifts promoted the flow breakup and increased the spray deceleration rate with an increase in the axial distance. Then, a detailed morphological study of the emerging flows was performed using two-hole nozzles under low injection pressures to slow down the flow breakup in order to figure out the intrinsic nature of the emerging flows associated with the nozzle internal flow. The phase-contrast images revealed clear morphologies of several branching flows inside the spray having different flowing directions and stretching the spray three-dimensionally that originate from complex nozzle internal flow pattern. The degree of flow expansion associated with the branching flows appeared differently with the needle lift with formation of various flow structures: cone shaped, stretched thin, and cylindrical. At certain needle lifts, the branching flows sometimes formed a couple of microwavelets inside the spray having different instability frequencies, indicating different origins of each flow associated with nozzle internal flow. Increasing ambient gas density did not alter the branching characteristics of the flows significantly, while increasing injection pressure and reducing the fuel viscosity significantly altered the branching flow characteristics.