The flow in the injector's sac volume has been reported to influence diesel-injector nozzle flow, but few studies have characterized sac volume. Our study modeled flow in the sac volume using a large Eddy simulation (LES) approach to gain better insight into the complexity of the flow dynamics. It focused on the effect of fixed needle lifts on sac-volume internal flow of a single-hole injector with emphasis on large-scale unsteadiness; three-dimensional proper orthogonal decomposition (POD) was used to analyze the flow. The near-wall turbulence resolution of the elaborated computational fluid dynamics (CFD) model has been validated with direct numerical simulation (DNS) results in the canonical case of fully developed channel flow. The main findings are: (1) an enlarging flow jet entering the sac volume with decreasing small scales of turbulence was observed as needle lift increased. (2) three-dimensional POD revealed that the mean flow energy was nearly constant at low needle lifts (6%, 8%, and 10%) and decreased twofold at the higher needle lift of 31%. (3) The analysis of fluctuating modes revealed that flow restructuring occurred with increasing needle lift as three different energy distributions were observed with the lowest (6%), intermediary (8%, 10%, and 16%), and highest needle lifts (31%). (4) Finally, the analysis of the POD-reduced-order model has shown that the lowest frequency of mode 1, which carries the highest fluctuating energy, is responsible for the oscillation of the main rotating structure within the sac volume that causes fuel-jet enlarging/narrowing with time. This oscillation of the main structure was found to decrease with increased needle lift.

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