Time-resolved measurement of the near-nozzle air entrainment of high-pressure diesel spray by high-speed micro-PTV technique

Abstract

The high-speed micro-particle tracking velocimetry (micro-PTV) technique was applied to investigate the near-nozzle air entrainment into the high-pressure diesel spray at 28 kHz by analyzing the 1024 × 768 pixel2 image with the spatial resolution of 1.92 micrometer per pixel. The gas velocity component normal to a control surface along the spray boundaries was measured during the entire injection period, including both the start-of-injection (SOI) and end-of-injection (EOI) transients. Thus, the unique micro-PTV technique enables the instantaneous air entrainment rate to be evaluated as a function of time in one shot. The spray jets were injected by a single-hole nozzle with the ambient gas density of 18 kg/m3 and the injection pressure of 120 MPa. The air entrainment and spray angle were simultaneously measured and their correlations were analyzed. The spray angle shows a double peak profile as a function of time, and it is remarkably larger during the SOI and EOI transients than during the quasi-steady state. The air entrainment velocity exhibits a close dependence upon the spray angle. It is the increased spray angle that leads to the enhanced air entrainment rate during the SOI and EOI transients.