Investigations of internal nozzle multiphase flow and its effects on diesel sprays

Cavitation has been known to occur in injector nozzles. This study focused on isolating the effects of nozzle cavitation and downstream ambient pressure on nozzle internal flow and spray using real diesel injectors at constant ambient density. Our initial studies revealed the distinctive effect of the cavitation number on flow discharge for sharp and rounded inlet nozzles.; The first phase of the spray experiments was conducted at two substantially different back pressures and the same ambient density using an in-line pump injection system with a single, inclined, sharp, inlet-hole nozzle. High magnification photographs were taken at the initial stages of the spray with different timings. High back pressure spray was observed to have smaller spray tip penetrations and larger Sauter Mean Diameters. The SMD for this case had a large decrease and the spray spreading angle had an increase when the cavitation number became smaller than about 1.4. This value was judged to be near the onset of cavitation for the nozzle and injection system used. The low discharge coefficients were judged to be caused by internal flow partial hydraulic flip for the high back pressure case. The low back pressure spray yielded larger spray spreading angle and smaller SMD.; In the second phase of the experiments, SMD measurements and momentum estimates for sharp and rounded inlet nozzles were performed using a common rail injection system for a range of back pressures and at the same back density. The nozzles otherwise had the same geometry. A Live Digital Light Extinction Technique (LDLET) was developed as the optical technique for spray SMD measurements. Calibration of the technique was improved for more accurate measurements. Discharge coefficient estimates were obtained by direct measurements of nozzle sac pressures. For the sharp inlet nozzle, multiphase flow choking was detected and the spray SMD was found to vary with the cavitation number for large cavitation number ranges. The rounded inlet nozzle yielded smaller SMD for the same injection conditions and its SMD was insensitive to the cavitation number. Results were compared and classifications of spray mixing were presented for the sharp and rounded inlet hole nozzles.