Characteristics of a transient diesel fuel spray.

摘要

The transient characteristics of a diesel fuel spray have been investigated with the simultaneous measurement of droplet sizes and velocities, and photographic images of the spray.; Measurement of droplet sizes and velocities have been performed at axial positions from the near nozzle (x/d = 41.7) to the far-field (x/d = 250) for a single hole nozzle (d = 0.24) mm). At each position, measurements at several radial positions were obtained. For all measurement locations, operating conditions included four gas-to-liquid density ratios, that cover the range from free atmospheric jets to conditions typical of diesel engines. Photographic images of the spray were taken for the entire spray and local portions of the spray with magnified photographs from two different nozzle holes (d = 0.24 mm and 0.41 mm) at ambient gas pressures ranging from near-vacuum to 1.34 MPa.; Increases in {dollar}rhosb{lcub}rm g{rcub}{dollar}/{dollar}rhosb{lcub}l{rcub}{dollar} resulted in greatly decelerated and extended fuel sprays. Near the nozzle, droplet sizes ranged from the lower limit of the measurement system (2{dollar}mu{dollar}m) to the order of nozzle diameter for all {dollar}rhosb{lcub}rm g{rcub}{dollar}/{dollar}rhosb{lcub}l{rcub}{dollar}. Along the spray axis, the droplet sizes slowly decrease with axial position. At the edge of spray, droplet velocities have shown wave-like behavior. Negative velocity components at the downstream edge at high {dollar}rhosb{lcub}rm g{rcub}{dollar}/{dollar}rhosb{lcub}l{rcub}{dollar} are observed because of vortices formed in the mixing layer.; Photographs showed surface waves at lower {dollar}rhosb{lcub}rm g{rcub}{dollar}/{dollar}rhosb{lcub}l{rcub}{dollar}. The near-nozzle spray angle was largest in the initial stage of the injection and decreased to a relatively constant value after 0.5 msec.; Detailed measurements of drop size and velocity were compared to established stability criteria for different droplet breakup mechanisms, specifically criteria for bag breakup and boundary layer stripping. Comparison with the stability criteria indicates that a majority of droplets in the spray are susceptible to both breakup mechanisms near the injector tip. However, downstream or on the edge of the spray, the spray appears to stabilize and any redistribution of droplet size must apparently be a result of collisions.; Experimentally-determined droplet size and velocity PDF's show characteristics that are strongly dependent on time and spatial position in the spray. Comparisons with computed PDF's show best agreement with the data for the multiparameter-dependent distributions, such as the chi-square or log-hyperbolic distributions.