ㄽ ᩥ ࡢ せ ᪨㢟┠  Droplet size and velocity characteristics of Diesel spray injected by common rail
injection systemࢥ࣮ࣔࣥࣞࣝᄇᑕ⣔࠿ࡽᄇᑕࡉࢀࡓࢹ࢕࣮ࢮࣝᄇ㟝ࡢ⢏ᚄ࡜㏿ᗘࡢ≉ᛶ ࡈࡏ࠸Ặ ྡ ྻΎ (WU, QING)
When a Diesel spray is injected into the cylinder with high injection pressure, it will form
small droplets after spray breakup. Droplet size distribution and its spatial distribution directly
determine the droplet evaporation speed and spatial concentration difference, which will further
influence the engine working and emission performances. The imaging-based technique is adopted
in this study to capture the droplet images before and after the end of fuel injection. Droplet
characteristic parameters, such as droplet velocity, droplet diameter and distance between droplets
can hereafter be obtained through imaging processing method. In addition to the experimental
analysis, simulation study of Diesel spray is also conducted.
The main points and conclusions of this study can be summarized as the following aspects:
In Chapter 2, the experimental setup and investigation methods are introduced. The method
adopted in the microscopic droplet imaging is a kind of imaging-based technique, compared withLDSA and PDPA, can capture the original images of spray droplets, and the droplet size can beobtained without any empirical functions. Besides, the droplets captured through imaging-based
technique are considerable, which ensures the reliability of subsequent analysis results such as
droplet size distribution curve.
In Chapter 3, the variations of droplet size, droplet velocity and We number of spray dropletsin the spray tip region and spray middle periphery region within a short duration before and after
the end of injection under different injection pressure and ambient pressure are studied to find out
their influence on the spatial droplet characteristics. For a certain spray, the droplet velocity in Y
direction will be more pronounced than the velocity in X direction over time. Increasing ambientpressure and will speed up the spray momentum dissipation. Besides, spray droplets in the spraytip region have a wider range than the middle periphery region. The droplet size distributiondifference of different measurement locations in the spray tip region will become more obvious ifinjection pressure or ambient pressure increases. But in the spray middle periphery region,
increasing injection pressure or ambient pressure will weaken the difference between spatial
droplet size distribution curves. This is accounted for the interaction between droplets and ambientgas. In a certain region, either droplet inertial momentum or vortex of air flow is dominant, thespatial variation in this region will become evident. Compared with the change in injection
pressure, altering ambient pressure will make greater change in the spatial droplet size
distribution.
In Chapter 4, the spray droplets along spray axis at 10 ms ASOI are studied to better
understand droplet distribution characteristics of the whole spray. Droplet size analysis is the focus

since the droplet velocity at this timing is fully dissipated. It can be found from comprehensive
analysis that increasing injection pressure will promote spray atomization but increasing ambientpressure will deteriorate spray atomization. The droplet size distribution curve along spray axis
under higher ambient pressure first changes from unimodal to bimodal, then shifts back tounimodal. However, the droplet size distribution curve along spray axis under near-atmosphericpressure remains unimodal. Correspondingly, the SMD along the spray axis first increases, thenstabilizes for a certain range, and finally decreases under higher ambient pressure. But thevariation in the SMD along the spray axis under near-atmospheric pressure tends to be linear. This
indicates under higher ambient pressure, high concentration mixture is likely to form in the spray
middle region, while under low ambient pressure, it tends to form in the spray tip region. Inaddition, among the three droplet size distribution functions R-R, N-T and L-N, L-N is the best
one in predicting droplet size distribution. What is more, the SMD calculated from the modified
SMD empirical formula is in good agreement with experimental results.
In Chapter 5, the simulation of Diesel spray development is conducted and compared withthe experimental results. Though the simulation result cannot be completely consistent with theexperimental result, it can describe the spray development process, and the change regularity ofspray tip penetration and SMD along spray axis under different injection pressure and ambientpressure. In the KHRT model, changing the constant which determines the breakup time ofKH-WAVE model will make great impact on the simulation result. Poor agreement between
experimental and simulation results is the most obvious in the spray tip and tail regions. This isowing to fuel spray introduction method that cannot match the reality and the idealized model indetermining the interaction between spray droplets and ambient gas.