Measurement of high-speed and high-number-density droplets by micro-probe L2F with mega-hertz data acquisition

A laser 2-focus velocimeter (L2F) has been applied for the measurements of velocity and size of droplets in the core region of diesel spray. The L2F has a micro-scale probe which consists of two foci. The focal diameter is about 2 μm, and the distance between two foci is 20 μm. The feature of this L2F is that the focus is reduced to near the diffraction limit. Investigated was the fuel spray injected intermittently into the atmosphere from an injector nozzle with the orifice diameter of 0.113 mm. The injection pressure was set at 40 MPa by using a common rail system. Measurement positions were located at 10 to 30 mm apart from the orifice exit along the spray axis. Valid data were extracted by a conditional sampling method based on the fact that a droplet passes through both upstream and downstream foci. Measurement result shows that the velocity and size of droplets decrease with the distance along the spray axis. It is clearly shown that the distance between droplets in isotropically dispersed condition decreases with the distance along the spray axis.