Measurement of High–Number–Density Droplets in Diesel Spray using 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 had a micro–scale probe which consists of two foci. The distance between two foci was 14 µm and the focus size was nominally 3 µm. Valid data were extracted by the criteria that a downstream signal was sequentially observed after an upstream signal. The maximum data sampling rate of the L2F was set at 15 MHz. Fuel was intermittently injected into the atmosphere by using a common rail injector. The nozzle orifice diameter of the injector was 0.113 mm and the injection pressure was set at 70 MPa. Five measurement positions were located in the plane of 15 mm from the nozzle orifice. The L2F could detect droplets just after the spray tip reached the measurement position. Measurement result showed that the velocity and size of droplets in the spray core decreased at the off–axis region. It was clearly shown that the distance between droplets decreased towards the off–axis direction within 0.6mm. It could be mentioned that short distances between droplets after disintegration can be measured by a mega–hertz data–rate sampling.