Investigation of Droplets and Ambient Gas Interaction in a Diesel Spray Using a Nano-Spark Photography Method

A single nano-spark back light photography method has been developed to record the image of non-evaporating diesel sprays injected into high pressure nitrogen gas. Relatively clear image of fine droplets and spray was obtained. An image analysis method has been developed to quantify the droplet characteristics which are in focus, such as droplet size and shape. Spatial and temporal distribution of droplets has been clarified. It was observed that the number of droplets around the nozzle tip region decreases by time, however a large number of droplets were observed at X=13∼25 mm from nozzle tip at t=300∼700 μs from injection start. Double-nano spark photography of diesel sprays was carried out and relatively clear double exposure images of droplets were obtained on the same film. Two dimensional size and velocity measurement of droplets were simultaneously carried out based on these photographs. Distribution of droplets velocity was clarified and used to describe the droplets distribution. It was observed that at early period of injection droplets fly away from the spray at a certain angle. However, by time they lose their radial velocity and start to entrain into the spray.

Variation of droplets velocity persuaded the authors to find out a relation between droplets and ambient gas velocity. Ambient gas around the diesel sprays was seeded by light particles and two-dimensional velocity measurement of ambient gas was carried out using a Particle Tracking Velocimetry (PTV) method. Velocity information of droplets and ambient gas was used to clarify the droplets-gas interaction. According to the results, there are two types of ambient gas flow around the diesel sprays. Around the nozzle tip region, ambient gas entrains steeply into sprays, however a vortex of ambient gas was observed around the spray tip. Maximum velocity of ambient gas was very low and about 1/5 of maximum droplet velocities. The velocity of droplets was also much lower than the penetration velocity of spray.