1997-05-01

Air Entrainment in a High Pressure Diesel Spray 971620

This paper presents some experimental results of air velocity measurements near high pressure diesel sprays. The measurements were made using a moderately high pressure (90 MPa) common rail injector in a pressurized spray chamber. The chamber was operated at ambient temperature (25°C) and was pressurized with Argon to produce a chamber gas density of about 27 kg/m3, similar to densities found in a large turbocharged diesel near TDC. The gas phase was tagged using water droplets doped with Stilbene 420, with an estimated droplet size of 18 μm. The atomized water-Stilbene droplets were illuminated with the third harmonic of a pair of Nd:YAG lasers which caused the Stilbene to fluoresce at about 420 nm. To reduce the competing fluorescence from the injected fuel, the injector was fueled with Jet-A fuel.
Using the two lasers, double exposures of the small droplets were recorded on film. The laser pulse lengths were about 6 ns, and typical times between pulses were 100 μs. The resulting images were analyzed as Particle Image Velocimetry images to determine the gas phase velocity magnitude and direction near the fuel spray.
The results of the measurements show variation in gas velocities normal and tangential to the outer envelope of the spray. At early times after the start of injection, velocities normal to the spray plume are highest near the injector tip and decrease quickly along the spray axis. At later times, the gas velocity near the injector tip is somewhat higher than at earlier times. As a result, while downstream velocities are much higher, the decrease along the axis of the spray is much smaller.
Estimates of air mass flux show that the mass flux of air into the spray region is small soon after the start of injection, except near the injector tip where the liquid velocities are high. At later times after the start of injection, the air mass flux into the spray is much larger and is nearly constant along the axis of the spray. At the end of injection, the estimated local mass flux of air into the spray is about 10% of the local mass flux of fuel in the spray.

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