ID 206

Tomographic Stereo PIV and High Speed Visualization of String Cavitation in a Multi-Hole Diesel Injector

 

Abstract:

It is essential to control fuel spray characteristics in diesel engines to reduce exhaust gas emissions and improve thermal efficiency. Not only turbulent cavitation flow in a nozzle but also string cavitation in a sac of multi-hole fuel injectors plays an important role in fuel spray characteristics. However, string cavitation occurs randomly, and its appearing condition, mechanism and effects on spray characteristics remain unknown. In this study diesel fuel with appropriate amount of α-methyl naphthalene is injected at various constant flow rates and various needle lifts through an acrylic multi-hole mini-sac injector with three holes of 2 mm in diameter into ambient air at room temperature to match the refractive index of the acrylic wall and that of the liquid for clear visualization and measurement of turbulent cavitating flow. Cavitation in the holes, string cavitation in the sac, and injected fuel sprays are observed simultaneously using a high speed camera to clarify the occurring condition of unsteady string cavitation and its effects on injected spray. Three components of liquid velocity in three-dimensional space inside the sac is measured by tomographic stereo PIV (Particle Image Velocimetry) using for digital cameras and string cavitation in the sac and an injected fuel near a nozzle is captured simultaneously using a high speed camera. As a result, it is found that string cavitation often occurs at low needle lift randomly, and is caused by a turbulent strong vortex flow around it in the sac, while at high needle lift, a thin and short string cavitation rarely occurs due to a weak vertical flow in the sac, which does not strongly influence discharged spray. Just after the appearance of string cavitation, a swirling flow in the nozzle forms a hollow cone spray, which increases the spray angle drastically.