Ignition of Individual Droplets in a Reactive Fuel/Air Mixture behind Reflected Shock Waves

Multiphase-induced ignition is frequently discussed as a trigger for early ignition in internal combustion engines. In this context, we investigated the ignition process of single lubricant-oil droplets and their interaction with the bulk air/fuel mixture in a high-pressure shock tube, mimicking oil-fuel interaction in turbocharged internal combustion engines at the end of the compression stroke. A fast micro-dispensing injector released single fuel or lubricant oil droplets with a diameter of 200±50 µm into shock-heated fuel/air mixtures consisting of PRF95 and synthetic air. The injector was flush-mounted in the sidewall of the shock tube. The droplets were released into the gas after the passage of the reflected shock waves at post-shock conditions of 2 MPa and 750-950 K. With a high-frame-rate color camera, the entire evolution of droplet injection and ignition was traced in space and time through a large sapphire window in the endwall of the shock tube. A high-repetition-rate laser at 532 nm was used to illuminate the droplets. The light scattered by the droplets was detected in the green channel of the camera, whereas ignition and soot luminosity were detected in the blue and red channel, respectively. From the camera images the time for the appearance of first detected kernel related to local ignition (originating from either droplet or gaseous ignition) was determined. Additionally, the volume ignition that proceeds after an ignition delay time was determined from the OH* emission time-trace from a photomultiplier tube. It was found that the oil-droplet injection had an influence on the local ignition whereas the ignition delay time was reduced in the low temperature range only.