Holographic Investigations of a Diesel Jet injected into a high‐pressure test chamber

Abstract

AbstractA Diesel spray injected into a high‐pressure test chamber was investigated with two different holographic techniques. The usual off‐axis recording geometry was used to investigate the very early beginning of the injection. Double pulsing of the recording laser facilitated velocity measurements. By using two different reference angles for the two recording the two images can be separated upon reconstruction of the holograms. Thus superior image quality could be achieved. Processes leading to jet formation could be identified: the spray tip is periodically axially decelerated and radially accelerated. Consequently, liquid moves at the tip off axis. The spray consists of fast central region surrounded by a slower outer jet region. The outer jet region exhibits periodic droplet concentration fluctuations which are interpreted as a result of the periodic processes at the spray tip. Some high concentration regions in the outer jet region were investigated using a dark‐field holographic technique. This technique permits single droplet velocity measurements and local droplet concentration determination at extremely high droplet concentrations. The most important result of these investigations is that no correlation between the local droplet concentration and the droplet velocity could be found.