High-resolution microscopic imaging
The direct visualisation of diesel sprays using high-resolution microscopy can give novel insight into the complex processes involved in the initial stage of spray formation.
The light source and imaging optics are optimised to produce shadowgraphic images of sprays with unprecedented sharpness. Sub-micron resolutions and ultra-short exposure times allow the observation of previously unreported shearing instabilities and the stagnation point on the tip of diesel jets.
An oblate spheroidal cap was observed for a wide range of conditions, which may consist of residual fuel from the previous injection. These findings suggest that vapourised fuel may remain trapped in the injector holes after the end of the injection process, and would support the theory that the formation of deposits in the holes of diesel injector nozzles may be linked to the degradation of such residual fuel.
Ultra high speed microscopic video
The complex interaction between the spheroidal cap and the jet that follows were observed using an ultra high-speed camera, operated at up to 50 million images per second.
Although still apparent, the spheroidal cap is much less distinguishable at evaporating conditions. At elevated in-cylinder temperatures the spheroidal cap is in vapour state, and can be observed due to the refraction of the light caused by density gradients.
A vortex ring motion within the vapour phase was identified, and resulted in a slipstream effect, which led to a central ligament being propelled ahead of the liquid jet. This phenomenon had been reported in the literature, but had remained unexplained until our research.