Fluid Mechanics and Acoustics Laboratory - UMR 5509

LMFA - UMR 5509
Laboratoire de Mécanique des Fluides et d’Acoustique
Lyon
France


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Cavitation

In diesel engines, fuel is injected into the combustion chamber under the form of a fine droplets spray. The engine efficiency in terms of consumption and pollution is strongly dependent on this spray’s characteristics. Fuel is injected at very high pressure (up to 2000 bar) through the nozzle holes, whose diameters typically range between 80 and 150 µm. The subsequent pressure usually drops down to the vapor pressure, leading to cavitation [1] inception.

Images simultanées de l’écoulement interne et du jet liquide. En haut, dans la buse, la cavitation et les bulles d’air apparaissent en noir et le liquide en clair. En bas, c’est le jet liquide et le spray qui apparaissent en noir. Le léger décentrement de l’orifice induit un développement asymétrique des poches de cavitation et du spray.

Cavitation is often regarded as a disadvantage. Cavitation is a source of noise, it can cause damage and vibrations. In diesel injection in particular, it also limits the flow rate. On the other hand, cavitation can prevent injectors from coking, which is a growing challenge in the context of decreasing hole diameters and increasing of the bio-fuel proportion. In addition, it is now commonly admitted that cavitation can enhance the efficiency of atomization processes. However, involved mechanisms are not well understood yet.
Injector nozzles are made of metal and are therefore opaque. Optical techniques, non-intrusive and highly accurate, can be applied only in transparent nozzle models. Such transparent nozzles have been developed in our laboratory during the 2000s. Recent progresses in nozzle fabrication and optics allow us to perform high-quality images of the flow, under cavitation condition and close to real scale condition (hole’s diameter down to 200 µm). An injection bench facility has been developed to study the influence of key parameters (geometrical parameters of the hole, pilot injection timing or needle lift,...) on cavitation inception and development through the nozzle.


[1Cavitation is a phase change (from liquid to vapor) induced by a pressure drop