buletinul institutului politehnic din iaşi - Universitatea Tehnică ...

Bul. Inst. Polit. Iaşi, t. LVIII (LXII), f. 4, 2012 183
A very important factor in direct injection diesel engines is the quality of
the diesel spray. An improvement in the air fuel mixing leads to a better
combustion process that results in higher performance, but also modifies
pollutant emissions (Pierpont&Reitz, 1995). The spray characteristics are
strongly influenced by the nature of the flow inside the injection nozzle hole
(Sazhin et al., 2001, Pavri et al., 2004). This internal flow is largely dependent
on the presence of the cavitation phenomenon (Soteriou et al., 1995, Chaves et
al., 1995). On the other hand, momentum flux is a very important parameter for
predicting the mixing potential of injection processes. Despite this, there are
very few studies that use this kind of measurement (Kampmann et al., 1996;
Siebers, 1999). Important factors such as spray penetration, spray cone angle
and air entrainment depend largely on spray momentum (Rajaratnam, 1974).
With the simultaneous determination of the momentum and mass flux it is
possible to determine the velocity in the outlet section of the nozzle hole. In Fig.
5, point i corresponds to the upstream point where velocity could be neglected.
Point b corresponds to the outlet section of the hole. (Payri et al., 2004). It can
be seen in Fig. 5 how the cavitation influence injection.
Fig. 5 – Fluid separation inside a Diesel nozzle hole.
The phase transition from liquid to vapour of a fluid due to low pressure
is called cavitation. Cavitation in a diesel nozzle appears at the inlet of the
nozzle hole. In this region, and due to the strong change in cross-section and
flow direction, the boundary layer tends to separate from the hole wall and a socalled
“vena contracta” is established. As a consequence, a recirculation zone
appears between the vena contracta and the orifice wall. In this zone, there is a