It is the objective of this work to characterize mixture
formation in the sprays emanating from Multi-Layer (ML) nozzles
under approximately engine-like conditions by quantitative,
spatially, and temporally resolved fuel-air ratio and temperature
measurements. ML nozzles are cluster nozzles which have more than
one circle of orifices. They were introduced previously, in order
to overcome the limitations of conventional nozzles. In particular,
the ML design yields the potential of variable spray interaction,
so that mixture formation could be controlled according to the
operating condition. In general, it was also a primary aim of the
cluster-nozzle concepts to combine the enhanced atomization and
pre-mixing of small nozzle holes with the longer spray penetration
lengths of large holes.
The applied diagnostic, which is based on 1d spontaneous Raman
scattering, yields the quantitative stoichiometric ratio and the
temperature in the vapor phase. The measurements are conducted in
non-reacting sprays slightly downstream of the
liquid-phase-penetration length, because flame-lift-off
stabilization generally occurs in the vicinity of the liquid tip in
comparable combusting sprays under quasi-steady, engine-like
conditions. It is well established that the stoichiometric ratio in
the region of flame lift-off significantly affects the soot
formation in diesel sprays.
The measurements are conducted in a high-temperature,
high-pressure vessel. N-decane is used as the fuel, because it is a
commonly applied model fuel for standard diesel. The investigated
diesel-like sprays emanate from a state-of-the-art piezo
injector.
In the present work, the results of two ML nozzles with two
circles of orifices are compared. The plane of the two holes in
each cluster is parallel to the injector axis. The clustered nozzle
holes are convergent (-4°) for one of the ML nozzles, whereas they
are divergent (+4°) for the other one. Two conventional nozzles
with one circle of orifices are also investigated, one with the
same flow number as the ML nozzles and the other one with halved
flow number, corresponding to a single hole of the ML nozzles. The
temporal and spatial evolution of the quantitative stoichiometric
ratio and temperature is determined and discussed. Furthermore, the
shot-to-shot variability in these quantities is analyzed. These
measurements essentially show that the ensemble-averaged
fuel-air-ratio distributions are very similar for both ML nozzles
and the reference nozzle with large orifices, but they are
significantly different for the reference nozzle with small
orifices. The shot-to-shot variability in the fuel-air ratio is
generally very similar for the ML nozzles as compared to a
previously investigated cluster nozzle with only one orifice
circle, indicating that the particularly complex in-nozzle flow
does not lead to enhanced fluctuations of the outcome of the
mixture formation process. The results also lead to conclusions on
soot formation in comparable combusting sprays emanating from ML
nozzles. Apparently, the soot-reduction potential cannot be
improved by enhancing evaporation and penetration of the free spray
simultaneously using an ML nozzle. Thus, previously observed
reduced engine-out soot emissions for ML nozzles could be explained
by wall impingement or differences in flame lift-off.