Quantitative Fuel-Air-Mixing Measurements in Diesel-Like Sprays Emanating from Convergent and Divergent Multi-Layer Nozzles 2012-01-0464

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.