Influence of in-nozzle flow on spray morphology

Abstract

The diesel combustion process is strongly dependent on the rate of introduction of fuel and hence the quality of the atomization process, which in turn is significantly influenced by effects caused by the geometry of the fuel injection equipment itself as well as the potential occurrence of cavitation. In particular, the injector geometry of large marine two-stroke diesel engines differs substantially from the configurations used in most other diesel engine applications, as the injector orifices are distributed in a highly non-symmetric fashion. In order to simulate, respectively experimentally assess the impact of key features of such orifice arrangements on spray morphology, a generic nozzle design has been introduced in [1]. This specimen consists of an elongated tip with two orifices: The first one for producing the spray to be observed, the second one significantly far downstream of the first and with a diameter corresponding to the area of the remaining four orifices in a typical production injector, in order to simulate the same flow behaviour up to of the spray orifice. It was extended for the purpose of the present study by adding an insert, thus mimicking the flow conditions inside more recent injector designs. Selected configuration variants corresponding to isolated variations of key design parameters were investigated experimentally as well as by means of CFD. For this purpose, the IFP-C3D code, which includes an advanced cavitation model, has been utilised. The results obtained in those simulations help understanding how fluid-dynamic effects occurring in the injector influence the propagation of sprays. In particular, the observed non-symmetric spray structure can be clearly attributed to high levels of flow inhomogeneity at the exit of the orifice, a feature that is even more pronounced with eccentric arrangements of the orifice and is also to some extent depending on the height of the flow channel below the orifice inlet. For eccentric orifices, the generation of a swirling flow inside the orifice seems to contribute to clearly wider spray angles and sprays from such orifices are moreover characterised by non-negligible deflection from their theoretical axis. It could also be shown that it is essential to properly account for the actual flow conditions inside the injector, as single-spray tests neglecting the flow through the other orifices of a multi-hole injector yielded distinctly different spray behaviour, which would lead to erroneous conclusions when applied the results as is to multi-orifice configurations. Introduction The diesel combustion process and emission formation are not only highly dependent on hydro-carbon oxidation kinetics but also strongly influenced by turbulence and two-phase flow effects. In particular, this is related to the rate of introduction of fuel and hence the quality of the atomization process, which in turn is significantly affected by the geometry of the fuel injection equipment itself as well as the potential occurrence of cavitation. The injection system design and its operational parameters are decisive factors with respect to these processes. Nozzle internal phenomena, their impact on the fuel flow characteristics at the orifice outlet, and the associated effects on the sub-sequent spray formation and combustion processes are far from being fully understood today. As a consequence, they are commonly not accounted for in the spray-related sub-models applied in CFD simulations. An improved understanding of the liquid break-up physics is highly required and the spray boundary conditions at the nozzle exit in particular are of supreme importance for spray modelling purposes. In order to obtain such better understanding of those influencing factors, Wärtsilä Switzerland Ltd., PSI and the IFP worked together in a project focusing on the investigation of in nozzle flows in generic injector configurations. These nozzle designs allow the isolated study of the effect of key design parameters of actual large two-stroke engine injectors on fuel spray characteristics at relevant operating conditions. To this end, extensive computational fluid dynamic (CFD) investigations of the injection system internal flow have been performed, which help assessing the liquid conditions at the nozzle exit and, therefore, the boundary conditions for spray development. In parallel, the same generic injector designs have been applied on the Spray Combustion Chamber (SCC) test facility in order to characterize the associated spray behaviour by means of Mie-scattering measurements.