[EN] Flashboiling denotes the phenomenon of rapid evaporation and atomization at nozzles, which occurs when fluids
are injected into ambient pressure below their own vapor pressure. It happens in gasoline direct injection (GDI)
engines at low loads, when the cylinder pressure is low during injection due to the closed throttle valve. The fuel
temperature at the same time approaches cylinder head coolant temperature due to the longer dwell time of the
fuel inside the injector. Flash boiling is mainly beneficial for atomization quality, since it produces small droplet
sizes and relative broad and homogenous droplet distributions within the spray. Coherently, the penetration depth
normally decreases due to the increased aerodynamic drag. Therefore the thermal properties of injectors are
often designed to reach flash boiling conditions as early as possible. At the same time, flash boiling significantly
increases the risk of undesired spray collapsing. In this case, neighbouring jets converge and form a single jet.
Due to the now concentrated mass, penetration depth is enhanced again and can lead to liner or piston wetting in
addition to the overall diminished mixture formation.
In order to understand the underlying physics, it is important to study the occurring phenomena flashboiling and
jet-to-jet interacting i.e. spray collapsing separately. To this end, single hole injectors are built up to allow for an
isolated investigation of flashboiling. The rapid expansion at the nozzle outlet is visualized with a microscopic high
speed setup and the forces that lead to the characteristic spray expansion are discussed. Moreover, the results
on the macroscopic spray in terms of penetration, cone angles and vapor phase are shown with a high speed
Schlieren setup. Resulting droplet diameters and velocities are measured using LDA/PDA.
As a result, we find a comprehensive picture of flash boiling. The underlying physics can be described and
discussed for the specific case of high pressure injection at engine relevant nozzle geometries and conditions, but
independently from neighbouring jets. These findings provide the basis to understand and investigate flashboiling
and jet-to-jet interaction as distinct, but interacting subjects rather than a combined phenomenon.The authors would like to thank Continental for providing the experimental injectors used in this paper and
Specialised Imaging for providing parts of the equipment used for the shown measurements.
Additionally the authors gratefully acknowledge the financial support for parts of their work from the Erlangen
Graduate School in Advanced Optical Technologies (SAOT) within the framework of the German Excellence
Initiative by the German Research Foundation (DFG).Bornschlegel, S.; Conrad, C.; Eichhorn, L.; Wensing, M. (2017). Flashboiling atomization in nozzles for GDI engines. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 321-328. https://doi.org/10.4995/ILASS2017.2017.4750OCS32132
