In-nozzle multi-phase flow patterns in flash boiling atomization and their impacts on external sprays

Flash boiling atomization is a promising method to produce finer droplets with a higher vapor concentration for fuel combustion purposes. The vapor phase is regarded as a key component to enable better atomization efficiency in flash boiling atomization, but the exact mechanisms are missing. Typical two-dimensional slit nozzle measurements can reveal in-nozzle superheated flow characteristics, but volumetric features cannot be approximated by enlarged two-dimensional slit nozzles. In this work, we fabricate a rectangular, optical accessible nozzle with dimensions close to real fuel injector nozzles. The single-component fuel of n-pentane was used to trigger superheated in-nozzle flows and consecutive flash boiling sprays. Microscopic and macroscopic features of the two-phase in-nozzle flows were captured at different fuel temperatures and injection pressures. It is found that the flow patterns inside the nozzle during flash boiling are similar to those observed in vertical tube boiling flows. It is also observed that the external spray atomization is closely related to the in-nozzle two-phase flow patterns. Finally, new criteria for categorizing flash boiling regimes based on the flow pattern are proposed.