Visualization and analysis of the characteristics of transitional underexpanded jets

Underexpanded jets can be formed when high-pressure gaseous fuel is injected directly into an engine cylinder. In such conditions, shock waves are formed immediately near the nozzle exit. In the present study, the flow structure and turbulent mixing of pulsed jets issuing from a circular nozzle is investigated using acetone planar laser-induced fluorescence (PLIF). By monitoring axial and various radial cross-sections under different injection pressure conditions, different features of gaseous jets are visualized and interpreted. The temporal development of the axial cross-sections reveals three typical jet flow patterns (subsonic, moderately underexpanded, and highly underexpanded) during the injection. These stages are (1) well described with the observed shock structures and (2) noted to take a considerably long portion of the full injection process. The visualizations from the radial cross sections show how the nozzle inflow conditions may influence the primary and the azimuthal (secondary) instability of the jet which influences the turbulence transition process and the mixing process. The results indicate the importance of inner nozzle flow on the flow behavior. For example, systematic asymmetries in the mean concentration fields are observed. In addition to PLIF data, numerical simulations can be used to support the experimental picture of the jet behavior. We give examples of large-eddy simulations (LESs) in order to further explore the behavior of the underexpanded jets. Results show that LES is able to reproduce the basic physics of underexpanded jets. LES and PLIF compare favorably in terms of the barrel shock structures and the description of the normal shocks. LES also provides detailed flow field information including temperature, Mach number, concentration and scalar dissipation rate (SDR).