Characterization of recirculating structures in the near field of underexpanded swirling jets

Recirculation in underexpanded supersonic jet flow with varying degrees of swirl was experimentally investigated due to its potential for enhanced flame holding in supersonic combustion engines. Recirculation caused by swirl is free of solid walls, avoiding the challenge of wall cooling due to high heat fluxes. Schlieren imaging, planar laser Mie scattering, and planar particle image velocimetry were used to observe and quantify shock cell and vortex structure, and where applicable, to map velocity in and around the recirculation zone. Case study showed that different patterns of tangential injection, pressure ratios, and nozzle sizes were more or less favorable to flow recirculation. Various metrics of recirculation and shock structure were correlated with the onset, variability, characteristic length, and characteristic time of recirculation vortex. Statistical analysis of instantaneous velocities indicated that (1) recirculation velocity and recirculation zone width had standard deviations of ~ 30% and ~ 20%, respectively (in part due to unsteady precession of the vortex around the axis of mean flow); and (2) the average velocity within the recirculation zone was seen to be insensitive to the pressure ratio (and hence, to the fully expanded Mach number). The recirculation zone width was slightly increased by doubling the pressure ratio, within the range investigated. Thus, under conditions that most favored recirculation, the net effect of (1) and (2) was the formation of a toroidal vortex structure within which the average velocity was sufficiently slowed (by a factor of 3 relative to the bulk velocity) to measurably enhance flame holding in supersonic combustor applications. Graphic abstract