Vortex-flame interactions and extinction in turbulent jet diffusion flames

Abstract

Unsteady vortex-flame interactions and subsequent local extinction have been investigated in double-concentric turbulent methane jet diffusion flames stabilized on a thick-walled fuel tube using joint Mie scattering/thin-filament pyrometry (MS/TFP) and two-color particle image velocimetry (PIV). Particle images visualized the evolution and development of large-scale vortices in the internal fuel jet shear layer and external annulus air jet boundary. The real-time MS/TFP technique captured the events when the internal vortices poked through the flame zone, and the PIV measurements revealed the instantaneous velocity fields in flames interfered by the internal or external vortices. The visible (blue) flame zone turns luminous due to soot formation downstream (>5 jet diameter), where the flame bulges out in response to a radial movement of the internal or external vortices. By contrast, in the near-jet region where the flame zone was strained and relatively immobile, two modes of local extinction were observed: (1) interference by the internal jet-fluid vortices extending outward to the flame zone and (2) interference by the external annulus-fluid yortices perturbing air entrainment. In both cases, the entrainment velocity increased locally, and cold air passed through the quenched holes in the flame zone into the internal vortical structure in the intermittent mixing layer. Local flame extinction is attributed to two distinct unsteady mechanisms: (1) a sudden increase in the fuel diffusive influx as a result of a large Peclet number for mass transfer associated with the internal vortex and (2) an increase in the convective influx as a result of enhanced air entrainment due to the external vortex's squeezing motion.