Swirling turbulent non-premixed flames of methane: Flow field and compositional structure

This paper introduces a new swirl burner which has simple, well-defined boundary conditions and which stabilizes complex, turbulent, unconfined flames that are not unlike those found in practical combustors. Two flames with identical swirl numbers but differing bulk jet velocities (U) over bar (j) are selected for further investigations. Flow field measurements reveal that a second recirculation zone may exist on the centerline of the flames further downstream of the primary recirculation zone. This is attributed to vortex breakdown. The measurements also show the presence of highly rotating collar-like flow features present between the primary and secondary recirculation zones. These regions of the flow are characterized by high tangential shear stresses (u'w'). The compositional structures of these methane flames are measured using the simultaneous Raman-Rayleigh laser-induced fluorescence (LIF) technique. The LIF technique is used to measure concentrations of OH, CO, and NO. Results are presented as scatter plots and radial Favre mean profiles of temperature, mixing, and composition fields. As the fuel jet velocity increases and the flame approaches blowoff, a higher proportion of fluid samples shifts away from fully burned conditions and closer to a mixing asymptote. An interesting feature of these flames is that these locally extinguished samples originate mostly from regions of the flow near the high-shear, collar-like region, which is not found in similar bluff-body flames.