An experimental investigation of low-speed nonpremixed flames and buoyant jets using particle tracking

Low-speed non-premixed flames are dominated by large-scale vortical structures and are characterized by a natural "flickering" frequency. Flickering is a phenomenon marked by flame roll-up and is believed to occur in response to buoyancy forces acting on the hot regions of the flow. The objective of this research is to better understand flickering and the underlying flame-flow interactions. A laminar co-flowing axisymmetric methane-air flame, exhibiting many of the same structural features as a turbulent flame, was selected for use in this research. A helium-air jet was also studied as a possible model of the flame without the complexities of combustion and heat release.;Various flow diagnostics were used to study the flows including: schlieren photography, planar Mie scattering flow visualization, and planar laser induced fluorescence of OH. The primary data is in the form of planar velocity vector fields measured using particle track velocimetry. These measurements were made over the first twenty jet diameters of the flame and over the first five diameters of the helium-air jet. The periodicity of the flow field was fixed using an acoustic excitation of the jet fluid and the velocity measurements were phase-locked to this excitation signal. When the excitation frequency coincides with the natural flickering frequency, flame pinch-off is observed. The velocity data was interpolated to a uniform grid and differentiated to obtain vorticity and strain fields. The non-premixed flame is characterized by a vortex birth-growth cycle which begins when the annular flame sheet contracts toward the centerline causing local fluid acceleration. A region of high compressive strain associated with this vortex development has been identified in a region where flame extinction is observed. The direction of compressive strain tends to be aligned normal to the flame sheet. The vortices observed in the helium-air jet are unlike the flame in that they exhibit a much smaller growth rate and originate in the near-exit region of the jet. Phase-plane analysis has been used to describe these flows in terms of elementary flow patterns. The flow topology is readily drawn from the planar velocity measurements and shows the primary flow structure of the flame to consist of three critical points: a closed center bounded by two saddle points. The invariants of the deformation tensor at these critical points show a marked increase prior to flame pinch-off, followed by relaxation after the pinch-off. The topology of the helium-air jet consists of closed centers and saddles with a central alleyway on the centerline. The differences in the initial buoyancy distribution in these two flows is believed to be responsible for the differences in flow development.