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Methane-air partially premixed flames subjected to grid-generated turbulence are stabilized in a two-slot burner with initial fuel concentration differences leading to stratification across the stoichiometric concentration. The fuel concentration gradient at the location corresponding to the flame base is measured using planar laser induced fluorescence (PLIF) of acetone in the non-reacting mixing field. Simultaneous PLIF of the OH radical and particle image velocimetry (PIV) measurements are performed to deduce the flow velocity and the flame front. These flames exhibit a convex premixed flame front and a trailing diffusion flame, with flow divergence upstream of the flame, as indicated by the instantaneous OH-PLIF, Mie scattering images, and PIV data. The mean streamwise velocity profile attains a global minimum just upstream of the flame front due to expansion of a gases caused by heat release. The flame speed measured just upstream of the flame leading edge is normalized with respect to the turbulent stoichiometric flame speed that takes into account variations in turbulent intensity and integral length scale. The turbulent edge flame speed exceeds the corresponding stoichiometric premixed flame speed and reaches a peak at a certain concentration gradient. The mean tangential strain at the flame leading edge locally peaks at the concentration gradient corresponding to the peak flame speed. The strain varies non-monotonically with the flame curvature unlike in a non-stratified curved premixed flame. The mechanism of peak flame speed is explained as the competition between availability of hot excess reactants from the premixed flame branches to the flame stretch induced due to flame curvature. The results suggest that the stabilization of lifted turbulent partially premixed flames occurs through an edge flame even at a relatively gentle concentration gradient. The strain is also evaluated along the flame front; it peaks at the flame leading edge and decreases gradually on either side of the leading edge. The present results also show qualitatively similar trends as those of laminar triple flames. � 2014 The Combustion Institute.

This study focuses on the thermal-diffusive oscillations of the full spectrum of co-flow laminar flames-from premixed to non-premixed through partially premixed flames for Lewis numbers sufficiently greater than unity. A premixedness parameter is identified using the analytical solution to the mixing field downstream of a splitter plate. Different premixedness levels ranging from zero (fully non-premixed) to unity (fully premixed) are then used as Dirichlet data for inlet concentrations of the reactants to the flame. The initial value problem is numerically solved by adopting a thermo-diffusive model and single-step finite rate chemical reaction and using higher-order compact schemes. A thumb-shaped region in the Damk�hler number - premixedness parameter space is identified as corresponding to limit cycle oscillations of the flame. This fills the gap between what is reported for the pulsating instability of non-premixed edge flames recently and fully premixed flames earlier. The thumb-shaped region enlarges with increase in the Lewis number in the range of 1.5-1.7, with marked increase in the oscillatory amplitude of the total heat release. A subcritical Hopf bifurcation is seen to occur at the boundary of this region in the above parametric space. The mechanism of oscillations is studied: hysteresis is found between the upstream and downstream propagation of the flame due to the thermal-diffusive imbalance. The oscillations are plotted in the phase space of the domain-integrated mass and heat diffusive fluxes of and to the reactants. The area of the limit cycle peaks at a particular Damk�hler number and premixedness parameter and drops to zero on both sides in this parametric space. � 2012 The Combustion Institute.