A burner for investigating lean stratified premixed flames propagating in intense
isotropic turbulence has been developed. Lean pre-mixtures of methane at different
equivalence ratios are divided between two concentric co-flows to obtain annular
stratification. Turbulence generators are used to control the level of turbulence intensity
in the oncoming flow. A third annular weakly swirling air flow provides the flame
stabilization mechanism. A fundamental characteristic is that flame stabilization does
not rely on flow recirculation. The flames are maintained at a position where the local
mass flux balances the burning rate, the result is a freely propagating turbulent flame
front. The absence of physical surfaces in the vicinity of the flame provides free access
for laser diagnostics.
Stereoscopic Planar Image Velocimetry (SPIV) has been applied to obtain the three
components of the instantaneous velocity vectors on a vertical plane above the burner
outlet where the flames propagate. The instantaneous temperature fields have been
determined through Laser Induced Rayleigh (LIRay) scattering. Planar Laser Induced
Fluorescence (PLIF) on acetone has been used to calculate the average equivalence ratio
distributions. Instantaneous turbulent burning velocities have been extracted from SPIV
results, while flame curvature and flame thermal thickness values have been calculated
using the instantaneous temperature fields. The probability distributions of these
quantities have been compared considering the separate influence of equivalence ratio
stratification and turbulence. It has been observed that increased levels of turbulence
determine higher turbulent burning velocities and flame front wrinkling. Flames
characterized by stronger fuel stratification showed higher values in turbulent burning
velocities. From the curvature analysis emerged that increased fuel concentration
gradients favour flame wrinkling, especially when associated with positive small radius
of curvature. This determines an increased surface area available for reaction that
promotes a faster propagation of the flame front in the oncoming combustible mixtures
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