Measured properties of turbulent premixed flames for model assessment, including burning velocities, stretch rates, and surface densities

Several previously unreported properties of turbulent premixed flames were measured because they are especially useful for the future assessment of direct numerical simulations and models. These new properties include local stretch rates, a wrinkling parameter, the degree of flamelet extinction, and the reaction layer thickness, which were quantified using simultaneous CH planar laser-induced fluorescence/particle image velocimetry (CH PLIF-PIV) diagnostics. Other reported properties that are useful for model assessment are flame surface density (Σ) and global consumption speed, which is one type of turbulent burning velocity. Also measured was the Meneveau-Poinsot stretch efficiency function (ΓK), which plays a central role in the coherent flamelet model. Some images of the flame-eddy interactions show how eddies exert strain and how flamelets “merge.” A highly wrinkled (corrugated) flame with well-defined boundary conditions was stabilized on a large two-dimensional slot Bunsen burner. It was found that the turbulent burning velocity of Bunsen flames depends on the mean velocity U¯, which was varied independently of turbulence intensity. It is concluded that conventional relations for the turbulent burning velocity of Bunsen flames are inadequate because they should include two additional parameters: mean velocity U¯ and burner width W. These parameters affect the residence times of the flame-eddy interactions. A scaling analysis is presented to explain the observed trends. It indicates that if the burner width is sufficiently large, the long flame will experience significant flamelet merging, which is one factor leading to the “bending” (nonlinear behavior) of the burning velocity curve. Images of CH layers show that flame surface area is lost by flamelet merging, but is not lost due to local extinction, as no extinction was observed. The stretch efficiency function increases with increasing integral scale, indicating that large eddies are more efficient in exerting flame stretch than small eddies.