Talks at Nordita Programs [before October 2010]
Imaging of turbulent eddies interacting with flamelets: To assess concepts of Damköhler and to quantify stretch efficiency
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Europe/Stockholm
Nordita seminar room 132:028
Nordita seminar room 132:028
Description
High-speed movies were obtained that show the time-history of turbulent eddies as they stretch and wrinkle the flamelets within a turbulent premixed flame. The goal is to assess classical ideas of Damkohler and Landau who predicted how eddies increase the flame surface area and increase the turbulent burning velocity.
Within a turbulent slot Bunsen flame, sub-grid properties were measured with Cinema PIV diagnostics using the same methodology that is used in Large Eddy Simulations. The measurement plane was divided into a grid; within each cell the vorticity (3), the strain-rate (S12) in the reactants, and the flamelet stretch rate (at) were spatially-averaged and relations between these quantities were determined. Strain-rate structures and vortical structures were tracked in a Lagrangian manner as they crossed the flamelet surface.
Images show that the strain-rate structures in the reactants and the vorticity structures (eddies) are separated in space. The strain-rate exerted on the flame (at) is better correlated with the strain-rate structures (S12) than the vorticity structures (3). That is, the strain structures in the reactants primarily strain the flame without wrinkling it (similar to a counter flow flame) while the vorticity structures primarily wrinkle the flame without straining it. Measurements yield the Stretch-Efficiency Function which is needed in several LES sub-grid models and has not been measured previously. A similar quantity, called the Strain-Rate Transfer function, also was measured and it provides new sub-grid relation that more accurately represents the measured eddy-flame interactions.