On the structural sensitivity of purely strained planar premixed flames to strain rate variations

The effects of aerodynamic straining on the structure and response of adiabatic, unrestrained, equidiffusive, planar premixed flames were experimentally and computationally studied via the counterflow, twin-flame configuration formed by oppositely directed identical jets of nitrogen-diluted, near-stoichiometric methane/air mixtures. Experimentally, the velocity, temperature and major species concentration profiles were determined as functions of the applied strain rate by using LDV and spontaneous Raman scattering. Computationally, the experimental situation was simulated with detailed reaction mechanisms and transport properties. Both the experimental and computational results show that the temperature and species structure of the flame in the direction normal to the flame surface remains largely similar in response to variations in strain rate as long as the flame is sufficiently far away from the stagnation surface so that incomplete reaction is minimal. These results substantiate the concepts that the scalar structure of the flame, and thereby the flame thickness, are insensitive to strain rate variations for these purely strained flames, and that these flames cannot be extinguished by straining alone. The computed results are further shown to agree quantitatively with the experimental data, hence supporting the usefulness of the computational model for the simulation of strained flames. Implications of present findings on the concept of the local flow time, the extinction of strained flames, the modelling of turbulent flames through the concept of laminar flamelets, and flame stabilization and blowoff, are discussed.