Genesis, Evolution and Annihilation of Premixed Flames in Turbulence
10:00 - 10:30
Level 0 lecture hall between Al-Jazri and Al-Kindi (buildings 4 and 5)
Flames interacting with turbulence are continuously generated and annihilated by stretching and folding processes over a range of length-scales and time-scales. In this work, we address: from where and how do the complex topology and physico-chemical state of a fully developed turbulent premixed flame generate and evolve in time by analyzing the motion of flame particles. Flame particles are points that co-move with reactive isoscalar surfaces, which are representative of turbulent premixed flames. Direct Numerical Simulation (DNS) of H2-air turbulent premixed flames with detailed chemistry is combined with a computational methodology called the Backward Flame Particle Tracking (BFPT) algorithm. Uniformly distributed flame particles that entirely span isotherms at time tf are tracked backward to an earlier time ti (ti < tf). On backtracking, the once uniformly distributed flame particles form multiple clusters at some specific leading locations of the corresponding isotherms. Since Zeldovich, such leading locations or leading points have remained an enigmatic concept in combustion literature inducing strong hypotheses without concrete proofs of their role. The critical observation that entire flame surface evolves from multiple clusters of leading points at earlier times allows a Finite Strain Theoretic description of the turbulent flame in terms of these points. From this analysis, we observe that at the leading points the direction of minimum curvature is preferentially aligned with the most extensive direction of the left Cauchy-Green strain-rate tensor and vorticity. This, combined with stretching induced by flame propagation along the direction of maximum curvature cause the leading regions to become finite sized surfaces, several of which subsequently join together generating the complete surface at tf. The mean local flame displacement speed of the leading points is shown to be related to the global turbulent flame speed, albeit with a finite time-lag. Furthermore, mean local displacement speeds show a generalized response to stretch parameters in this Lagrangian viewpoint, from ti to all the way upto annihilation of the flame surfaces. Finally, the dispersing flame particles are shown to follow Batchelor's pair dispersion law which yields a ratio of generation to annihilation time-scales for turbulent premixed flames.