SUMMARY

One major challenge in turbulent premixed combustion theory is to model the influence of fuel/oxidizer composition on turbulent burning rates. Leading points concepts suggest that the turbulent burning velocity of premixed flames is controlled by the velocity of the points on the flame that propagate farthest out into the reactants: in this framework, it is postulated that modifications of the overall turbulent combustion speed due to fuel/oxidizer composition depend solely on modifications of the burning rate at the leading points. To investigate this proposed physical mechanism, first this thesis details a comparison between numerical simulations of quasi-steady one-dimensional flames in different configurations and statistics from a database of direct numerical simulations (DNS); second, the dynamics of flame propagation in simplified flow geometries is studied theoretically by means of the G-equation. The results obtained in this thesis validate some basic ideas from leading points arguments, but also modify them appreciably. In particular, the DNS study show that the leading portions of the flame front display a structure that on average can be reproduced reasonably well by results obtained from model geometries with the same curvature. However, the comparison between model laminar flame computations and highly curved flamelets is complicated by the presence of gas expansion across the flame which prevents the flame from becoming too curved for a given level of turbulent intensity. Furthermore, the theoretical results clearly show that only for sufficiently strong and steady flow perturbations, the front displacement speed is controlled by velocity field characteristics at discrete points on the flame. Besides, these points do not generally lie on the farthest forward point of the front. Finally, the effects of flame curvature sensitivity in modifying the front displacement speed can be successfully interpreted in term of leading point concepts.