Influence of a wall cooling air film on the behavior of a turbulent premixed flame

Thermal durability in aero-engines is a key challenge with the advent of downsized and high-power density combustion chambers. Liner effusion cooling maintains appropriate heat fluxes, thereby improving flame-cooling air interaction (FCAI) processes. Beyond the thermal effectiveness of the wall, FCAI requires a better understanding compared to conventional flame–wall interactions, to assess the modifications of the flame topology. This study intends to examine how the thermal wall cooling performance and the flame topology are impacted when the momentum of a cooling air film (i.e. blowing ratio) is varied. Experiments are performed in a lab-scale combustion chamber operating at atmospheric pressure. A V-shaped turbulent lean premixed CH 4 /air flame stabilized on a ceramic rod is used to interact with an air-cooled steel wall. The cooling air film is generated by a splash cooling system located at the bottom of the wall. The flame topology, the 2D aerodynamic flow field and the 2D wall temperature distribution are measured simultaneously by Planar Laser-induced Fluorescence on hydroxyl radicals (OH-PLIF), Particle Image Velocimetry (PIV) and surface Phosphor Thermometry (PT). Results reveal various FCAI processes, depending on the blowing ratio induced by the cooling air film. For blowing ratios below unity, the cooling air film provides a limited impact on the thermal protection of the wall, while the flame topology exhibits a flame–wall interaction pattern with positive flame curvatures. For blowing ratios larger than unity, the cooling air film gives an additional insulation layer, reinforcing the wall thermal protection. The strong shear flow layer then governs the level of flame wrinkling. Located in the outer region of the shear layer, the flame is subjected to the effects of negative flow strain, while heat loss/dilution processes remain negligible. Novelty and significance statement Wall thermal management in combustion systems is critical in terms of safety and durability, but also essential to achieve high-efficient combustion systems. Routinely used in combustor liners, cooling air films create a thermal protection of the wall. However, the intrusive cooling air can influence the behavior of a flame, which still remains unclear. Differing from aerothermal studies and classical flame–wall interaction, this study considers the configuration of a flame cooling-air interaction, and systematically elucidates the role of a parietal cooling air film on the dynamics of a turbulent premixed flame. Taking advantage of laser-based planar diagnostics, this study highlights for the first time different regimes of flame-cooling air interaction, being driven by the cooling air momentum. Interestingly, the higher cooling air film efficiency is found to be unfavorable for the flame dynamics, with important flame straining and reduced flame activity.