Pressure influence on the flame front curvature of turbulent premixed flames: comparison between experiment and theory

Abstract

In gas turbines, lean premixed combustion is executed in strongly turbulent flow fields and under high-pressure to allow large thermal loads within small-size combustors. Previous research on turbulent premixed flames has revealed the vital importance of flame-vortex interactions, but most of these investigations have been performed only at atmospheric pressure disregarding the large pressure dependency of the flame front dynamics. We report about spatially high-resolved laser-induced predissociation fluorescence imaging of OH ( OH- LIPF) in premixed, high-pressure bluff-body stabilized methane/air flames. For each of the two measurement series with different equivalence ratio (φ = 0.7 and φ = 1.0), the planar flame topology at different pressures (0.1 to 1.1 MPa) but constant exit velocity was detected and stored for analysis. As the pressure was increased, the flame front contour of both equivalence ratios became strongly wrinkled with formation of highly curved flame front elements. For quantification of this phenomenon, the probability density function of flame curvature was evaluated with definition of the mean curvature radius as representative folding scale. To discuss different mechanisms of flame front disturbances according to their relevance, the flame curvature is compared with characteristic turbulence scales of the flow field and with the expected folding scale derived with Sivashinsky‘s formulation of linear flame instability theory. Significant changes become obvious especially if the pressure is increased up to 0.5 MPa. The mean curvature radius decreases distinctly and can be linked to the decreasing size of the Taylor length. Additionally, the formation of highly convoluted flame front elements is enforced by the increasing flame instability behavior. As the results show, the flame stoichiometry has a strong impact on the flame front topology at increasing pressures due to the differences of their flame dynamics.