Experimental investigation of the nonlinear response of turbulent premixed flames to imposed inlet velocity oscillations

Abstract

This paper describes an experimental investigation of acoustically forced lean premixed turbulent bluff-body-stabilised flames in an enclosure short enough so that no coupling of the combustor downstream acoustics occurred for the frequencies studied here, which allows an unambiguous examination of the flame response to inlet velocity fluctuations. Special emphasis was placed on the amplitude dependence of this response. Measurements of the heat release rate were performed with OH^∗ and CH^∗ chemiluminescence, planar laser-induced fluorescence (PLIF) of OH from which the flame surface density (FSD) was computed, and simultaneous CH₂O and OH PLIF imaging from which the local heat release rate (RX) was estimated. The global heat release measured with chemiluminescence and that integrated from the local FSD measurements were in close agreement, while a comparison between FSD and high-resolution RX imaging also showed good agreement. This suggests that estimates of the flame area are sufficient to determine heat release rate for this flow. The heat release response became nonlinear after inlet velocity amplitudes of around 15% of the bulk velocity. This value depended on the forcing frequency and the equivalence ratio. The nonlinearity was found to occur when the shear layers rolled up into vortices. The vortices induced by the inlet velocity fluctuations not only generated flame area when the flame wrapped around them, but also caused cusps and even large-scale flame annihilation events, as observed in time-resolved OH PLIF images. Such events occurred when parts of the flame stabilised on the inner shear layer close to the recirculation zone collapsed on parts of the flame stabilised on the outer recirculation zone, a phenomenon that was made more prominent with increasing forcing amplitude. A further nonlinearity occurred at high amplitudes and at some equivalence ratios, where a significant leakage of energy to higher harmonics was observed, but the origin of this is not yet clarified. The present results suggest that the flame sheet kinematics play a major role in the saturation mechanism of lean premixed flame response, hence extending previous experimental and analytical results from laminar to turbulent flames. Heat release fluctuations due to local fluctuations of strain rate and curvature were less significant, while no localised extinction has been observed even at large forcing amplitudes.