Experimental investigation of the response of premixed and non-premixed turbulent flames to acoustic forcing

This paper describes an experimental investigation of acoustically forced turbulent bluff-body stabilised flames with swirl and with three different degrees of premixing: fully premixed, non-premixed with radial fuel injection and non-premixed with axial fuel injection flames. The flame was imaged using OH* chemiluminescence at 5 kHz. The heat release response of these flames to acoustic forcing at 160 Hz, which was the frequency that gave the maximum oscillation amplitude, was studied quantitatively with the calculation of the Nonlinear Flame Transfer Function (NFTF) for various forcing amplitudes, equivalence ratios, and air velocities. The post-processing analysis also consisted of phase-averaged OH* chemiluminescence images. It was found that non-premixed flames with radial fuel injection exhibited a much greater response to acoustic forcing, followed by premixed and non-premixed flames with axial fuel injection. In the premixed system, the magnitude of the heat release response was greater for higher air velocities, however the effect of equivalence ratio was more complex. Both non-premixed systems showed a reduced sensitivity to input air velocity and global equivalence ratio. Also, it was found that for the conditions studied, all three systems showed a nonlinear response. The qualitative analysis of the flame dynamics showed that in the premixed system the flame roll-up at the bluff body edge plays an important role in the flame response. In the non-premixed system with radial fuel injection, apart from the flame roll-up, the temporal variation in equivalence ratio constitutes an important phenomenon

Response of flames with different degrees of premixedness to acoustic oscillations

The response of three flames with different degrees of premixedness (fully premixed, non-premixed with radial, and non-premixed with axial fuel injection) to acoustic oscillations is studied experimentally. The flames were imaged using OH* chemiluminescence and OH planar laser-induced fluorescence at 5 kHz. In addition to a flame kinematics analysis, the amplitude dependence of the transfer function was calculated. The dominant spatial structures of the heat release and their periodicity were examined using the proper orthogonal decomposition (POD) method. The Non-Premixed system with Radial fuel injection (NPR) showed the highest response to acoustic forcing, followed by the fully premixed and the Non-Premixed system with Axial fuel injection (NPA). In addition, the response of the non-premixed system with radial fuel injection was greater than that of the fully premixed system for various bulk velocities U, global equivalence ratios φ, forcing amplitudes A, and forcing frequencies f. In the fully premixed system, the heat release modulation was mainly through flame surface area modulation, while in the NPR system, both the flame area and the equivalence ratio modulations were found to be important mechanisms of the heat release oscillations. About 70% of the energy of the total fluctuations in the NPR case was contained in the first four POD modes, a percentage that decreased with overall equivalence ratio, but only this dropped to about 40% for the NPA flame. The frequency spectra of the coefficients of the POD modes exhibited peaks at the forcing frequency, with increasing broadband contributions in higher modes and for the NPA flame

Dynamics of acoustically forced non-premixed flames close to blow-off

The effect of forced oscillations on the behaviour of non-premixed swirling methane flames close to the lean blow out limits was investigated using experiments in a lab-scale burner. Two different fuel injection geometries, non-premixed with radial -NPR- and non-premixed with axial -NPA- fuel injection, are considered. The flame behaviour is studied using 5 kHz OH∗ chemiluminescence and OH Planar Laser Induced Fluorescence (OH PLIF) imaging. In both systems, acoustic forcing reduces the stability of the flame, and in particular, the stability was found to decrease with the increase in forcing amplitude. Flame lift-off was observed in both configurations, with the magnitude of the effect of forcing depending on the fuel injection configuration. The results provide insight on the effect of superimposed flow field fluctuations in systems operating close to the lean blow out limits and offer useful data for the development and validation of numerical models for the prediction of the dynamic behaviour of flames of industrial interest

Proper orthogonal decomposition analysis of a turbulent swirling self-excited premixed flame

Thermoacoustic oscillations constitute a serious threat to the integrity of combustion systems. The goal of the present work is to determine the effect of the equivalence ratio (φ), inlet flow velocity (U), and burner geometry on the characteristics of the self-excited oscillations and to reveal the dominant mechanisms. Experiments were conducted with a fully-premixed air/methane flame stabilized on a conical bluff body. Self-excited acoustic instabilities were induced by extending the length of the combustion chamber downstream of the bluff body. The flame was visualised using OH* chemiluminescence and OH PLIF at 5 kHz. For the data post-processing, apart from a Fast Fourier Transform analysis, the Proper Orthogonal Decomposition technique was applied on the imaging data aiming at extracting information about the dynamics that are not captured through classical ensemble-averaging. A strong effect of the chamber length was found, which primarily drove the generation of acoustic oscillation and flame-vortex interaction. Significant differences in the flame roll-up were found when either the burner geometry or the equivalence ratio was altered. Changes were detected in the frequency of oscillations, which showed a general trend to increase with φ and U and decrease with the length of the duct. Analysis of the POD modes allowed an estimate of the convection speed of the flame structures associated with the dominant frequency and it was found that this convection speed was about 1.5 U for most conditions studied

Numerical investigation of the response of turbulent swirl non-premixed flames to air flow oscillations

The response of swirl non-premixed flames to air flow oscillations is studied using Large-Eddy Simulation (LES) and the Conditional Moment Closure (CMC) combustion model, focusing on the physical mechanisms leading to the heat release rate oscillations observed in a parallel experimental study. Cases relatively close to blow-off and characterized by different amplitude of the flow oscillations are considered. Numerical results are in good agreement with the experiment in terms of both mean flame shape and heat release rate response. Simulations show that the oscillation of the air flow leads to an axial movement and fragmentation of the flame that are more pronounced with increasing amplitude of the forcing. The flame response is characterized by fluctuations of the flame area, time-varying local extinction and lift-off from the fuel injection point. LES-CMC, due to the inherent capability to capture burning state transitions, predicts properly the flame transfer function as a function of the amplitude of the air flow oscillations. This suggests that the response mechanism for this flame is not only due to time-varying flame area, but also local extinction and re-ignition. This study demonstrates that LES-CMC is a useful tool for the analysis of the response of flames of technical interest to large velocity oscillations and for the prediction of the flame transfer function in conditions close to blow-off. EPSRC Grant EP/R029369/