The current research focuses on the thermoacoustic instability of lean premixed combustion, which is a promising technique to inhibit Nitrogen Oxides (NOx) emission. Thermoacoustic instability describes the condition that the pressure oscillation is unusually high in the combustion device. It results from the coupling between pressure fluctuation and heat release oscillation, which experiences significant temporal and spatial variations. These variations are closely related to the flame shape deformation and critical in determining the trend of the global instability. Therefore, the current study aims to examine both the global and local flame features created by thermoacoustic instability.
The first part of the work is studying the unstable flame induced by artificial acoustic perturbation. The particular focus is on the global and local heat release rate oscillation. In the experiment, the global heat release rate oscillation was indicated by the hydroxyl (OH*) chemiluminescence captured with a photomultiplier tube (PMT). On the other hand, the flame shape and the local mean heat release rate were examined with flame surface density (FSD), which was calculated with the images captured with the planar laser-induced fluorescence of the hydroxide radical (OH-PLIF) method. The main analysis methods used in the current research are Rayleigh criterion and proper orthogonal decomposition (POD), which can efficiently capture the dominant oscillation mode of the flame.
The acoustic perturbation study first examined the effect of pressure variation (0.1 - 0.4 MPa) on the flame response to the acoustic perturbation. Results show that the elevated pressure intensifies the fundamental mode of heat release oscillation when the heat release oscillation is in phase with the pressure fluctuation; otherwise, the fundamental oscillation tends to be inhibited. The pressure affects both the strength and the distribution of the local fundamental and the first harmonic oscillations. Furthermore, the effect of the pressure on the distribution is larger than that on the strength.
The study also investigated the role of Strouhal numbers in characterizing the flame oscillation induced by acoustic perturbation. Results show that the Strouhal number can characterize the changing trend of the oscillation amplitude, whereas the oscillation phase-delay is less dependent on the Strouhal number. The local analysis reveals that the nonlinear flame behavior results from the flame rollup induced by acoustic perturbation. Furthermore, the reconstruction of the global heat release shows that the cancellation of out-of-phase local oscillations can cause a low-level global oscillation. Results also demonstrate that the local heat release oscillation contains intense harmonic oscillations, which are closely associated with the flame rollup. However, the harmonic oscillation is less likely the main reason causing nonlinear flame behavior.
Besides the study with acoustic perturbation, the current study also conducted experimental and modeling studies on the self-excited thermoacoustic instability. The particular focus is examining the effects of hydrogen addition on the instability trend. Results demonstrate that the hydrogen concentration can affect both the oscillation frequency and amplitude. Pressure analysis shows that the low-frequency mode is triggered when the hydrogen concentration is low, whereas a high hydrogen concentration tends to excite a high-frequency mode. Moreover, the frequency tends to increase with an increasing hydrogen concentration. Modeling results illustrate that the change of the oscillation mode, which is determined by the turbulent flame speed, is mainly affected by the delay time between the heat release oscillation and the velocity fluctuation. The modeling work shows that the one-dimensional model is not very efficient in capture the instability trend of the high-frequency mode. It may result from the lack of the knowledge of the mechanism of acoustic damping and flame dynamics.