Pulsation of Burner-Stabilized CH4-O2 Flames Moderated by CO2 Addition

TL;DR

This study experimentally characterizes pulsating instabilities in burner-stabilized CH4-O2 flames with CO2 dilution, using a porous-plug burner and a comprehensive diagnostic suite to capture spatiotemporal flame dynamics. The authors extract the primary oscillation frequency $f_{osc}$ via harmonic power analysis and reveal a secondary low-frequency component $f_{FI}$ that modulates the high-frequency thermo-diffusive pulsation, leading to sidebands and, at higher heat release, multi-mode oscillations. Regime diagrams map instability boundaries across equivalence ratio $ extphi$, CO2 dilution $ extEta$, and CH4 flow, showing instability only under fuel-rich conditions and a non-monotonic dependence on flow with CO2 dilution; the effective Lewis number $Le$ is found to span $0.96$–$1.09$, with pulsations absent for $Le<1$. The work advances understanding of CO2-moderated oxy-combustion flame dynamics and provides a framework that can extend to other fuels and boundary conditions, aiding both fundamental flame theory and practical instability mitigation.

Abstract

This study investigated the pulsating instability of burner-stabilized premixed CH4-O2 flames at various levels of CO2 dilution. Experiments were conducted using a water-cooled porous-plug burner of 18 mm diameter over a wide range of mixture compositions and flow rates, during which time-resolved measurements of flame chemiluminescence and gas temperature were obtained. The primary oscillation frequencies of the pulsating flames were determined using fast Fourier transform and harmonic power analysis. Phase-locked analysis of the chemiluminescence images revealed an interesting mode-transition phenomenon of the flame oscillations. Under fuel-rich conditions with relatively low heat release rates and low flow rates, the flames exhibited quasi-periodic single-mode oscillations. At elevated flow rates, these oscillations were modulated by low-frequency flame flickering instabilities, which created sidebands around the primary oscillation frequency. At higher heat release rates, the flickering instability further triggered mode splitting, eventually leading to multi-mode oscillations. Regime diagrams of the flame oscillation modes, as well as the stability boundaries, were obtained under various fuel flow rates. These findings can be useful for both fundamental research on flame dynamics and practical applications of CO2-moderated oxy-combustion.

Figures (11)

• Figure 1: The current experimental setup. (a) Configuration of the porous-plug burner. (b) Example of a burner-stabilized flame. (c) Detailed view of the water cooling channel. (d) Photograph of the static mixer. (e) Schematic of the flame chemiluminescence and the tunable diode laser diagnostics. (f) The side-view schlieren diagnostic. (g) The side-view OH-PLIF diagnostic system.
• Figure 2: Example measurement of a premixed CH$_4$-O$_2$ flame pulsating in a single axisymmetric mode under the conditions of $\dot{m}_{\rm CH_{4}}$ = 0.20 SLPM and $\dot{m}_{\rm O_{2}}$ = 0.30 SLPM. (a) A spectrogram of the absorbance signal at different beam locations, with data obtained from various oscillation cycles at a fixed relative time of 10 ms. (b) The integrated absorbance of an isolated feature at different beam locations. (c) The Spatial distribution of gas temperature reconstructed from integrated absorbances under the assumption of axial symmetry.
• Figure 3: (a) Representative OH* NPSD spectra of pulsating flames at three different conditions. Top (blue): $\phi$ = 1.72, $\eta$ = 1.00; middle (green): $\phi$ = 1.72, $\eta$ = 0.71; bottom (red): $\phi$ = 1.47, $\eta$ = 0.71. $f_{\text{OSC}}$: the primary oscillation frequency; $f_{\text{FI}}$: the flickering instability frequency. (b) Total harmonic power analysis of the three pulsating flames.
• Figure 4: Example low-frequency oscillation of a methane-oxygen flame at $\phi$ = 1.20, $\eta$ = 1.00. (a) the FFT spectrum of OH* normalized power spectral density (NPSD), (b) sequential side-view images of the flame, (c) side-view schlieren images of the toroidal vortex formation and shredding.
• Figure 5: Scaling of the measured non-dimensional flame flickering frequency (St) versus the inverse Froude number (1/Fr). Results are shown for purely flickering flames without thermo-diffusive pulsations.