Effects of Acoustic Excitation on the Combustion Instability of Hydrogen–Methane Lean Premixed Swirling Flames

Abstract

Lean premixed flames are useful for low nitrogen oxide (NO x ) emissions but more prone to induce combustion instability in gas turbines. Combustion instability of a lean premixed swirling flame (LPSF) with hydrogen–methane was investigated experimentally. The effects of hydrogen addition on combustion instability with equivalence ratios 0.75–1 were investigated with acoustic frequencies (90–240 Hz) and acoustic amplitudes (the ratio of velocity fluctuation to an average velocity of 0–0.5), respectively, which are characterized by the gain and phase of the flame describing function (FDF). The evolution of vortex and the flame morphologies were observed by the particle image velocimetry (PIV), intensified charge-coupled device (ICCD), photomultiplier tube (PMT), and Cassegrain optical systems. The global and local heat release fluctuations of the LPSF were shown by CH*/OH* chemiluminescence and temperature measurements. Results show that the FDF features maximum and minimum gain values in the acoustic frequency range of 90–240 Hz and reaches local maximum peaks at 110 and 180 Hz and local minimum peaks at 160 Hz. It can also be observed that varying velocity amplitudes (0–0.5) have greater effects on the gain and phase of FDF than changing equivalence ratios (0.75–1) for lean swirling flames. Higher velocity amplitudes more effectively intensified the compression of the flame length, which enhanced the mixing of the high-burning gas and the unburned gas, and then heat release fluctuations increased. However, it is more interesting that the effects of hydrogen addition on the combustion instability of the LPSF show a completely opposite phenomenon due to acoustic frequency under all experimental conditions. The FDFs were compared at typical frequencies of 140 and 180 Hz, and it was found that combustion instability enhanced with increasing hydrogen content at 140 Hz while weakened at 180 Hz. The flow field of PIV images shows that it is related to the location and development of vortices in the flame with varying acoustic frequencies. The intensity of OH*/CH* chemiluminescence, local temperature, and heat release rate show the same changing trend with the flame morphology for two acoustic parameters with the increasing hydrogen content in the LPSF. This directly affects the compression and curvature of the LPSF and thereby changes the mixture and temperature of the combustible gas, which influence the heat release fluctuation of the LPSF.

Analysis of Combustion Instability of Methane–Air Premixed Swirling Flame Based on OH-PLIF Measurements

“…The flame transfer function is an important method to quantify the degree of combustion instability, and the author has done a lot of research in this area. − According to the definition of the flame transfer function, the amplitude | H | represents the response of heat release pulsation to velocity pulsation. − The increase in amplitude represents that the ability of the flame to resist external disturbance is weakened, and the flame combustion state is more unstable. Figure shows the variation of flame transfer function amplitude | H | of lean premixed swirl flame at different equivalence ratios.…”

“…The flame fronts are calibrated in the flame binary images, and the stepped chemical reaction progression variables c = 0 (unburned zone) and c = 1 (burned zone) can be assumed at the flame fronts. The average reaction progress variable < c > can be obtained by superimposing all flame fronts under this operating condition and then averaging the N moment progress variables < c > = ∑ i = 1 N C i ( x , y ) / N where C i ( x , y ) represents the position of the flame brush ( x , y ) at a certain point in time and takes the value of 0 or 1.…”

“…The average reaction progress variable <c> can be obtained by superimposing all flame fronts under this operating condition and then averaging the N moment progress variables. 33…”

Combustion Instability of Swirl Premixed Flame with Dielectric Barrier Discharge Plasma

“…When the acoustic frequency is less than 100 Hz, the gain gradually increases with the acoustic frequency; when the acoustic frequency is between 100 and 220 Hz, the gain in flame transfer function tends to decrease first and then increase with the acoustic frequency; when the frequency is greater than 220 Hz, the gain gradually decreases with the acoustic frequency. This is related to the different response phases of the flame upstream and downstream of the acoustic frequency [19]. The flame response is controlled by two types of fluctuations, the first being related to azimuthal velocity perturbations generated by the swirler and convected by the flow, the second being associated with axial velocity perturbations propagating in the injector exhaust.…”

Optical Analysis of Blast Furnace Gas Combustion in a Laboratory Premixed Burner

“…Second, research on combustion chemiluminescence can also be developed with imaging techniques. For that purpose, cameras for UV, visible (vis), and IR ranges are set up with narrow-band filters to only measure light emissions related to the relevant radicals. ,− For example, the measurement of OH* emissions with imaging techniques enables the characterization of premixed flame fronts. ,, In addition, hyperspectral cameras can also be used to measure light emissions of several radicals simultaneously …”