Analysis of combustion instability of hydrogen fueled scramjet combustor on high-speed OH-PLIF measurements and dynamic mode decomposition

This paper investigated the combustion instability of spanwise positions in a hydrogen fueled scramjet combustor with a cavity flame holder. High-speed OH-PLIF technique was performed on a direct-connect supersonic combustion facility, and dynamic mode decomposition (DMD) as postprocessing. Combustion instability was investigated by characterizing the dominant frequencies and growth factors. By changing the equivalence ratio of hydrogen, the peak frequencies of scramjet mode and ramjet mode were obtained. Scramjet mode tended to have small oscillation at 150–200 Hz reflected by negative growth factors due to the stable flame structure. At ram-to-scram transition, oscillations at 80–120 Hz were remarkably enhanced due to the positive growth factors. In ramjet mode, the large differences of frequency characteristics in spanwise positions were observed. The dominant DMD modes near the cavity wall appeared to have negative growth factors leading to a stable flame with small oscillations. Besides, the characteristics of frequency-shift were affected by the positions of injector.     

Non-premixed acoustically perturbed swirling flame dynamics

An investigation into the response of non-premixed swirling flames to acoustic perturbations at various frequencies (f_p = 0-315 Hz) and swirl intensities (S = 0.09 and 0.34) is carried out. Perturbations are generated using a loudspeaker at the base of an atmospheric co-flow burner with resulting velocity oscillation amplitudes |u′/U_avg| in the 0.03-0.30 range. The dependence of flame dynamics on the relative richness of the flame is investigated by studying various constant fuel flow rate flame configurations. Flame heat release rate is quantitatively measured using a photomultiplier with a 430 nm bandpass filter for observing CH∗ chemiluminescence which is simultaneously imaged with a phase-locked CCD camera. The flame response is observed to exhibit a low-pass filter characteristic with minimal flame response beyond pulsing frequencies of 200 Hz. Flames at lower fuel flow rates are observed to remain attached to the central fuel pipe at all acoustic pulsing frequencies. PIV imaging of the associated isothermal fields show the amplification in flame aspect ratio is caused by the narrowing of the inner recirculation zone (IRZ). Good correlation is observed between the estimated flame surface area and the heat release rate signature at higher swirl intensity flame configurations. A flame response index analogous to the Rayleigh criterion in non-forced flames is used to assess the potential for a strong flame response at specific perturbation configurations and is found to be a good predictor of highly responsive modes. Phase conditioned analysis of the flame dynamics yield additional criteria in highly responsive modes to include the effective amplitude of velocity oscillations induced by the acoustic pulsing. In addition, highly responsive modes were characterized by velocity to heat release rate phase differences in the ±π/2 range. A final observed characteristic in highly responsive flames is a Strouhal number between 1 and 3.5 based on the burner co-flow annulus diameter (St = f_pU_avg/d_m). Finally, wavelet analyses of heat release rate perturbations indicate highly responsive modes are characterized by sustained low frequency oscillations which accompany the high amplitude velocity perturbations at these modes. Higher intensity low frequency heat release rate oscillations are observed for lean flame/low pulsing frequency conditions.     

An experimental study of fuel–air mixing section on unstable combustion in a dump combustor

The main objective of this study is effect of the various fuel–air mixing section geometries on the unstable combustion. For the purpose of observing the combustion pressure oscillation and phase difference at each of the dynamic pressure results, the multi-channel dynamic pressure transducers were located on the combustor and inlet mixing section. By using an optically accessible quartz-type combustor, we were able to OH* measurements to characterize the flame structure and heat release oscillation with the use of a high-speed ICCD camera. In this study, we observed two dominant instability frequencies. Lower frequencies were measured around 240–380 Hz, which were associated with a fundamental longitudinal mode of combustor length. Higher frequencies were measured around 410–830 Hz. These were related to the secondary longitudinal mode in the combustion chamber and the secondary quarter-wave mode in the inlet mixing section. These second instability mode characteristics are coupled with the conditions of the combustor and inlet mixing section acoustic geometry. Also, these higher combustion instability characteristics include dynamic pressure oscillation of the inlet mixing section part, which was larger than the combustor section. As a result, combustion instability was strongly affected by the acoustically coupling of the combustor and inlet mixing section geometry.     

A study on acoustically modulated bunsen flame and its impingement heat transfer

This study experimentally examines acoustic-field-flame-interaction by using a low-power loudspeaker to actuate the oscillation of a Bunsen flame. It is observed that under acoustic forcing, the flow dynamics are altered, different patterns of the flame front are triggered, and both flame temperature field and heat transfer characteristics are changed. Moreover, impingement heat transfer is found to be increased when the flame is under acoustic modulation, indicating that acoustics can be used to promote heat transfer for flame impingement heating applications.There is a threshold forcing frequency of 300 Hz, beyond which no interaction between the sound and flame exists. The response of the flame to acoustic excitation exhibits a double-cone structure to naked eyes, and is found to be convectively bubbling, wrinkling and shrinking flame front under high-speed photography. The oscillating flame front height has exactly the same frequency as the sound, but the waveform is non-sinusoidal. Both symmetric and asymmetric distorted flame fronts are observed, with the former occurring at low frequencies while the latter at relatively higher frequencies.The effect of acoustic field on the thermal field is to lower the high-temperature region of the flame. Therefore, the cool core in the centre is narrowed, leading to higher local heat transfer. A ten percent increase in total heat transfer rate is obtained when the optimum nozzle-to-plate distance is coupled with the most effective forcing frequency of 50 Hz. Therefore, acoustic modulation is a feasible technique for promoting heat transfer.     

Effects of different preheated CO2/O2 jet in cross-flow on combustion instability and emissions in a lean-premixed combustor

To reveal the suppression mechanism of thermoacoustic instability flames under CO₂/O₂ jet in cross flow. Experiments on the effects of different preheated CO₂/O₂ jet in cross-flow (JICF) on combustion instability and NO_x emissions in a lean-premixed combustor were conducted in a model gas turbine combustor. Two variables of the JICF were investigated—the flow rate and the temperature. Results indicate that combustion instability and NOx emissions could be suppressed when the JICF flow rate increases from 1 to 5 L/min. The average pressure amplitude decreases from 18.6 Pa to 1.6 Pa, and the average NOx emission decreases from 26.4 ppm to 12.1 ppm. But the average pressures amplitude and NOx emissions increase as the JICF temperature grows up. The sound pressure and the flame heat release rate exhibits different mode-shifting characteristics. The oscillation frequency of the sound pressure almost unchanged under JICF injection. However, the oscillation frequency of the heat release rate jumps from 95 Hz to 275 Hz under different JICF temperatures. As the CO₂/O₂ JICF flow rate arrived 3 L/min, the oscillation frequency of flame heat release rate jumps from 85 Hz to 265 Hz. The color of the flame fronts and roots were changed by the JICF injection. The average length of flame under CO₂/O₂ JICF cases is shorter than the N₂/O₂ JICF cases. There are three different modes of flames when the CO₂/O₂ JICF flow rate varies, and two different modes of flames when the CO₂/O₂ JICF temperature varies. This article explored the joint effects of different CO₂/O₂ or N₂/O₂ JICF on combustion instability and NOx emissions, which could be instructive to the designing of safely and clean combustors in industrial gas turbines.     

Combustion dynamics of a low-swirl combustor

The methodology for the measurement of dynamic combustor behavior has never been clearly established, due to the complexities associated with unsteady premixed flames and the difficulties in their measurement. The global and local distribution of Rayleigh index and the flame response functions are the main parameters normally employed to quantify and describe combustion dynamics. The Rayleigh index quantifies the thermoacoustic coupling, while the flame response function is a measure of the response of the system to outside disturbances. The primary objective of this work is to investigate the combustion dynamics of a commonly used low-swirl burner and to develop tools and methods for examining the dynamics of a combustion system. To this end, the effect of acoustic forcing at various frequencies on flame heat release behavior has been investigated. The current work uses OH-PLIF imaging of the flame region to produce phase-resolved measurements of flame behavior at each frequency. The response of the flame to the imposed acoustic field over the range of 22-400 Hz is then calculated from the processed images. This provides a starting point for an extension/extrapolation to practical acoustic ranges (∼5000 Hz). It was found that the thermoacoustic coupling was mainly evident in the shear mixing zone, producing a toroidal Rayleigh index distribution pattern. The phase shift of the flame fluctuation from the imposed acoustic wave seems to be very closely coupled to the vortices generated at the flame boundary due to shear mixing (Kelvin-Helmholtz instability), thus inducing the alternating toroidal structures. The peak value of the flame response function coincides with the peak absolute value of the Rayleigh index.     

Effects of hydrogen enhancement on mesoscale burner array flame stability under acoustic perturbations

Partial substitution of hydrocarbon fuel with hydrogen can effectively improve small-scale combustion system stability and performance, potentially opening the way for novel compact power generation and/or propulsion systems in the future. In this study, the effects of hydrogen enhancement between 0% and 40% hydrogen volumetric fractions in methane fuel were experimentally observed in a mesoscale burner array subjected to external acoustic perturbations. The mesoscale burner array utilizes an array of swirl-stabilized burner elements and their interactions with neighboring elements to improve the overall flame stability and simultaneously reduces the combustor length scale. OH1 chemiluminescence and OH planar laser-induced fluorescence (OH-PLIF) were used to image various hydrogen-enriched flames at an equivalence ratio of 0.7, subjected to transverse acoustic perturbations at 320 Hz. Two acoustic modes were imposed by controlling the phase difference between two speakers perturbing the flow. OH1 chemiluminescence images exhibited flame length scale reduction, leading to a denser flame array. Also, flame arrays with higher hydrogen enrichment were found to be more robust against transverse acoustic perturbations, demonstrated by reduced fluctuations in the global heat release rate. OH-PLIF images showed that flames with higher hydrogen enrichment initiated V- to M-shaped flame shape transition even under fuel lean conditions, thereby improving the combustion stability. OH-PLIF images were also used for flame stability analysis through spectral proper orthogonal decomposition (SPOD). The SPOD analysis showed hydrogen enrichment diminished flame fluctuation structures under fuel lean operation.     

An experimental study on the oscillation of the propagating syngas-air flame in a duct

In this paper, premixed syngas-air flame propagating from the open end to the closed end were experimentally investigated. The effects of equivalence ratios, 0.8 ≤ Ф ≤ 1.2, and hydrogen volume fractions, 10% ≤ α(H₂) ≤ 90%, on flame deformation and oscillation had been discussed in detail. The tulip-like flame was observed because of the large pressure gradient. Results indicate that the pressure wave plays an important role in the flame deformation and oscillation. The flame oscillates as hydrogen volume fraction varies. There are two oscillation modes. When the flame oscillates as mode Ⅰ, the flame first oscillates smoothly, then the oscillation is gradually enhanced, and finally the oscillation decays. The interaction of flame and pressure waves continuously stimulates the flame deformation and oscillation, finally the violent flame folding emerges in the later stage. When the flame oscillates as mode Ⅱ, the flame just oscillates violently in the early stage.     

Experimental study of explosion dynamics of syngas flames in the narrow channel

This article introduced the experimental study of the propagation of a syngas premixed flame in a narrow channel. The structural evolution, flame front position and velocity characteristics of lean and rich premixed flames were investigated at different hydrogen volume fractions as the flame was ignited at the open end of the pipe and propagated to the closed end. The comparative study of the syngas fuel characteristics, flame oscillation frequency and overpressure oscillation frequency prove that the syngas explosion flame oscillation in the narrow passage has a strong coupling relationship with overpressure and fuel heat release rate. The results was shown that the flame structure was strongly influenced by the hydrogen volume fraction of the syngas and the fuel concentration. The distorted tulip flame only appears in lean mixture. At 30% of hydrogen volume fraction, the flame exhibits intense and unstable propagation, manifested as the reciprocating and alternating movement of the flame front. As the volume fraction of hydrogen increased, the velocity of flame propagation and the frequency of oscillation increased. When the hydrogen volume fraction γ ≥ 0.4 at the equivalence ratio of Φ = 0.8, the pressure oscillation amplitude gradually increases and reaching the peak after 200–320 ms. Significantly, when γ = 0.3, the pressure peak increases abnormally. This work can provide support for the safe use of syngas in industry by experimental study of various explosion parameters in the narrow channel.     

Experimental investigation of thermoacoustic coupling using blended hydrogen–methane fuels in a low swirl burner

In this study, experimental testing and analysis were performed to examine the combustion instability characteristics of hydrogen–methane blended fuels for a low-swirl lean premixed burner. The aim of this study is to determine the effect of hydrogen addition on combustion instability, and this is assessed by examining the flame response to a range of constant amplitude, single frequency chamber acoustic modes. Three different blends of hydrogen and methane (93% CH₄–7% H₂, 80% CH₄–20% H₂ and 70% CH₄–30% H₂ by volume) were employed as fuel at an equivalence ratio of 0.5, and with four different acoustic excitation frequencies (85, 125, 222 and 399 Hz). Planar laser induced fluorescence of the hydroxyl radical (OH-PLIF) was employed to measure the OH concentration at different phases of acoustic excitation and a Rayleigh Index was then calculated to determine the degree of thermoacoustic coupling. It was found, as has been previously reported, that the combustion characteristics are very sensitive to the fraction of hydrogen in the fuel mixture. The flame shows significant increases in flame base coupling and flame compaction with increasing hydrogen concentration for all conditions. While this effect enhances the flame response at non-resonant frequencies, it induces only minimal compaction and appears to decreases the coupling intensity at the resonant frequency.     

The combined dynamics of swirler and turbulent premixed swirling flames

The dynamics of premixed confined swirling flames is investigated by examining their response to incident velocity perturbations. A generalized transfer function designated as the flame describing function (FDF) is determined by sweeping a frequency range extending from 0 to 400 Hz and by changing the root mean square fluctuation level between 0% and 72% of the bulk velocity. The unsteady heat release rate is deduced from the emission intensity of OH^* radicals. This global information is complemented by phase conditioned Abel transformed emission images. This processing yields the distribution of light emission. By assuming that the light intensity is proportional to the heat release rate, it is possible to deduce the distribution of unsteady heat release rate in W m⁻³ and see how it evolves with time during the modulation cycle and for different forcing frequencies. These data can be useful for the determination of regimes of instability but also give clues on the mechanisms which control the swirling flame dynamics. It is found from experiments and demonstrated analytically that a swirler submitted to axial acoustic waves originating from the upstream manifold generates a vorticity wave on its downstream side. The flame is then submitted to a transmitted axial acoustic perturbation which propagates at the speed of sound and to an azimuthal velocity perturbation which is convected at the flow velocity. The net result is that the dynamical response and unsteady heat release rate are determined by the combined effects of these axial and induced azimuthal velocity perturbations. The former disturbance induces a shedding of vortices from the injector lip which roll-up the flame extremity while the latter effectively perturbs the swirl number which results in an angular oscillation of the flame root. This motion is equivalent to that which would be induced by perturbations of the burning velocity. The phase between incident perturbations is controlled by the convective time delay between the swirler and the injector. The constructive or destructive interference between the different perturbations is shown to yield the low and high gains observed for certain frequencies.     

Fuel variability effect on flickering frequency of diffusion flames

It is known that fuel variability of different gas suppliers may cause combustion instability in a gas turbine combustor. Mechanisms that control the time scale of the heat release oscillations and acoustic pressure perturbations are both physical and chemical in nature, and thus can be influenced by changes in fuel composition. The intent of this study is to investigate the fuel variability on the flickering frequency of diffusion flames in the hope of understanding some of the fundamental aspects of fuel variability effect on the dynamics of combustion. Experiments were conducted at atmospheric pressure with a matrix of methane and propane blends. An optical fibre system was applied to capture simultaneously the flame flickering at two different light frequencies (430 nm and 516 nm), which provided a means of comparing the chemistry change. It was found that the low frequency oscillation of flow and flame structures depended only weakly on the exit velocities of the fuel, while ambient conditions had a significant effect on flickering frequencies and spectrum. The results of using CH₄ and C₃H₈ as test fuels at different flow rates showed very little variations, with peak frequencies at 11–13 Hz. When the jet flame was not disturbed, harmonics to at least the third mode were obtained in most of these cases. However, the cases which included CH₄/C₃H₈ splits of 90/10, 85/15 and 80/20 by volume showed that unstable flickering frequencies and flame harmonics were not observed. When a mixture of methane/propane at a ratio of 1:1 was used the peak flickering frequency was around 6 Hz, and slight disturbance in the environment would cause the harmonics to disappear. Mechanisms thought to produce changes in the dynamic response and frequency harmonics were discussed.     

Experimental study on the premixed syngas-air explosion in duct with both ends open

Premixed flame of stoichiometric syngas-air mixture with various hydrogen volume fractions, 10% ≤ X (H₂) ≤ 90%, propagating in a duct with both ends open is experimentally investigated in this study. Two representative ignition locations, i.e., Ig-1, locating at the center of the duct, and Ig-2, locating at the right open end, are considered. Results show that the tulip flame is first attained in the duct with both ends open at 10% ≤ X (H₂) ≤ 50% as the flame is ignited at Ig-1. However, the flame maintains the convex shape with the cellular structure on the flame surface as the flame is ignited at Ig-2. The cellular structure results from Darrieus-Landau instability, but the Darrieus-Landau instability cannot invert the convex flame front. The flame tip and pressure dynamics have been examined. When the flame is ignited at Ig-1, the flame oscillates violently, and the overpressure profiles oscillate as a Helmholtz-type. When the flame is ignited at Ig-2, the left flame front propagates in an atmospheric pressure with a nearly constant speed. The prominent flame acceleration and oscillation are not observed at Ig-2 because of lacking flame acoustic interaction. What's more, the characteristic time of flame propagation has been compared. The time t_w is shorter while the time t_p is longer than the calculated value, and the time t_e has been delayed by both open ends. The flame propagation process is moderated as the flame propagates in the duct with both ends open.     

Heat release and flame structure measurements of self-excited acoustically-driven premixed methane flames

An open-open organ pipe burner (Rijke tube) with a bluff-body ring was used to create a self-excited, acoustically-driven, premixed methane-air conical flame, with equivalence ratios ranging from 0.85 to 1.05. The feed tube velocities corresponded to Re = 1780-4450. Coupled oscillations in pressure, velocity, and heat release from the flame are naturally encouraged at resonant frequencies in the Rijke tube combustor. This coupling creates sustainable self-exited oscillations in flame front area and shape. The period of the oscillations occur at the resonant frequency of the combustion chamber when the flame is placed ∼¼ of the distance from the bottom of the tube. In this investigation, the shape of these acoustically-driven flames is measured by employing both OH planar laser-induced fluorescence (PLIF) and chemiluminescence imaging and the images are correlated to simultaneously measured pressure in the combustor. Past research on acoustically perturbed flames has focused on qualitative flame area and heat release relationships under imposed velocity perturbations at imposed frequencies. This study reports quantitative empirical fits with respect to pressure or phase angle in a self-generated pressure oscillation. The OH-PLIF images were single temporal shots and the chemiluminescence images were phase averaged on chip, such that 15 exposures were used to create one image. Thus, both measurements were time resolved during the flame oscillation. Phase-resolved area and heat release variations throughout the pressure oscillation were computed. A relation between flame area and the phase angle before the pressure maximum was derived for all flames in order to quantitatively show that the Rayleigh criterion was satisfied in the combustor. Qualitative trends in oscillating flame area were found with respect to feed tube flow rates. A logarithmic relation was found between the RMS pressure and both the normalized average area and heat release rate for all flames.     

The non-linear thermo-acoustic response of a small swirl burner

The non-linear response of a swirl stabilised, lean premixed flame (CH₄/air) was determined by forcing the flame acoustically at frequencies between 40 and 200 Hz with increasing amplitude. Measuring the chemiluminescent emission from OH¹ with a photodiode sensor and calculating the flame transfer function, a linear response to increasing amplitude was observed at 40 and 60 Hz for all amplitudes with an equivalence ratio ? = 0.56. However, between 80 and 200 Hz the flame response exhibited non-linear characteristics for r.m.s velocity fluctuations greater than 20% of the mean flow velocity. With ? = 0.48, even 60 Hz became non-linear. Phase-locked Particle Image Velocimetry and Intensified CCD imaging were deployed at three amplitudes for detailed study of the flame and flow field response to forcing. At low frequencies the flow field was characterised by a pulsating inner recirculation zone, whilst at all frequencies the outer recirculation zone was modified by vortices rolling up the annular jet. As the forcing amplitude was increased, the effect on the flame shape became more pronounced, with large variations in flame volume at low frequencies and flame extinction due to stretching of the flame around the roll-up vortices at the higher frequencies. The results indicate different driving mechanisms behind the flame response at low and high frequencies. At low frequencies the flame response is governed by equivalence ratio fluctuations due to the ‘stiff’ fuel system and the volumetric fluctuations of the input air. At the higher frequencies the response is governed by flow field features such as vortex roll-up.     

Experimental investigation of the flame characteristics of a fuel mixture with high hydrogen content enriched with oxygen under the externally acoustic enforcement conditions

Pollutant emissions are one of the major problems for the World. In this regard, researchers focus on the studies on emission reduction. Hydrogen is an alternative solution for this problem. Hydrogen produces only water as a result of combustion with oxygen. Therefore, this study examines the combustion stability and emissions of a high hydrogen content fuel mixture. The fuel mixture containing 45% H₂ by volume was supported with 5% CH₄ in order to provide stable combustion. In addition, in order to reduce the instabilities caused by the high laminar burning rate of hydrogen, it was diluted with 50% CO₂ which equal volume with the fuel mixture. After the fuel mixture containing 45% H₂ - 5% CH₄ - 50% CO₂ was burned with air containing 21% O₂, enrichment was applied at the rates of 24% and 27% O₂. The flame that contains different oxygen ratios was acoustically forced through the speakers around the combustion chamber. The stability data, dynamic pressure, and light intensity fluctuation of the flame were recorded under different acoustic resonance frequencies (110 Hz, 190 Hz, 260 Hz). In this way, the oxygen enrichment performance and flame characteristics of hydrogen in a premixed burner, which is promising in zero-emission studies, were investigated. As a result, when the combustion condition of 21% O₂ and 24% O₂ ratios are compared, the instability increased slightly from 801 Pa to 887 Pa, respectively. However, at 27% O₂, the flame could not perform a stable combustion under acoustic enforcement. The flame flashbacked with a dynamic pressure fluctuation of 1577 Pa under an acoustic frequency of 110 Hz. In addition, it was observed that CO emissions have decreased with the increase in oxygen enrichment rate. CO emission measured at 1080 ppm at 21% O₂ decreased to 542 ppm and 276 ppm respectively at 24% and 27% oxygen enrichment levels. While NOx emission was measured at 10 ppm in the case of combustion with air, it was observed that decreased to 4 ppm at the rate of 27% O₂.     

Flame dynamics of a meso-scale heat recirculating combustor

The dynamics of premixed propane–air flame in a meso-scale ceramic combustor has been examined here. The flame characteristics in the combustor were examined by measuring the acoustic emissions and preheat temperatures together with high-speed cinematography. For the small-scale combustor, the volume to surface area ratio is small and hence the walls have significant effect on the global flame structure, flame location and flame dynamics. In addition to the flame–wall thermal coupling there is a coupling between flame and acoustics in the case of confined flames. Flame–wall thermal interactions lead to low frequency flame fluctuations (∼100 Hz) depending upon the thermal response of the wall. However, the flame–acoustic interactions can result in a wide range of flame fluctuations ranging from few hundred Hz to few kHz. Wall temperature distribution is one of the factors that control the amount of reactant preheating which in turn effects the location of flame stabilization. Acoustic emission signals and high-speed flame imaging confirmed that for the present case flame–acoustic interactions have more significant effect on flame dynamics. Based on the acoustic emissions, five different flame regimes have been identified; whistling/harmonic mode, rich instability mode, lean instability mode, silent mode and pulsating flame mode.     

Effects of channel length on propagation behaviors of non-premixed H2-air flames in Y-shaped micro combustors

In the present study, dynamics of non-premixed hydrogen-air flames in two Y-shaped cylindrical micro combustors of different horizontal channel lengths (L = 100 and 200 mm) were experimentally compared. The inner diameters of the micro-combustors are 2 mm. Unburned mixture was ignited by heating the near-exit wall with a butane torch. The results show that six and three flame propagation modes in the 200-mm and 100-mm micro-combustors were observed, respectively. Moreover, it is found that the flame oscillation duration is much longer with a larger noise intensity in the 200-mm micro-combustor. As a result, the mean propagation speed under L = 100 mm is much larger. In addition, the edge flame is longer on the lean side under L = 100 mm and almost identical on the rich side for the two combustors. Furthermore, the luminosity of edge flame in the 100-mm micro-combustor is much brighter. Numerical analysis reveals that the deflection of propagating flame in the Y-shaped micro-combustor is determined by the stoichiometric line. In summary, the short combustor has a smaller heat loss ratio and a stronger flame-wall thermal coupling, which can enhance the combustion intensity and increase the flame propagation speed.     

Thermoacoustic combustion instability of propane-oxy-combustion with CO2 dilution: Experimental analysis

In this study, the effect of CO₂ dilution on the thermoacoustic stability of propane-oxyfuel flames is studied in a non-premixed, swirl-stabilized combustor. The results, obtained at a fixed combustor power density (4 MW/m³ bar) and global stoichiometric equivalence ratio (Φ = 1.0), show that the oxy-flame is stable at 0% and low CO₂ concentrations in the oxidizer. A self-amplifying coupling between heat release and pressure fluctuations was observed to occur at the CO₂ concentration of 45%, which matches the point of flame transition from a jet-like to a V-shaped flame resulting from the formation of inner recirculation zone. The observed frequency for both the pressure and heat release oscillations is 465 Hz and the ensuing thermoacoustic instability is believed to have been resulted from vortexes and flame interactions. Subsequent to the coupling of the oscillations at the CO₂ concentration of 45%, their amplitudes grew at 50% to 60% CO₂ dilution levels. The maximum amplitude was observed at 60% CO₂ concentration after which, as CO₂ dilution level increases, the acoustic amplitude and that of its counterpart in the heat release spectrum decreased due to damping (energy dissipation) arising from heat loss and viscous dissipation. An increase in hydrogen concentration in the fuel and a decrease in the combustor power density were observed to lower the acoustic amplitude. Furthermore, a frequency shift is observed with a change in the combustor firing rate, which shows that the mode scales with the flow velocity, and therefore, unlikely to be a natural acoustic mode of the combustor. This study, therefore, reveals thermoacoustic instability in non-premixed oxy-combustion driven by changes in flame dynamics and macrostructures as the CO₂ concentration in the oxidizer mixture varies.     

Nonlinear interaction between a precessing vortex core and acoustic oscillations in a turbulent swirling flame

The interaction of a helical mode with acoustic oscillations is studied experimentally in a turbulent swirl-stabilized premixed flame. In addition to a precessing vortex core (PVC), the helical mode features perturbations in the outer shear layer of the burner flow. Measurements of the acoustic pressure, unsteady velocity field and flame emission are made in different regimes including self-sustained combustion oscillations and stable regimes with and without acoustic forcing. The acoustic oscillation and the helical mode create a pronounced rotating heat release rate perturbation at a frequency corresponding to the difference of the frequencies of the two individual mechanisms. Measurements over a wide range of operating conditions for different flow rates and equivalence ratios show that while the helical mode is always present, with a constant Strouhal number, self-excited thermoacoustic oscillations exist only in a narrow region. The interaction can be observed also in cases of thermoacoustically stable conditions when external acoustic modulation is applied to the system. The evolution of the helical mode with the forcing amplitude is examined. High-speed imaging from the downstream side of the combustor demonstrates that the heat release rate perturbation associated with the nonlinear interaction of the helical mode and the acoustic oscillations produces a ”yin and yang” -type pattern rotating with the interaction frequency in the direction of the mean swirl. At unstable conditions, the oscillation amplitude associated with the interaction is found to be significantly stronger in the heat release rate than in the velocity signal, indicating that the nonlinear interaction primarily occurs in the flame response and not in the aerodynamic field. The latter is, however, generally possible as is demonstrated under non-reacting conditions with acoustic forcing. Based on a second-order analysis of the G-equation, it is shown that the nonlinear flame dynamics necessarily generate the observed interaction component if the flame is simultaneously perturbed by a helical mode and acoustic oscillations.