Strategy for PLIF single-shot HCO imaging in turbulent methane/air flames

Formyl (HCO) has since long been recognized as a common intermediate species and a potential local indicator of the major heat release in hydrocarbon combustion. Consequently, the detection of HCO is desirable especially in turbulent flames of practical relevance. However, due to the low concentration and low fluorescence quantum yield, single-shot based detection of HCO with planar laser-induced fluorescence (PLIF) has been a real challenge for experimentalists. In the present paper, a series of systematic investigations have been performed in order to develop a strategy for single-shot HCO PLIF detection in methane/air premixed flames. Potential spectral interference and applicable combustion conditions were analyzed in stable laminar flames employing fluorescence detection with high spectral and spatial resolution for different laser wavelengths. The wavelength 259.004 nm was identified as optimum in giving the maximum signal and minimum spectral interference from other species (e.g., OH and hot O₂). Photolytically generated HCO from formaldehyde (CH₂O) was also observed, which restricts the applicable laser fluence to below 2.5 J/cm² in order to diminish the influence of CH₂O down to 5%. Besides, large hydrocarbon species generated in rich flames were found to contribute a considerable interference which can hardly be screened out. This limits the application of the HCO PLIF technique to lean premixed flames. Finally, by employing an optimized alexandrite laser system, single-shot HCO PLIF imaging in a turbulent methane/air flame is demonstrated, indicating the feasibility of further application of this technique to turbulent combustion systems.     

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.     

Effect of hydrogen enrichment on flame broadening of turbulent premixed flames in thin reaction regime

An experimental study to identify the effect of hydrogen enrichment and differential diffusion on the flame broadening is conducted. Turbulent lean premixed flames in the Broadened Preheat–Thin Reaction (BP-TR) regime are obtained. The flames are stabilized on a Bunsen burner and CH₄/H₂/air mixtures are adopted with three hydrogen fractions of 0, 30% and 60%. The preheat zone and heat release zone are captured with the multi-species Planar Laser-Induced Fluorescence (PLIF) of OH and CH₂O radicals. Flame thicknesses of the preheat and heat release layers are measured. Results show broadened preheat zone and thin heat release layers for the flames, as predicted by the BP-TR regime. The preheat zone thickness can be increased to about 3–6 times compared to the laminar preheat thickness. An apparently decreased preheat zone thickness with hydrogen addition is observed. The differential diffusion is anticipated to locally thicken the heat release zone along the flame front. The mean heat release thickness is nearly not affected by the turbulence or hydrogen addition.     

Heat release rate variations in high hydrogen content premixed syngas flames at elevated pressures: Effect of equivalence ratio

Three-dimensional direct numerical simulations with detailed chemistry were performed to investigate the effect of equivalence ratio on spatial variations of the heat release rate and flame markers of hydrogen/carbon monoxide syngas expanding spherical premixed flames under turbulent conditions at elevated pressures. The flame structures and the heat release rate were analysed and compared between fuel-lean, stoichiometric and fuel-rich centrally ignited spherical flames. The equivalence ratio changes the balance among thermo-diffusive effects, Darrieus–Landau instability and turbulence, leading to different flame dynamics and the heat release rate distribution, despite exhibiting similar cellular and wrinkling flames. The Darrieus–Landau instability is relatively insensitive to the equivalence ratio while the thermo-diffusive process is strongly affected by the equivalence ratio. As the thermo-diffusive effect increases as the equivalence ratio decreases, the fuel-lean flame is more unstable than the fuel-rich flame with the stoichiometric flame in between, under the joint effects of the thermo-diffusive instability and the Darrieus–Landau instability. The local heat release rate and curvature display a positive correlation for the lean flame, no correlation for the stoichiometric flame, and negative correlation for the rich flame. Furthermore, for the fuel-lean flame, the low and high heat release rate values are found in the negative and positive curvature zones, respectively, while for the fuel-rich flame, the opposite trends are found. It is found that heat release rate markers based on species concentrations vary strongly with changing equivalence ratio. The results suggest that the HCO, HO₂ concentrations and product of OH and CH₂O concentrations show good correlation with the local heat release rate for H₂/CO premixed syngas-air stoichiometric flame under turbulent conditions at elevated pressures.     

Investigating the effect of local addition of hydrogen to acoustically excited ethylene and methane flames

While lean combustion in gas turbines is known to reduce NOx, it makes combustors more prone to thermo-acoustic instabilities, which can lead to deterioration in engine performance. The work presented in this study investigates the effectiveness of secondary injection of hydrogen to imperfectly premixed methane and ethylene flames in reducing heat release oscillations. Both acoustically forced and unforced flames were studied, and simultaneous OH and H atom PLIF (planar laser induced fluorescence) was conducted. The tests were carried out on a laboratory scale bluff-body combustor with a central V-shaped bluff body. Two-microphone method was used to estimate velocity perturbations from pressure measurements, flame boundary images were captured using high speed Mie scattering, while global heat release fluctuations were determined from OH* chemiluminescence.The results showed that hydrogen addition considerably reduced heat release oscillations for both methane and ethylene flames at all the forcing frequencies tested, with the exception of methane flames forced at 315 Hz, where oscillations increased with hydrogen addition. The addition of hydrogen reduced the extent of flame roll-up for both methane and ethylene flames, however, this reduction was larger for methane flames. NOx exhaust emissions were observed to increase with hydrogen addition for both methane and ethylene flames, with absolute NOx concentrations higher for ethylene flames, due to higher flame temperatures.     

Turbulence and combustion interaction: High resolution local flame front structure visualization using simultaneous single-shot PLIF imaging of CH, OH, and CH2O in a piloted premixed jet flame

High resolution planar laser-induced fluorescence (PLIF) was applied to investigate the local flame front structures of turbulent premixed methane/air jet flames in order to reveal details about turbulence and flame interaction. The targeted turbulent flames were generated on a specially designed coaxial jet burner, in which low speed stoichiometric gas mixture was fed through the outer large tube to provide a laminar pilot flame for stabilization of the high speed jet flame issued through the small inner tube. By varying the inner tube flow speed and keeping the mixture composition as that of the outer tube, different flames were obtained covering both the laminar and turbulent flame regimes with different turbulent intensities. Simultaneous CH/CH₂O, and also OH PLIF images were recorded to characterize the influence of turbulence eddies on the reaction zone structure, with a spatial resolution of about 40 μm and temporal resolution of around 10 ns. Under all experimental conditions, the CH radicals were found to exist only in a thin layer; the CH₂O were found in the inner flame whereas the OH radicals were seen in the outer flame with the thin CH layer separating the OH and CH₂O layers. The outer OH layer is thick and it corresponds to the oxidation zone and post-flame zone; the CH₂O layer is thin in laminar flows; it becomes broad at high speed turbulent flow conditions. This phenomenon was analyzed using chemical kinetic calculations and eddy/flame interaction theory. It appears that under high turbulence intensity conditions, the small eddies in the preheat zone can transport species such as CH₂O from the reaction zones to the preheat zone. The CH₂O species are not consumed in the preheat zone due to the absence of H, O, and OH radicals by which CH₂O is to be oxidized. The CH radicals cannot exist in the preheat zone due to the rapid reactions of this species with O₂ and CO₂ in the inner-layer of the reaction zones. The local PLIF intensities were evaluated using an area integrated PLIF signal. Substantial increase of the CH₂O signal and decrease of CH signal was observed as the jet velocity increases. These observations raise new challenges to the current flamelet type models.     

Effect of the composition of the hot product stream in the quasi-steady extinction of strained premixed flames

The extinction of premixed CH₄/O₂/N₂ flames counterflowing against a jet of combustion products in chemical equilibrium was investigated numerically using detailed chemistry and transport mechanisms. Such a problem is of relevance to combustion systems with non-homogeneous air/fuel mixtures or recirculation of the burnt gases. Contrary to similar studies that were focused on heat loss/gain, depending on the degree of non-adiabaticity of the system, the emphasis here was on the yet unexplored role of the composition of counterflowing burnt gases in the extinction of lean-to-stoichiometric premixed flames. For a given temperature of the counterflowing products of combustion, it was found that the decrease of heat release with increase in strain rate could be either monotonic or non-monotonic, depending on the equivalence ratio φ_b of the flame feeding the hot combustion product stream. Two distinct extinction modes were observed: an abrupt one, when the hot counterflowing stream consists of either inert gas or equilibrium products of a stoichiometric premixed flame, and a smooth extinction, when there is an excess of oxidizing species in the combustion product stream. In the latter case four burning regimes can be distinguished as the strain rate is progressively increased while the heat release decreases smoothly: an adiabatic propagating flame regime, a non-adiabatic propagating flame regime, the so-called partially-extinguished flame regime, in which the location of the peak of heat release crosses the stagnation plane, and a frozen flow regime. The flame structure was analyzed in detail in the different burning regimes. Abrupt extinction was attributed to the quenching of the oxidation layer with the entire H-OH-O radical pool being comparably reduced. Under conditions of smooth extinction, the behavior is different and the concentration of the H radical decreases the most with increasing strain rate, whereas OH and O remain comparatively abundant in the oxidation layer. As the profile of the heat release rate thickens, the oxidation layer is quenched and the attack of the fuel relies more heavily on the OH radicals.     

Effects of strain rate,turbulence, reactant stoichiometry and heat losses on the interaction of turbulent premixed flames with stoichiometric counterflowing combustion products

Intense strain, turbulence, heat transfer, and mixing with combustion products can affect premixed flames in practical combustion devices. These effects are systematically studied in turbulent premixed CH₄/N₂/O₂ flames using a reactant versus product counterflow system and independently varying bulk strain rate, turbulent Reynolds number, equivalence ratio of the reactant mixture, and temperature of the stoichiometric counterflowing combustion products. The flow field and the turbulent flames are investigated using particle image velocimetry (PIV) measurements and laser-induced fluorescence (LIF) imaging of OH. The OH-LIF images are used to identify the interface between the counterflowing streams, referred to here as the gas mixing layer interface (GMLI). The flame response for different flow conditions is compared in terms of the probability of localized extinction along the GMLI, the turbulent flame brush thickness, and flame position relative to the GMLI, by using an OH-LIF-based progress variable. The probability of localized extinction at the GMLI increases as the separation between the turbulent flame brush and the GMLI decreases. Flame fronts in the vicinity of the GMLI are more likely to extinguish as a result of heat losses, dilution of the reaction zone by the product stream, and large local strain rates. A higher probability of localized extinction at the GMLI is induced by either a larger bulk strain rate or a slower flame speed. As the turbulent Reynolds number increases, the corresponding increase in turbulent flame brush thickness enhances the interactions of the flame fronts with the GMLI. Heat losses are substantially less significant for cases in which the turbulent flame brush is sufficiently separated from the GMLI. For flames in close proximity to the GMLI, the effects of the product stream on the flame front differ for lean and rich reactant mixtures. These disparities are attributed in part to differences in the ignitibility of the reactant mixtures by the hot product stream.     

Stratified jet flames in a heated (1390 K) air cross-flow with autoignition

Measurements are reported of the heat release profiles, the flame lengths, flame structure and other properties of a reacting jet-in-cross-flow (JICF) for two fuels. The air was heated to a static temperature of 1390 K, which is above the autoignition temperature, and the air velocity was 468 m/s, which is much larger than values that were considered previously. Aerodynamic strain rates are so large that the flame was expected to fall into either the “distributed reaction”, “thickened flamelet”, or “shredded flamelet” regimes. Fluorescence images of CH, OH and formaldehyde identified the flame structure. The jet-in-cross-flow is a unit physics problem that occurs in turbojets and scramjets. While scaling relations are known for the non-reacting case, more information about the reacting case is needed, especially when autoignition and strain rates become important. Three regions were identified. In the liftoff region autoignition reactions occur which create a strong formaldehyde PLIF signal. However, flames and heat release do not occur in the liftoff region since CH and CH¹ signals were negligible. The second region is the lifted flame base, which has the character of a premixed flame, as evidenced by a very rapid rise in the heat release rate as indicated by the CH¹ and OH¹ signals. The third region contains a turbulent non-premixed flame and the CH images indicate the presence of thickened and shredded flamelets. The 2–3 mm thickness of each CH layer is more than 10 times the laminar flamelet thickness. In the third region the heat release rate decays slowly downstream, which is typical of a non-premixed flame. Because both upstream autoignition and downstream thickened flamelets were observed, we classify this combustion to be an “autoignition-assisted flame”. Flame lengths increase linearly with fuel mass flow rate, indicating that mixing is controlled by the air velocity rather than the fuel velocity.     

The regulation effect of methane and hydrogen on the emission characteristics of ammonia/air combustion in a model combustor

Ammonia, made up of 17.8% hydrogen, has attracted a lot of attention in combustion community due to its zero carbon emission as a fuel in gas turbines. However, ammonia combustion still faces some challenges including the weak combustion and sharp NOx emissions which discourage its application. It was demonstrated that the combustion intensity of ammonia/air flame can be enhanced through adding active fuels like methane and hydrogen, while the NOx emission issue will emerge in the meantime. This study investigates regulation effect of methane and hydrogen on the emission characteristics of ammonia/air flame in a gas turbine combustor. The instantaneous OH profile and global emissions at the combustion chamber outlet are measured with Planar Laser Induced Fluorescence (PLIF) technique and the Fourier Transform Infrared (FTIR), respectively. The flames are also simulated by large eddy simulation to further reveal physical and chemical processes of the emissions formation. Results show that for NH₃/air flames, the emissions behavior of the gas turbine combustor is similar to the calculated one-dimensional flames. Moreover, the NOx emissions and the unburned NH₃ can be simultaneously controlled to a proper value at the equivalence ratio (φ) of approximate 1.1. The variation of NO and NO₂ with φ for NH₃/H₂/air flames and NH₃/CH₄/air flames at blending ratio (Z_f) of 0.1 are similar to the NH₃/air flames, with the peak moving towards rich condition. This indicates that the NH₃/air flame can be regulated through adding a small amount of active fuels without increasing the NOx emission level. However, when Z_f = 0.3, we observe a clear large NOx emission and CO for NH₃/CH₄/air flames, indicating H₂ is a better choice on the emission control. The LES results show that NO and OH radicals exhibit a general positive correlation. And the temperature plays a secondary role in promoting NOx formation comparing with CH₄/air flame.     

A new burner for studying auto-ignition in turbulent dilute sprays

This paper introduces a new burner to study the auto-ignition characteristics of dilute turbulent spray flames. The spray is formed upstream of the exit plane and carried with air or nitrogen into a hot co-flowing stream of vitiated combustion products. The fluid dynamics is kept intentionally simple and the boundary conditions are well-defined such that the burner lends itself easily to computations. The stability characteristics are presented here for a range of liquid fuels as a function of the controlling parameters. For each fuel, three types of flames are identified based on their visual appearance. These flames are all lifted but differ in the shape of the leading edge and heat release zones. These characteristics are similar for all fuels and three flames, one from each type, are selected for further study. Measurements of mean temperature, velocity and droplet fields as well as high-speed LIF images of OH, Mie scattering from droplets and chemiluminescence of CH^∗ are made in the selected flames of ethanol. It is found that the leading edge of the flame, which is a hardly visible light-blue zone close to the jet exit plane, is marked by the presence of OH (but not CH^∗), and slight increases in peak temperature above the levels of the co-flow. Further downstream, there exists an intense blue region that is characterized by higher OH levels, significant emission of CH^∗, and temperatures that are much higher than those of the co-flow. This is the region where the bulk of the heat release is taking place. Pockets of CH^∗ are seen to appear in this region and grow as they are advected further downstream.     

Structure of laminar sooting inverse diffusion flames

The flame structure of laminar inverse diffusion flames (IDFs) was studied to gain insight into soot formation and growth in underventilated combustion. Both ethylene-air and methane-air IDFs were examined, fuel flow rates were kept constant for all flames of each fuel type, and airflow rates were varied to observe the effect on flame structure and soot formation. Planar laser-induced fluorescence of hydroxyl radicals (OH PLIF) and polycyclic aromatic hydrocarbons (PAH PLIF), planar laser-induced incandescence of soot (soot PLII), and thermocouple-determined gas temperatures were used to draw conclusions about flame structure and soot formation. Flickering, caused by buoyancy-induced vortices, was evident above and outside the flames. The distances between the OH, PAH, and soot zones were similar in IDFs and normal diffusion flames (NDFs), but the locations of those zones were inverted in IDFs relative to NDFs. Peak OH PLIF coincided with peak temperature and marked the flame front. Soot appeared outside the flame front, corresponding to temperatures around the minimum soot formation temperature of 1300 K. PAHs appeared outside the soot layer, with characteristic temperature depending on the wavelength detection band. PAHs and soot began to appear at a constant axial position for each fuel, independent of the rate of air flow. PAH formation either preceded or coincided with soot formation, indicating that PAHs are important components in soot formation. Soot growth continued for some time downstream of the flame, at temperatures below the inception temperature, probably through reaction with PAHs.     

Numerical and experimental study on silica generating counterflow diffusion flames

Temperatures and concentrations of OH radicals in silica generating counterflow oxy-hydrogen diffusion flames are measured using a broadband coherent anti-Stokes Raman spectroscopy (CARS) and a planar laser induced fluorescence (PLIF) techniques to study thermo-chemical effects of SiCl₄ addition to flames. Numerical analysis considering detailed chemical reactions including silica generating reactions is also conducted. The experimental results demonstrate that temperatures decrease in preheated zone due to the increase in specific heat of the gas mixture while the decrease is mitigated in particle formation zone due to the heat release through hydrolysis and oxidation reactions of SiCl₄. Also, OH concentrations significantly decrease in silica formation flame, which can be attributed to the consumption of oxidative radicals during the silica generating reactions of SiCl₄ and depletion of OH by HCl. The numerical simulation agrees well for flames having relatively low flame temperatures of 1750 K but underestimates the decrease in OH concentration for high temperature flame over 2700 K. The disagreement for the high temperature flames would imply possible OH consumption via direct reactions between OH radicals and silicon chlorides, which is expected to be highly sensitive to temperature.     

Flame speed and tangential strain measurements in widely stratified partially premixed flames interacting with grid turbulence

Methane-air partially premixed flames subjected to grid-generated turbulence are stabilized in a two-slot burner with initial fuel concentration differences leading to stratification across the stoichiometric concentration. The fuel concentration gradient at the location corresponding to the flame base is measured using planar laser induced fluorescence (PLIF) of acetone in the non-reacting mixing field. Simultaneous PLIF of the OH radical and particle image velocimetry (PIV) measurements are performed to deduce the flow velocity and the flame front. These flames exhibit a convex premixed flame front and a trailing diffusion flame, with flow divergence upstream of the flame, as indicated by the instantaneous OH–PLIF, Mie scattering images, and PIV data. The mean streamwise velocity profile attains a global minimum just upstream of the flame front due to expansion of a gases caused by heat release. The flame speed measured just upstream of the flame leading edge is normalized with respect to the turbulent stoichiometric flame speed that takes into account variations in turbulent intensity and integral length scale. The turbulent edge flame speed exceeds the corresponding stoichiometric premixed flame speed and reaches a peak at a certain concentration gradient. The mean tangential strain at the flame leading edge locally peaks at the concentration gradient corresponding to the peak flame speed. The strain varies non-monotonically with the flame curvature unlike in a non-stratified curved premixed flame. The mechanism of peak flame speed is explained as the competition between availability of hot excess reactants from the premixed flame branches to the flame stretch induced due to flame curvature. The results suggest that the stabilization of lifted turbulent partially premixed flames occurs through an edge flame even at a relatively gentle concentration gradient. The strain is also evaluated along the flame front; it peaks at the flame leading edge and decreases gradually on either side of the leading edge. The present results also show qualitatively similar trends as those of laminar triple flames.     

A numerical study on the ability to predict the heat release rate using CH chemiluminescence in non-sooting counterflow diffusion flames

Numerical studies on 1-D, non-sooting counterflow diffusion flames were performed to determine the precision with which the total heat release rate can be calculated using light emission, namely, chemiluminescence, from the reaction zone. A detailed reaction mechanism, incorporating sub-reaction models for excited state radicals (CH^* and OH^*, where ^* denotes the excited state), was employed in this study. A set of 1-D, steady state conservation equations was solved under standard atmospheric conditions over a counterflow configuration utilizing the CHEMKIN-PRO package. A variety of fuels (CH₄ and C₃H₈), velocities (0.1 m/s − near the extinction condition), diluents (N₂, H₂O, CO₂, and Ar), detailed reaction mechanisms for C₁–C₃ hydrocarbons (GRI-Mech 3.0, Hai Wang’s-, and San Diego-mechanism), different sub-reaction models for excited radicals, and excited radical transport properties were examined for the current purpose. It was found that a one-to-one correlation between total chemiluminescence from CH^* and the total heat release rate cannot be sustained when the flame experiences a relatively high stretch and dilution, even though the condition is still far away from extinction. This trend is consistent with the different types of fuels, and it is understood that the reduction of the ethynyl radical (C₂H), a potential precursor of CH^*, is the main cause of the one-to-one correlation not being sustained. To this end, it was concluded that the observable light emission can only be used to predict the total heat release rate when non-sooting diffusion flames exist under velocity conditions from 0.1 m/s to 1.5 m/s. In other words, the chemiluminescence intensity does not always correlate with the total heat release rate of highly stretched flames found in practical combustors.     

Influence of fuel composition on chemiluminescence emission in premixed flames of CH4/CO2/H2/CO blends

The growing concern about pollutant emissions and depletion of fossil fuels has been a strong motivator for the development of cleaner and more efficient combustion strategies, such as the gasification of coal, biomass or waste, which have increased the interest in using a new type of fuels, mainly composed of CH₄, H₂, CO and CO₂.These new fuels, commonly called syngas, display a wide range of compositions, which affects their combustion characteristics and, in some cases, are more prone to instabilities or flashback. Since flame properties have been demonstrated to be strongly related to equivalence ratio, a precise measurement of the flame stoichiometry is a key pre-requisite for combustion optimization and prevention of unstable regimes. In particular, chemiluminescence emission from flames has been largely tested for stoichiometry monitoring for methane flames, but its use in syngas flames has been far less studied. Consequently, the main goal of this work is analyzing the effect of fuel composition on the chemiluminescence vs. equivalence ratio curves for different fuel blends, as a first approach for a wide range of syngas compositions. The experimental results revealed that the ratio OH*/CH*, which had been widely demonstrated to be the best option for methane, may not be suitable for monitoring with certain fuels, such as those with a high percent of hydrogen. Alternatively, other signals, in particular the ratio OH*/CO₂*, appear as viable stoichiometry indicators in those cases.The analysis was also completed by numerical predictions with CHEMKIN. The comparisons of calculations with different flame models and experimental data reveals differences in the chemiluminescence vs. equivalence ratio curves for the different combustion regimes, depending on the range of the equivalence ratio ranges and fuel compositions. This finding, which confirms previous observations for a much narrower range of fuels, could have important practical consequences for the application of the technique in real combustors.     

Syngas combustion radiation profiling through spectrometry and DFCD processing techniques

Experimental investigations of H₂ and H₂-enriched syngas flame radiation properties have been conducted through spectroscopic and DFCD (Digital Flame Colour Discrimination) techniques. A spectrograph was employed to quantify the emission profile of H₂-based flames in the UV–visible spectral domain. The OH* emission was found to be the strongest in reactants with highest amount of H₂. Further addition of CO and/or CO₂ resulted in the reduction of OH* intensity with the addition of CO₂ causing greater radical intensity-loss than that of CO. The decrease in OH* intensity is accompanied by a corresponding increase in the CO–O* broadband continuum in the short-wavelength domain of the visible spectrum. Such reduction of OH* along with increase in CO–O* intensity can be related to the endothermic reaction mechanism of CO + OH => CO₂ + H, which describes the role of CO/CO₂ addition in H₂-enriched syngas flames. Comparison with direct imaging results provided additional credence to the effect of temperature reduction as flames with CO and/or CO₂ additions resembled colourations closer to typical bluish premixed hydrocarbon flame. The employment of DFCD processing effectively characterised different syngas combustion conditions by combining aspects of digital flame colour intensities with spatial combustion distributions. This colour signal quantification method was shown to yield useful characterisation of H₂-based flames, similar to the use of OH* chemiluminescence intensity variation from spectrometry. Also, DCFD analysis was able to depict the variances between the burning of different syngas gaseous constituents. Thus, useful image-based parameters related to the H₂-based combustion can be derived and potentially applied as a practical monitoring and characterisation mean for syngas combustion.     

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.