CH4 Flames Research Articles

Experimental and numerical study on the combustion characteristic of H2 and CH4 in oxygen-enriched environment

Oxygen-enriched combustion could rise the hazard of unexpected fire and explosion in industry. This study experimentally and numerically analyzed the combustion characteristic of H2 and CH4 in oxygen-enriched environment (Ω = 21%–100 %). For H2 at Φ = 1 and Ω = 100 %, the maximum explosion pressure and maximum pressure rise rate were about 3.89 MPa and 1259.2 MPa/s. For CH4, these values were about 4.77 MPa and 6279.4 MPa/s. The upper flammability limit of gases increased sharply with the increase of Ω, however, the lower flammability limit was almost unchanged. Besides, the increasing of Ω also could result in the rising of aforementioned combustion parameters. The sensitivity of burning velocity was also analyzed. For H2 flame at Φ = 0.8, R35 could slow down burning velocity when Ω = 21 %, but promote it when Ω = 30 % and 70 %. For CH4 flame at Φ = 1.4, the sensitivity coefficients of R11, R35, R43 and R84 changed from negative to positive when Ω increased from 21 % - 30 %–70 % - 100 %. It was concluded that the oxygen concentration not only affect the inert gas but also affect the reaction kinetic of flame.

Comparative study on the local hybrid combustion characteristics of the CH4-NH3 laminar premixed flames with/without the wall effects

The CH4-NH3 (50%:50%) premixed flame with/without the wall effects are simulated and compared at the stoichiometric condition in this study. The effects of cold wall and local flame dynamics on the flame temperature, heat release rate (HRR), CH4/NH3 oxidations and local CO/NO formations are analyzed quantitatively. Results show that the flame temperature and HRR are decreased and increased respectively along the flame front towards the axis of free flame, but they are both declined significantly towards the wall owing to suppressed chemical kinetics by the heat loss. The negative flame stretch and differential diffusion at the flame tip suppress the CH4 oxidation and the fast path (NH3→NH2→NH→N2) of NH3 oxidation effectively, but affect the slow path (NH3→NH2→HNO→NO→N2) of NH3 oxidation moderately. This contributes to suppressed CH4/NH3 oxidations and improved NO production at the flame tip of free flame. Based on NRR variations and ROP analysis, compared with CH4 oxidation, wall heat loss exerts stronger suppressions on the fast path of NH3 oxidation, while it decelerates the slow path of NH3 oxidation less evidently. Since the fast path is the primary pathway of the NH3 oxidation, the NH3 oxidation suffers more effective suppression of the wall heat loss than the CH4 oxidation. Furthermore, considering the significance of the slow path to the NO formation, the relatively enhanced importance of the slow path to the NH3 oxidation in the near-wall region predominates the comparatively effective NO formation in the CH4-NH3 flame compared with the pure CH4 flame under the influence of wall heat loss. For the CO/NO formations, the local CO production/oxidation in the wall-impinging CH4-NH3 flame are decelerated simultaneously due to the strong heat loss, while the local NO production/destruction are inhibited/improved by the wall heat loss.

Effects of buoyancy on the spherical flame and explosion pressure of a CH4 mixture under dilution conditions

To determine the effects of buoyancy on the spherical flame and explosion pressure under dilution conditions, a series of explosion experiments were conducted in a 20-L spherical container. The schlieren technique was used to record CH4 flames to assess the buoyancy effect. The change in the explosion pressure was recorded and correlated with the flame shape to analyze the explosion characteristics. The methane explosion occurred in two stages: initial ignition and flame development. During initial ignition, the addition of CO2 caused the flame buoyancy to increase considerably. Flames with different buoyancies had different shapes, such as spherical, ellipsoidal, and “ω” shapes. The radius of the flame at 12 mm may serve as the threshold for the onset of buoyancy effects; once the radius exceeds this value, the flame could deform due to buoyancy effects. Increasing the added CO2 concentration caused the buoyancy effect to be enhanced and appear at a very low radius of the flame. The combined effects of buoyancy and dilution reduced the flame expansion speed. The difference between the horizontal and upward expansion speeds linearly increased with the CO2 concentration. At a high CO2 concentration, the flame stagnated at speeds below 0.5 m/s. During flame development, CO2 addition made the flame surface smoother. Suppressing the cellular flame structure delayed the maximum explosion pressure rise rate and reduced the pressure peak. A coupled analysis of the flame shape and the maximum explosion pressure rise rate showed a dense cellular structure for a high explosion pressure rise rate and a smooth flame surface for a low maximum explosion pressure rise rate. The size and number of cellular structures for different mixtures were similar at a fixed maximum explosion pressure rise rate.

Experimental Investigation on Oxy-Hydrogen Gas Flame Injecting Coal Powder Gasification and Combustion

Hydrogen energy is an important carrier for energy terminals to achieve green and low-carbon transformation. Hydrogen, as a carbon-free fuel, has great research and development value in the field of thermal power generation. This article proposes a solution for the stable combustion of coal powder using Oxy-hydrogen Gas ignition technology. An Oxy-hydrogen Gas flame injection coal powder combustion testing device was constructed to experimentally study the temperature distribution in the combustion chamber under Oxy-hydrogen Gas ignition technology, with primary air coal powder concentrations of 0.27, 0.32, and 0.36 (kg coal powder/kg air), as well as the concentration changes of volatile CO emissions during the ignition of coal powder using both Oxy-hydrogen Gas and CH4 flames. The sensitivity of the NO generation during coal gasification combustion under the Oxy-hydrogen Gas ignition was simulated and analyzed. The results show that at a coal powder concentration of 0.32 (kg coal/kg air) and an Oxy-hydrogen Gas flow rate of 2.1 L/min, the combustion effect of coal powder is the best, and the highest combustion chamber temperature can reach 1156 K; when the concentration of coal powder varies within a range from 0.32 to 0.27, the combustion chamber temperature can be maintained at around 850K, achieving stable combustion conditions for coal powder. The only product generated by the Oxy-hydrogen Gas combustion is high-temperature water vapor, which helps the rapid gasification of coal powder and releases a large amount of volatile CO, which is beneficial for the ignition and stable combustion of coal powder.

Experimental and numerical investigations of the laminar burning velocities of premixed fuel-rich methane oxy-fuel and oxygen-enhanced flames

In the context of Oxy-fuel combustion, there is a strong industrial interest in highly fuel-rich flames, characterized by an equivalence ratio (Φ) exceeding 2.5, for the production of H2-CO-rich synthesis gas. The validation of existing chemical reaction mechanisms for this specific regime pose a significant challenge due to the scarcity of experimental data for this particular range of equivalence ratio.The scope of this study was therefore the determination of the laminar burning velocity sL of ultra-rich CH4-O2 flames under varying conditions, such as high equivalence ratios and preheating temperatures using the heat flux burner method. The laminar burning velocity was determined for equivalence ratio within the range of 2.3 < Φ < 3.6 and preheating temperature ranging from 300 K to 455 K. Additionally, the impact of the of reducing the oxygen content in the oxidizer on sL was investigated by argon addition in molar percentages ranging from 40 % to 100 %.The results of our study reveal laminar burning velocities ranging from 5 cm/s to 45 cm/s for pure oxy-fuel CH4 flames. Preheating is shown to increase sL and this effect can be represented by a power law correlation sL = sL,0(T/T0)α with α = 1.53. Furthermore, the dilution of the oxidizer with argon leads to a significant decrease in sL of approximately 50 % compared to pure oxy-fuel flames. It is worth noting that detailed reaction mechanisms in combination with molecular transport data are typically not validated within the selected equivalence ratio range.To assess the performance of such reaction schemes in predicting laminar burning velocity under the specified selected conditions, we evaluated 13 different reaction mechanisms sourced from the literature. We compared the laminar burning velocities obtained through calculations of laminar premixed 1D flames and, additionally compared these with experimental data. A sensitivity analysis of the laminar burning velocity were carried out for selected mechanisms. Among the mechanisms, the CalTech2.3 mechanism exhibited the most consistent and accurate performance, particularly at high equivalence ratios.

Influence of Variable Swirl on Emissions in a Non-Premixed Fuel-Flexible Burner at Elevated Ambient Conditions

Abstract As alternative fuels are designated for future energy applications, flexible combustor designs require considerable development to ensure stable operation with reduced NOx emissions. A nonpremixed variable swirl burner was used to experimentally appraise changes in NO production pathways, with CH4 NH3 and H2 flames, alongside intermediate fuel blends. Maintaining an equivalent thermal power and flame temperature between fuels, preheated reactants (500 K) were supplied to the burner, with parametric changes made to pressure (1–6 bara) and swirl number (0.8–2.0). NO production was characterized, alongside variations in flame structure and topology, with a correlation demonstrated for exhaust emissions. NO production was shown to be sensitive to combustor pressure, providing an expected increase for CH4 and H2 flames. Emission profiles from both NH3 and H2 flames are shown to be significantly augmented by a change in swirl number. As NH3 fractions were increased in the H2 blend, a decaying trend in NO emissions was observed with an increase in pressure, and as a function of mixture ratio. However, this behavior was markedly augmented by a change in swirl number and suggests that further reductions may be possible at increased pressure. At the low swirl/high pressure condition, the NH3/H2 blend outperformed pure H2, providing lower NO concentrations. Emissions data were normalized using the traditional dry/O2 correction, alongside mass scaled by thermal power, with a comparison provided. The corresponding differences in emission formation pathways were investigated, alongside high-speed OH* chemiluminescence to further elucidate findings.

Numerical Modeling of Swirl Stabilized Lean-Premixed H2–CH4 Flames With the Artificially Thickened Flame Model

Abstract The lean premixed technology is a very convenient combustion strategy to progressively move from natural gas to high hydrogen content fuels in gas turbines limiting the pollutants emissions at the same time. The enabling process that will allow the combustor to manage a full H2 operation requires relevant design modifications, and in this framework, the numerical modeling will be a pivotal tool that will support this transition. In this work, high-fidelity simulations of perfectly premixed swirl stabilized flames have been performed varying the H2 content in the fuel from 0 to 100% to investigate the effect of the hydrogen addition on the methane flame. The artificially thickened flame model (ATFM) has been used to treat the turbulent chemistry interaction. The numerical results have been compared with the detailed experimental data performed at Cardiff University's Gas Turbine Research Center. After the numerical model validation against experimental OH* chemiluminescence maps has been presented, a deep numerical investigation of the effect of the H2 addition on the flame has been performed. In this way, the work aims to highlight the good prediction capability of the ATFM, and, at the same time, highlight the change in the different contributions that govern the flame reactivity moving from 100% CH4 to 100% H2 in very lean conditions.

Macroscopic flame and flow structures in hydrogen and methane multi-regime combustion

The current experimental investigation focuses on the macroscopic structures of CH4/air and H2/air flames operated on the Darmstadt multi-regime burner adapted for hydrogen operation. Building upon previous research on lean-burn limits, this study utilizes simultaneous PIV and OH-PLIF measurements to examine notable differences in flame and flow structures. Six flame cases are studied, focusing on CH4/air and H2/air jet flames at equivalence ratios of 1.4, 2.2, and 3.5. It is observed that the hydrogen slot 2 flames exhibit unique behavior under ultra-lean conditions, demonstrating thermodiffusive effects that generate finger-like structures. Despite receiving less thermal support from the slot 2 flame, hydrogen jet flames burn faster and resist flame extinction in high-velocity regions. The extensive heat release from fuel-rich H2 jets maintains a stable location compared to CH4 jets at the same equivalence ratio, altering local flow dynamics. Additionally, the study identifies a reshaped primary inner recirculation zone (IRZ) and a secondary IRZ in H2/air flame cases, which is absent in CH4 flames. The interplay between the jet flame and the primary IRZ results in visible flame enhancement in slot 2, indicating preheating and fuel enrichment effects. Overall, this research provides comprehensive insights into the distinct combustion behavior and flow structures of CH4 and H2 flames on a multi-regime burner.

Effect of simultaneous H2 and NH3 addition on soot formation in co-flow diffusion CH4 flame

Simultaneous blending of hydrogen (H2) and ammonia (NH3) to hydrocarbon fuels can tackle the safety issues of H2 and improve burning efficiency of NH3. While this strategy brings challenges for soot prediction due to the promotion effect of H2 and the suppression effect of NH3, and the interactions between H2 and NH3. In this study, the simultaneous addition of NH3 and H2 on soot formation was experimentally and numerically investigated in a co-flow diffusion CH4 flame. The interactions between NH3 and H2, and how they impacted different soot formation processes were comprehensively revealed using a detailed soot sectional model. The decrease of peak SVF in CH4 flame caused by NH3 was 0.013 ppm, about 31.6 % smaller than that in CH4/H2 flame (0.019 ppm), indicating that the inhibitive effect of NH3 on soot formation was promoted by H2. The existence of H2 promoted the suppression effect of NH3 on soot nucleation, condensation and HACA processes in the CH4 flame. Compared with CH4/NH3 flame, the pyrolysis rates of NH3, NH2 and NH in the CH4/NH3/H2 flame were higher since more H and OH radicals were generated via H2 decomposition. This led to a larger consumption rate of H and OH radicals, which decreased the reaction rates of CH4+OH=CH3+H2O and C2H4+OH=C2H3+H2O, and promoted the combination of NO and C2H. Both factors accounted for a stronger suppression effect of NH3 on the formation of A1 in CH4/H2 flame than that in CH4 flame, and thus a stronger inhibitive effect on soot inception and condensation. Compared with the CH4 flame, NH3 resulted in a larger decline of H and OH radicals mole fractions in the CH4/H2 flame, which explained the stronger suppression effect of NH3 on the HACA surface growth process in the CH4/H2 flame.

Experimental study on effects of ammonia enrichment on the thermoacoustic instability of lean premixed swirling methane flames

Ammonia (NH3) has recently emerged as a promising carbon-free energy carrier. Further development and application of NH3 as fuel in the gas turbine industry can significantly reduce the emissions of carbon dioxide (CO2) and contribute to the achievement of a carbon–neutral society. This study experimentally examined the thermoacoustic instability characteristics of a laboratory-scaled lean premixed gas turbine model combustor operated with different NH3 blending ratios with methane (CH4). Experiments conducted under a wide range of inlet velocities and equivalence ratios suggest that NH3 concentration is critical in determining the characteristics of the instability. Specifically, when the NH3 proportion is less than 50 %, the addition of NH3 causes a mode transition of the instability. However, when the content of NH3 is greater than 50 %, it is shown that the instabilities are suppressed, indicating that the addition of a certain amount of NH3 can enhance the stability of CH4 flames. Additional analysis of flame dynamics reveals that the introduction of NH3 causes the lengthening of the flame front and weakens heat release rate fluctuations in the flame root regions. Further Proper Orthogonal Decomposition (POD) analysis of the flow field shows that the instability modes are strongly coupled with periodic vortex motions of the flow dynamics along the shear layers. Finally, the mode shifting phenomena is successfully predicted by low-order thermoacoustic network modeling. It is suggested that the change in convective time delay caused by NH3 addition is responsible for such transitions.

MILD combustion of a premixed NH3/air jet flame in hot coflow versus its CH4/air counterpart

Moderate or intense low-oxygen dilution (MILD) combustion is suitable for solving the problems of unstable flames and high NOx emissions (ENOx) of ammonia fuels; however, studies on this are rare. This paper numerically investigates the MILD combustion characteristics of a premixed NH3/air jet flame in hot coflow (JHC) under different jet equivalence ratios (ΦJ), coflow temperatures (TC), and oxygen levels (XO2,C). For comparison, similar CH4/air MILD-JHC flame characteristics are calculated. The results show that the NH3 flame generally has a lower temperature increase and heat release, as well as a larger reaction zone, than CH4. This suggests that the NH3 flame can develop into a MILD regime more easily than CH4. ENOx of the NH3 flame is two orders of magnitude higher than that of CH4 flame at all TC and XO2,C values for ΦJ < 1. At ΦJ > 1, ENOx of the NH3 flame rapidly decreases to nearly zero, but unburned NH3 emissions (ENH3) and H2 emissions (EH2) are significantly high (∼1000 ppm) and the combustion efficiency is low. Moreover, ENOx increases rapidly (gradually) with XO2,C (TC), whereas ENH3 and EH2 are considerably low at TC ≥ 1300 K and XO2,C ≥ 1% for the NH3 flame. ENOx primarily originates from NO; however, N2O is also important at low ΦJ and TC values. Therefore, considering ENOx, ENH3, EH2 and the burning efficiency, the optimal conditions for NH3 MILD combustion are ΦJ = 1, TC ≥ 1500 K, and XO2,C = 1%–3%. In addition, compared with conventional flames, NH3 MILD combustion can reduce ENOx by one to two orders of magnitude.

Experimental and kinetic study on aromatic formation in counterflow diffusion flames of methane and methane/ethylene mixtures

Aromatics are molecular soot precursors, yet detailed kinetic mechanisms describing their formation in flame environments are insufficient even for the simplest hydrocarbon fuel of methane. Considering the building roles of methane-related chemistry in current hierarchically-structured mechanisms, the present study is devoted to the chemistry of aromatic formation in CH4 flames. We established a series of incipiently sooting methane counterflow diffusion flames (CDF) and characterized their thermochemical structures including the scalar fields of temperature, concentrations of major species, C2–C4 minor intermediates, benzene, large aromatic species, and soot volume fraction. The measurement techniques include tunable diode laser absorption spectroscopy (TDLAS), gas chromatography–mass spectrometer (GC–MS) and laser induced incandescence (LII). The experimental dataset was then used to assess various recently proposed kinetic mechanisms. The results showed that the literature mechanisms tested would significantly overpredict benzene formation in CH4 CDFs (up to a factor of 10). In addition, none of the tested mechanisms could capture the experimentally observed suppressing effects of CH4 addition on the formation of aromatic species in C2H4 CDFs. Potential reactions responsible for the model failures are discussed. The main contributions of this study are to: 1) identify the limitations of existing aromatic mechanisms when applied to methane CDFs; 2) clarify potential reactions/pathways related to model deficiencies; 3) provide novel experimental dataset covering both gas-phase speciation and soot for methane CDFs. It is our hope that the present data and analysis would deepen our understanding on benzene formation chemistry, and contribute to the refinement of aromatic formation predictive models.

Investigating local growth conditions in the flame synthesis of molybdenum-oxide nanoplates from molybdenum substrates

Molybdenum-oxide nanoplates are grown directly from the surface of molybdenum substrates using a catalyst-free flame synthesis technique. To investigate the effects of oxidizing routes, e.g., from gas-phase O2, H2O, and CO2 as reactants, on the growth and morphology of the molybdenum-oxide nanostructures, quasi-1D counterflow diffusion flames are strategically utilized and probed at specific locations within their flame structures. Moreover, two separate flame structures are compared using different fuels (i.e., methane and hydrogen) and diluted with nitrogen to have near-identical temperature profiles. For example, the H2O oxidizing route is scrutinized by assessing growth at the same elevated temperature locations at the fuel sides of the reaction zones of a CH4 flame (comprising H2O and CO2 as synthesis reactants) and an H2 flame (comprising only H2O as synthesis reactant). The temperature effect on the resultant morphologies are examined at 1280 K, 1500 K, and 1720 K. Single-crystalline MoO2 nanoplates (of thicknesses of 60-80 nm, widths of 200-450 nm, and lengths of 1-2 µm) that are regularly distributed with high-surface-area-exposed packing are obtained at 1720 K on the air side of the CH4 flame (with O2, H2O, and CO2 as synthesis reactants) for 10 min growth durations. Based on the results and Gibbs free energy calculations, the CO2 route is deduced to seed the growth of the nanoplates at the nucleation stage heterogeneously, with subsequent condensation-based growth from volatile intermediate species, such as MoO2(OH)2, playing an important role. With such studies revealing fundamental growth mechanisms, scale-up of the process with different flame configurations is afforded.

A Numerical Study of CO2 Decoupled Effects on Extinction Limit and Flame Microstructure in CH4/Air Counterflow Diffusion Flame with Various Pressures

Extinction limits and flame microstructures of CH4 versus air (N2/O2) counterflow diffusion flames with additional CO2 to oxidizer stream under various pressure conditions were numerically investigated by a proposed decoupled strategy. The chemical and thermal effects of CO2 addition were isolated from the synergistic effect on extinction limits and flame microstructures. The results show that the extinction limits were decreased due to the CO2 addition and extended caused by the enhanced pressure, respectively. The changes of flame microstructures from far away from extinction to near extinction mainly are reflected in a narrower combustion zone and thinner flame thickness, while there is no visible displacement of the flame front location. Quantitative analyses manifest the thermal effect of CO2 addition on extinction limit is greater than chemical effect, and elevated pressure can enhance both thermal and chemical effects. An interesting phenomenon occurred between the thermal and chemical effects resulting from increasing CO2 mole fraction in an oxidizer on extinction limit; the former monotonically increase along with CO2 mole fraction, while the latter represents a kind of approximate quadratic-parabola tend along with CO2 mole fraction. The distributions of the production and consumption rate of hydroxyl (OH) radicals were presented to clarify the contribution of the kinetic term in the asymptotic solution to the extinction limits with increasing CO2 molar fraction and various pressures.

A RANS/EDC Simulation of the Lifted Turbulent Non-Premixed Round Jet of CH4 Flame

The objectives of this work is to determine the average rate of reaction by numerical study of the reacting flow inside an axisymmetric free jet, and the validation of the eddy dissipation concept (EDC) combustion model implemented in ANSYS Fluent. The validation of the eddy dissipation concept combustion model involved the reproduction of a lifted, non-premixed, turbulent free jet flame described experimentally in Mahmud (2007). Three chemical mechanisms were used along the combustion model, namely the 2-step Westbrook and Dryer mechanism, the skeletal Jones-Lindstedt mechanism and the Hyer mechanism. Turbulence was modeled with the standard k-ε model The flame lift-off height and the temperature profiles are reproduced accurately by the Hyer model.

Supersonic Combustion Mode Analysis of a Cavity Based Scramjet

Since flame stability is the key to the performance of scramjets, scramjet combustion mode and instability characteristics were investigated by using the POD method based on a cavity-stabilized scramjet. Experiments were developed on a directly connected scramjet model that had an inlet flow of Mach 2.5 with a cavity stabilizer. CH* chemiluminescence, schlieren, and a wall static pressure sensor were employed to observe flow and combustion behavior. Three typical combustion modes were classified by distinguishing averaged CH* chemiluminescence images of three ethylene fuel jet equivalence ratios. The formation reason was explained using schlieren images and pressure characteristics. POD modes (PDMs) were determined using the proper orthogonal decomposition (POD) of sequential flame CH* chemiluminescence images. The PSD (power spectral density) of the PDM spectra showed large peaks in a frequency range of 100–600 Hz for three typical stabilized combustion modes. The results provide oscillation characteristics of three scramjet combustion modes.

Flame Transfer Functions for Turbulent, Premixed, Ammonia-Hydrogen-Nitrogen-Air Flames

Abstract Ammonia is a promising hydrogen and energy carrier but also a challenging fuel to use in gas turbines, due to its low flame speed, limited flammability range, and the production of NOx from fuel-bound nitrogen. Previous experimental and theoretical work have demonstrated that partially dissociated ammonia (NH3/H2/N2 mixtures) can match many of the laminar flame properties of methane flames. Among the remaining concerns pertaining to the use of NH3/H2/N2 blends in gas turbines is their thermoacoustic behavior. This paper presents the first measurements of flame transfer functions (FTFs) for turbulent, premixed, and NH3/H2/N2-air flames and compares them to CH4-air flames that have a similar unstretched laminar flame speed and adiabatic flame temperature. FTFs for NH3/H2/N2 blends were found to have a lower gain than CH4 FTFs at low frequencies. However, the cutoff frequency was found to be greater, due to a shorter flame length. For both CH4 flames and NH3/H2/N2 flames, the confinement diameter was found to have a strong influence on peak gain values. Chemiluminescence resolved along the longitudinal direction shows a suppression of fluctuations when the flame first interacts with the wall followed by a subsequent recovery, but with a significant phase shift. Nevertheless, simple Strouhal number scalings based on the flame length and reactant bulk velocity at the dump plane result in a reasonable collapse of the FTF cutoff frequency and phase curves.

Impact of ammonia addition on soot and NO/N2O formation in methane/air co-flow diffusion flames

Ammonia (NH3) is receiving considerable attention as a potential substitute for carbon-containing fuels to reduce CO2 and soot emissions. Co-burning with hydrocarbons is a feasible solution to improve the combustion performance of NH3, which however, brings new challenges such as NO emissions. Hence, it is essential to investigate the detailed suppression/formation mechanisms of soot and NO during the co-burning of NH3 and hydrocarbons. In this study, the effects of NH3 addition in the fuel stream on soot and NO formations in a CH4/air co-flow atmospheric-pressure diffusion flame were numerically investigated using a 2D code and a combined chemical mechanism comprised of a model of polycyclic aromatic hydrocarbons (PAHs) up to five-rings and detailed nitrogen-containing reactions. The integral of reaction rates over the whole flame were obtained in both CH4 and CH4NH3 co-flow flames to quantitatively investigate how the NH3 addition affected the soot and NO formation pathways. The results showed that the addition of 10% NH3 in the fuel stream of a CH4/air diffusion flame had a strong suppression on soot formation, decreasing the peak soot volume fractions by 38.9%. It was found that the NH3 addition led to decreases in CH4 and H mole fractions close to the burner exit, and hence lowered the integral of reaction rate of R67 (CH4+H=CH3+H2) by 1.07×10−5 mol/s, which in turn reduced the A1 production mainly through the pathway of CH4→CH3→C2H6→C2H5→C2H4→C2H3→C2H2→C3H3→A1. NO and NH played an important role in the consumption of C1∼C2 species involved in A1 formation pathways. The NH3 addition led to an increase in the integrated rates of CH3+NH=CH2NH+H and C2H+NO=HCN+CO by 2.45×10−6 and 2.59×10−6 mol/s, respectively. The decrease of A1 mole fraction reduced the inception, condensation and HACA surface growth rates. The peak NO mole fraction was increased by two orders of magnitude, from 28 ppm in the CH4 flame to 3060 ppm in the CH4NH3 flame. A promoting effect of NH3 addition on N2O was observed in CH4NH3 mixtures, however, through different channels than that of CH4 flames. In addition, more CN species, such as CH2NH, H2CH and CH2CN, were produced in the CH4NH3 flame compared to the CH4 flame.

A comparison between turbulent non-premixed jet flames of CH4 and the 50%NH3+50%H2 blend

A challenge of application of ammonia in combustion devices is due to its low chemical reactivity. Addition of hydrogen into ammonia can increase drastically the premixed laminar flame speed of the blends. In practical combustion devices, however, turbulent non-premixed jet flames are usually employed, while researches on ammonia jet flames are scarce. In this study, firstly, the premixed laminar flame speeds of CH4 and the 50%NH3+50%H2 blend in volumetric fraction are investigated by the Schlieren imaging. It is confirmed that the laminar flame speeds of the 50%NH3+50%H2 blend are remarkably faster than those of CH4 for equivalence ratios above 1.1, while they are similar for lower equivalence ratios. Then, the Schlieren imaging and planar laser induced fluorescence imaging of hydroxyl of the burning turbulent gas jets are conducted. It is found that the flame propagation speed and OH intensity in the CH4 jets are remarkably higher than those in the 50%NH3+50%H2 jet. To unveil the mechanism, the equivalence ratio and fuel mole fractions along the jet axis are investigated by the laser induced breakdown spectroscopy. The equivalence ratios of the 50%NH3+50%H2 blend are significantly lower than those of CH4. Compared to that of CH4, the volumetric flow rate of 50%NH3+50%H2 blend is higher for the choked flow of the under-expanded jets. Therefore, the lower equivalence ratios are attributed to the higher stoichiometric fuel-air ratio of the 50%NH3+50%H2 blend. Stratification of NH3 and H2 due to their distinct molecule transfer properties is experimentally observed, which probably lowers the flame propagation speed in the jet. These findings are expected to be a valuable reference for development of simulation models and design of practical combustion devices with zero-carbon fuels.

Simultaneous stereo‐PIV and OH×CH2O PLIF measurements in turbulent ultra lean CH4/H2 swirling wall‐impinging flames

We present experimental results from turbulent low-swirl lean H2/CH4 flames impinging on an inclined, cooled iso-thermal wall, based on simultaneous stereo-PIV and OH×CH2O PLIF measurements. By increasing the H2 fraction in the fuel while keeping Karlovitz number (Ka) fixed in a first series of flames, a fuel dependent near-wall flame structure is identified. Although Ka is constant, flames with high H2 fraction exhibit significantly more broken reaction zones. In addition, these high H2 fraction flames interact significantly more with the wall, stabilizing through the inner shear layer and well inside the near-wall swirling flow due to a higher resistance to mean strain rate. This flame-wall interaction is argued to increase the effective local Ka due to heat loss to the wall, as similar flames with a (near adiabatic) ceramic wall instead of a cooled wall exhibit significantly less flame brokenness. A second series of leaner flames were investigated near blow-off limit and showed complete quenching in the inner shear layer, where the mean strain rate matches the extinction strain rate extracted from 1D flames. For pure CH4 flames (Ka ≈ 30), the reaction zone remains thin up to the quenching point, while conversely for the 70% H2 flames (Ka ≈ 1100), the reaction zone is highly fragmented. Remarkably, in all near blow-off cases with CH4 in the fuel, a large cloud of CH2O persists downstream the quenching point, suggesting incomplete combustion. Finally, ultra lean pure hydrogen flames were also studied for equivalence ratios as low as 0.22, and through OH imaging, exhibit a clear transition from a cellular flame structure to a highly fragmented flame structure near blow-off.