Experimental and numerical studies of laminar flame characteristics of ethyl acetate with or without hydrogen addition

Hydrogen (H₂) is an effective additive to improve the issue of low laminar burning velocity of some biofuels. In order to better understand the laminar burning characteristics of ethyl acetate (EA) with or without H₂ addition, experimental investigations of laminar burning characteristics were carried out by using the high-speed Schlieren photography technique in a constant volume combustion chamber. Tests were conducted under various equivalence ratios ranging from 0.5 to 1.4 with an initial temperature of 358 K, an initial pressure of 0.1 MPa and a H₂/air proportion of 0%, 4%, 8% and 12% by volume. Laminar burning velocities, together with other parameters such as laminar burning flux, flame thickness, Markstein length and Markstein number, were calculated and discussed. In addition, the experimental data were compared with numerical simulations based on the Dayma model. Results showed that the laminar burning velocity of EA was enhanced with the increase of H₂ addition, and the maximum value reached 95.09 cm/s at φ = 0.6 with 12% H₂, a value more than twice as fast as that of pure EA (39.3 cm/s). Moreover, H₂ was found to extend the lower flammability limit of EA. The laminar burning velocities simulated with the Dayma model agreed well with the experimental results of EA at various H₂ additions.     

Numerical investigation of the effects of H2/CO/syngas additions on laminar premixed combustion characteristics of NH3/air flame

Co-firing NH₃ with H₂/CO/syngas (SYN) is a promising method to overcome the low reactivity of NH₃/air flame. Hence, this study aims to systematically investigate the laminar premixed combustion characteristics of NH₃/air flame with various H₂/CO/SYN addition loadings (0–40%) using chemical kinetics simulation. The numerical results were obtained based on the Han mechanism which can provide accurate predictions of laminar burning velocities. Results showed that H₂ has the greatest effects on increasing laminar burning velocities and net heat release rates of NH₃/air flame, followed by SYN and CO. CO has the most significant effects on improving NH₃/air adiabatic flame temperatures. The H₂/CO/SYN additions can accelerate NH₃ decomposition rates and promote the generation of H and NH₂ radicals. Furthermore, there is an evident positive linear correlation between the laminar burning velocities and the peak mole fraction of H + NH₂ radicals. The reaction NH₂ + NH <=> N₂H₂ + H and NH₂ + NO <=> NNH + OH have remarkable positive effects on NH₃ combustion. The mole fraction of OH × NH₂ radicals positively affects the net heat release rates. Finally, it was discovered that H radicals play an important role in the generation of NO. The H₂/CO/SYN additions can reduce the hydrodynamic and diffusional-thermal instabilities of NH₃/air flame. The NH₃ reaction pathways for NH₃–H₂/CO/SYN-air flames can be categorized mainly into NH₃–NH₂–NH–N–N₂, NH₃–NH₂–HNO–NO(?N₂O)–N₂ and NH₃–NH₂(?N₂H₂)–NNH–N₂. CO has the greatest influence on the proportions of three NH₃ reaction routes.     

The synergistic effect of equivalence ratio and initial temperature on laminar flame speed of syngas

The laminar flame speed of syngas (CO:H₂ = 1:1)/air premixed gas in a wide equivalence ratio range (0.6–5) and initial temperature (298–423 K) was studied by Bunsen burner. The results show that the laminar flame speed first increases and then decreases as the equivalence ratio increasing, which is a maximum laminar flame speed at n = 2. The laminar flame speed increases exponentially with the increase of initial temperature. For different equivalent ratios, the initial temperature effects on the laminar flame speed is different. The initial temperature effects for n = 2 (the most violent point of the reaction) is lower than others. It is found that H, O and OH are affected more and more when the equivalence ratio increase. When the equivalence ratio is far from 2, the reaction path changes, and the influence of initial temperature on syngas combustion also changes. The laminar flame speed of syngas is more severely affected by H + O₂ = O + OH and CO + OH = CO₂ + H than others, which sensitivity coefficient is larger and change more greatly than others when the initial temperature and equivalence ratio change. Therefore, the laminar flame speed of syngas/air premixed gas is affected by the initial temperature and equivalence ratio. A new correlation is proposed to predict the laminar flame speed of syngas (CO:H₂ = 1:1)/air premixed gas under the synergistic effect of equivalence ratio and initial temperature (for equivalence ratios of 0.6–5, the initial temperature is 298–423 K).     

A computational study and correlation of premixed isooctane air laminar reaction fronts diluted with EGR

The current work investigates the propagation of premixed laminar reaction fronts for mixtures of isooctane–air and recirculated combustion products (or EGR) under high pressure and temperature conditions. The work uses a transient one-dimensional flame simulation with a skeletal 215 species chemical kinetic mechanism to generate laminar burning velocity and front thickness predictions. The simulation was exercised over fuel–air equivalence ratios, unburned gas temperatures, pressures and EGR levels ranging from 0.1 to 1.0, 400 to 1000 K, 1 to 250 bar, and 0% to 60% (by mass) respectively, a range extending beyond that of previous researchers. Steady reaction fronts with burning velocities in excess of 5 cm/s could not be established under all of these conditions, especially when burned gas temperatures were below 1450 K and/or when characteristic reaction front propagation times were on the order of the unburned gas ignition delay. For a given pressure, T_u and T_b, the burning velocity of an EGR dilute mixture was found to be lower than that of an air dilute mixture, with the decrease in burning velocity attributed primarily to the reduced oxygen concentration’s effect on chemistry. Steady premixed laminar burning velocities were correlated using a modified two-equation form based on the asymptotic structure of a laminar flame, which produced an average error of 3.4% between the simulated and correlated laminar burning velocities, with a standard deviation of 4.3%, while additional correlations were constructed for reaction front thickness and adiabatic flame temperature. Correlations are presented based on a non-product equivalence ratio φ and a fraction of stoichiometric combustion products X_SCP. Conversion factors are provided to facilitate application to modern direct injection internal combustion engines with inherent charge stratification where the local global Φ is different from the global Φ of the residual gas.     

Study on using hydrogen and ammonia as fuels: Combustion characteristics and NOx formation

This paper evaluates the potential of hydrogen (H₂) and ammonia (NH₃) as carbon‐free fuels. The combustion characteristics and NO_x formation in the combustion of H₂ and NH₃ at different air‐fuel equivalence ratios and initial H₂ concentrations in the fuel gas were experimentally studied. NH₃ burning velocity improved because of increased amounts of H₂ atom in flame with the addition of H₂. NH₃ burning velocity could be moderately improved and could be applied to the commercial gas engine together with H₂ as fuels. H₂ has an accelerant role in H₂–NH₃–air combustion, whereas NH₃ has a major effect on the maximum burning velocity of H₂–NH₃–air. In addition, fuel‐NO_x has a dominant role and thermal‐NO_x has a negligible role in H₂–NH₃–air combustion. Thermal‐NO_x decreases in H₂–NH₃–air combustion compared with pure H₂–air combustion. NO_x concentration reaches its maximum at stoichiometric combustion. Furthermore, H₂ is detected at an air‐fuel equivalence ratio of 1.00 for the decomposition of NH₃ in flame. Hence, the stoichiometric combustion of H₂ and NH₃ should be carefully considered in the practical utilization of H₂ and NH₃ as fuels. H₂ as fuel for improving burning performance with moderate burning velocity and NO_x emission enables the utilization of H₂ and NH₃ as promising fuels. Copyright © 2014 John Wiley & Sons, Ltd.     

The characteristics of flame propagation in hydrogen/oxygen mixtures

The extreme explosiveness and high flame velocity of hydrogen challenge its application. Overcoming these challenges requires improving the fundamental flame characteristics of H₂/O₂ mixtures. In this study, the propagation characteristics of H₂/O₂ flames are investigated. The laminar burning velocity (LBV) is evaluated using nonlinear extrapolation. The empirical relations of LBV are given with the equivalence ratio (ER) and initial mixture pressure (IMP). The LBV increases first and then decreases as the ER increases and reaches its maximum value at the ER slightly higher than 1.0 (φ = 1.1–1.2). The LBV increases monotonically with increasing IMP. The critical instability radius and Markstein length increase as the ER increases, while decreasing with the IMP increase. The flame thickness decreases significantly with increasing IMP. The flame remains stable and smooth throughout the propagation process for all examined ERs only at the lower IMPs of 0.1 atm and 0.3 atm.     

Effect of preferential diffusion and flame stretch on flame structure and laminar burning velocity of syngas Bunsen flame using OH-PLIF

By using OH-PLIF technique, experiments were conducted for laminar Bunsen flame of premixed CO/H₂/air mixtures with equivalence ratio ranging from 0.5 to 1.8. Reynolds number was varied from 800 to 2200, X_H2 = H₂/(H₂+CO) in the mixture was varied from 20% to 100% to study the effects of both preferential diffusion and flame curvature on flame structures and laminar flame burning velocities. Results showed that the combined effects of preferential diffusion and curvature gave an interesting phenomenon of the flame OH radical distributions on high hydrogen content flames. Furthermore, with the increase of H₂ fraction in fuel mixture, the effects of both preferential diffusion and flame curvature were increased. Interpretation of flame stretch effect on laminar burning velocity is also provided in this paper.     

Numerical investigation of premixed hydrogen/air combustion at lean to ultra-lean conditions and catalytic approach to enhance stability

Premixed combustion of hydrogen/air over a platinum (Pt) catalyst is numerically investigated in a planar channel burner with the aim of stabilising the flame at lean to ultra-lean conditions. A steady laminar species transport model is examined in conjunction with elementary heterogeneous and homogeneous chemical reaction schemes and validated against experimental results. A stability map is obtained in a non-catalytic burner for the equivalence ratios (φ) of 0.15–0.20, which serves as the basis for the catalytic flame analysis. Over the Reynolds numbers (Re) investigated in the non-catalytic burner, no flame is observed for φ ≤ 0.16, and flame extinction occurs at Re < 571 and Re < 381 for φ = 0.18 and 0.20, respectively. Moreover, a significant amount of unburned H₂ exits the burner in all cases. With the Pt catalyst coated on the walls, complete H₂ combustion is attained for 0.10 ≤ φ ≤ 0.20 where the contribution of gas phase (homogeneous) reaction increases with Re. Furthermore, radiation on the wall and at the inlet affects the combustion kinetics and flame temperature. Finally, NO_x emission is investigated under the same conditions and found to increase with equivalence ratio but has a negligible effect with the inflow Reynolds number.     

Chemical structure and laminar burning velocity of atmospheric pressure premixed ammonia/hydrogen flames

This paper presents experimental data on the flame structure of laminar premixed ammonia and ammonia/hydrogen flames at different equivalence ratios (φ = 0.8, 1.0 and 1.2) and the laminar flame speed of ammonia/hydrogen flames (φ = 0.7–1.5) at 1 atm. Experimental data were compared with modeling results obtained using four detailed chemical-kinetic mechanisms of ammonia oxidation. In general, all models adequately predict the flame structure. However, for the laminar burning velocity, this is not so. The main nitrogen-containing species present in the post-flame zone in significant concentrations are N₂ and NO. Experimental data and numerical simulations show that the transition to slightly rich conditions enables to reduce NO concentration. Numerical simulation indicate that increasing the pressure rise also results into reduction of NO formation. However, when using ammonia as a fuel, additional technologies should be employed to reduce NO formation.     

Laminar burning velocities and flame stability analysis of H2/CO/air mixtures with dilution of N2 and CO2

Experimental measurement of the laminar burning velocities of H₂/CO/air mixtures and equimolar H₂/CO mixtures diluted with N₂ and CO₂ up to 60% and 20% by volume, respectively, were conducted at different equivalence ratios and conditions near to the sea level, 0.95 atm and 303 ± 2 K. Flames were generated using contoured slot-type nozzle burners and Schlieren images were used to determine the laminar burning velocity with the angle method. Numerical calculations were also conducted using the most recent detailed reaction mechanisms for comparison with the present experimental results. Additionally, a study was conducted to analyze the flame stability phenomenology that was found in the present experiments. The increase in the N₂ and CO₂ dilution fractions considerably reduced the laminar burning velocity due to the decrease in heat release and increase in heat capacity. At the same dilution fractions this effect was higher for the case of CO₂ due to its higher heat capacity and dissociation effects during combustion. Flame instabilities were observed at lean conditions. While the presence of CO in the fuel mixture tends to stabilize the flame, H₂ has a destabilizing effect which is the most dominant. A higher N₂ and CO₂ dilution fraction increased the range of equivalence ratios where unstable flames were obtained due to the increase in the thermal-diffusive instabilities.     

The effect of hydrogen addition on premixed laminar acetylene–hydrogen–air and ethanol–hydrogen–air flames

In the present work, the laminar premixed acetylene–hydrogen–air and ethanol–hydrogen–air flames were investigated numerically. Laminar flame speeds, the adiabatic flame temperatures were obtained utilizing CHEMKIN PREMIX and EQUI codes, respectively. Sensitivity analysis was performed and flame structure was analyzed. The results show that for acetylene–hydrogen–air flames, combustion is promoted by H and O radicals. The highest flame speed (247 cm/s) was obtained in mixture with 95% H₂–5% C₂H₂ at λ = 1.0. The region between 0.95 < X_H2 < 1.0 was referred to as the acetylene-accelerating hydrogen combustion since the flame speed increases with increase the acetylene fraction in the mixture. Further increase in the acetylene fraction decreases the H radicals in the flame front. In ethanol–hydrogen–air mixtures, the mixture reactivity is determined by H, OH and O radicals. For X_H2 < 0.6, the flame speed in this regime increases linearly with increasing the hydrogen fraction. For X_H2 > 0.8, the hydrogen chemistry control the combustion and ethanol addition inhibits the reactivity and reduces linearly the laminar flame speed. For 0.6 < X_H2 < 0.8, the laminar flame speed increases exponentially with the increase of hydrogen fraction.     

Laminar flame speed of H2/CH4/air mixtures with CO2 and N2 dilution

The laminar flame speeds of H₂/CH₄/air mixtures with CO₂ and N₂ dilution were systematic investigated experimentally and numerically over a wide range of H₂ blending ratios (0–75 vol%) with CO₂ (0–67 vol%) and N₂ (0–67 vol%) dilution in the fuels. The experimental measurements were conducted via the Bunsen flame method incorporating the Schlieren technique under the condition of equivalence ratios from 0.8 to 2.0. To gain an insightful understanding of the experimental observations, detailed numerical simulation was carried out using Chemkin-Pro with GRI3.0-Mech. The experimental measurements were also used to validate the corresponding performance of a semiempirical correlation derived through asymptotic analysis method coupled with the reduced chemistry mechanism. The results showed that at lower H₂ fraction (x_H2 ≤ 0.5), the laminar flame speeds of H₂/CH₄/air mixtures displayed great linearly increase with the growth of H₂ fractions. The combustion of mixtures with low H₂ contents was dominated by CH₄ conversion which was mainly controlled by the increasing OH radicals produced from the key oxidation reactions of H + O₂ = O + OH. With the further increase of H₂ fractions, the methane-dominated combustion gradually transformed into the methane-inhibited hydrogen combustion, resulting to the growth of laminar flame speeds was dramatical and non-linear. Due to the larger heat capacity and chemical kinetic effect, CO₂ presented a stronger dilution effect on reducing the laminar flame speeds than N₂. With the addition of CO₂, the increasing stronger competition for H radical through CO + OH = CO₂ + H with H + O₂ = O + OH due to the significant reduction of H mole fractions, leading to the larger decrease of laminar flame speeds of mixtures. Besides, although the contribution of thermal effect of CO₂ decreased near the equivalence ratio, the thermal effect of CO₂ still preformed the dominated contribution to the total dilution effect. A comparison between the experimental data and estimated results using the semiempirical correlation showed that, the correlation using new modified coefficients provided the satisfactorily accuracy predictions on the laminar flame speeds of diluted H₂/CH₄/air mixtures at lower x_H2 (x_H2 ≤ 0.5) and lower x_dilution (x_dilution = 0.25). The estimated results were generally located within a deviation range of ±20% errors except for two unsatisfactory eases occurred at conditions of x_H2 = 0.75 and x_CO2 = 0.67. The considerably poor predictions were attributed to the significantly variation of the chemical kinetics under high H₂ content and large CO₂ dilution conditions.     

Non-monotonic behaviors of laminar burning velocities of H2/O2/He mixtures at elevated pressures and temperatures

Both experimental and calculated laminar burning velocities of H₂/O₂/He mixtures were obtained, with equivalence ratios of 0.6–4.0, initial pressures of 0.1 MPa–0.5 MPa, initial temperature of 373 K, and dilution ratio of 7.0. Laminar burning velocities changed non-monotonically with the increasing initial pressures at equivalence ratios of 1.0–3.0. The decrease of overall reaction orders can explain the non-monotonic relationship between the laminar burning velocities and initial pressures. Consumption and production of both H and HO₂ radicals were also obtained to explain the decrease of overall reaction order. The competition of H and HO₂ radical between elemental reactions were also discussed. The three body reaction R15 (H + O₂(+M) = HO₂(+M)) gained more H radical in the competition with R1 (H + O₂ = O + OH), producing more HO₂ radical. Through the reaction pathway analysis, the restraint in production of both OH and H leaded to a reducing radical pool. The poorer reaction pool would restrain the overall reaction and lead to the reduction of overall reaction order and the non-monotonic behavior of the laminar burning velocity.     

On the effective Lewis number formulations for lean hydrogen/hydrocarbon/air mixtures

Decades of research have underlined the undeniable importance of the Lewis number (Le) in the premixed combustion field. From early experimental observations on laminar flame propagation to the most recent DNS studies of turbulent flames, the unbalanced influence of thermal to mass diffusion (i.e. Le ≠ 1), known as nonequidiffusion, has shed the light on a wide range of combustion phenomena, especially those involving stretched flames. As a result the determination of the Lewis number has become a routine task for the combustion community. Recently, the growing interest in hydrogen/hydrocarbon (HC) fuel blends has produced extensive studies that have not only improved our understanding of H₂/HC flame dynamics, but also, in its wake, raised a fundamental question: which effective Lewis number formulation should we use to characterize the combustion of hydrogen/hydrocarbon/air blends? While the Lewis number is unambiguously defined for combustible mixtures with a single fuel reactant, the literature is unclear regarding the appropriate equivalent formulation for bi-component fuels. The present paper intends to clarify this aspect. To do so, effective Lewis number formulations for lean (φ = 0.6 and 0.8) premixed hydrogen/hydrocarbon/air mixtures have been investigated in the framework of an existing outwardly propagating flame theory. Laminar burning velocities and burned Markstein lengths of H₂/CH₄, H₂/C₃H₈, H₂/C₈H₁₈ and H₂/CO fuel blends in air were experimentally and numerically determined for a wide range of fuel compositions (0/100% → 100/0% H₂/HC). By confronting the two sets of results, the most appropriate effective Lewis number formulation was identified for conventional H₂/HC/air blends. Observed deviations from the validated formulation are discussed for the syngas (H₂/CO) flame cases.     

Numerical investigation on effects of high initial temperatures and pressures on flame behavior of CO/H2/Air mixtures near the dilution limit

This study investigates effects of initial temperatures and pressures on dilution limits of CO/H₂/air mixtures by numerical simulation of one-dimensional laminar premixed flames of CO/H₂/air mixtures (50%CO–50%H₂). Maximum flame temperatures, laminar flame speeds, mass burning rates and flame thickness near the dilution limits are analyzed. Results reveal that the dilution limits are extended at the elevated initial temperatures. The laminar flame speeds and mass burning rates at the dilution limits increase with the elevation of initial temperature, however, the flame thickness at the dilution limits decreases with increasing pressures and increases slightly with elevated initial temperature. The decreased flame thickness renders the flamelet modeling more favorable for turbulent combustion at elevated pressure conditions. The ratio of the flame thickness to the reaction thickness and the Zeldovich number increase first and then decrease with increasing pressure, but the non-monotonic trend of ratio of flame thickness to reaction thickness with the increasing pressures is unnoticeable. Sensitivity analysis suggested that the non-monotonic trend of the Zeldovich number could be caused by the combined effects of following elementary reactions: H + O₂ + M → HO₂ + M, 2HO₂ → H₂O₂ + O₂ and H₂O₂ + M → 2OH + M.     

Effects of high pressure,high temperature and dilution on laminar burning velocities and Markstein lengths of iso-octane/air mixtures

Spherically expanding flames are employed to measure flame velocities, from which are derived the corresponding laminar burning velocities at zero stretch rate. Iso-octane/air mixtures at initial temperatures between 323 and 473 K, and pressures between 1 and 10 bar, are studied over an extensive range of equivalence ratios, using a high-speed shadowgraph system. Effects of dilution are investigated with nitrogen and for several dilution percentages (from 5 to 25 vol% N₂). Over 270 experimental values have been obtained, providing an exhaustive data base for iso-octane/air combustion. Experimental results are in excellent agreement with recently published experimental data. An explicit correlation giving the laminar burning velocity from the initial pressure, the initial temperature, the dilution rate, and the equivalence ratio is finally proposed. Computed results using the two kinetic schemes and the Cantera code are compared to the present measurements. It is found that the mechanisms yield substantially higher values of laminar flame velocities than the present experimental results. Effects of oxygen enrichment are also investigated. A linear trend relating the percentage of oxygen in air and the unstretched laminar burning velocity is observed. Effects of high pressure, high temperature, and high dilution rate on Markstein lengths are also studied. As already done for the laminar burning velocity, an empirical correlation is proposed to describe the Markstein length for burned gases as a function of initial temperature and pressure, for equivalence ratios between 0.9 and 1.1, which has never been done before in the literature.     

Experimental and numerical study on the laminar burning velocity of hydrogen enriched biogas mixture

The laminar burning velocities of biogas-hydrogen-air mixture at different fuel compositions and equivalence ratios were determined and studied using the spherical flame method. The combined effects of H₂ and CO₂ on the laminar burning velocity were investigated quantitatively based on the kinetic effects and the thermal effects. The results show that the laminar burning velocities of the BG40, BG50 and BG60 are increased almost linearly with the H₂ addition owing to the improved fuel kinetics and the increased adiabatic flame temperature. The dropping trend of laminar burning velocity from the BG60-hydrogen to the BG40-hydrogen is primarily attributed to the decreased adiabatic flame temperature (thermal effects). The GRI 3.0 mechanism can predict the laminar burning velocity of biogas-hydrogen mixture better than the San Diego mechanism in this study. Whereas, the GRI mechanism still needs to be modified properly for the hydrogen-enriched biogas as the CO₂ proportion exceeds 50% in the biogas at the fuel-rich condition. The increased CO₂ exerts the stronger suppression on the net reaction rate of H + O₂=OH + O than that of H + CH₃(+M) = CH₄(+M), which contributes to that the rich-shift of peak laminar burning velocity of biogas-hydrogen mixture requires higher H₂ addition as the CO₂ content is enhanced. For the biogas-hydrogen fuel, the H₂ addition decreases the flame stability of biogas fuel effectively due to the increased diffusive-thermal instability and hydrodynamic instability. The improved flame stability of biogas-hydrogen fuel with the increased CO₂ content is resulted from the combined effects of diffusive-thermal instability and hydrodynamic instability.     

Laminar burning velocity of lean H2–CO mixtures at elevated pressure using the heat flux method

Laminar burning velocity measurements of 50:50 and 85:15% (by volume) H₂–CO mixtures with O₂–N₂ and O₂–He oxidizers were performed at lean conditions (equivalence ratio from 0.5 to 1) and elevated pressures (1 atm–9 atm). The heat flux method (HFM) is employed for determining the laminar burning velocity of the fuel–oxidizer mixtures. HFM creates a one-dimensional adiabatic stretchless flame which is an important prerequisite in defining the laminar burning velocity. This technique is based on balancing the heat loss from the flame to the burner with heat gain to the unburnt gas mixture, in a very simple way, such that no net heat loss to the burner is obtained. Instabilities are observed in lean H₂–CO flames with nitrogen as the bath gas for pressures above 4 atm. Stable flames are obtained with helium as the bath gas for the entire pressure range. With the aim to cater stringent conditions for combustion systems such as gas turbines, an updated H₂–CO kinetic mechanism is proposed and validated against experimental results. The scheme was updated with recent rate constants proposed in literature to suit both atmospheric and elevated pressures. The proposed kinetic model agrees with new experimental results. At conditions of high pressure and lean combustion, reactions H + O₂ = OH + O and H + O₂ (+M) = H₂ (+M) compete the most when compared to other reactions. Reaction H + HO₂ = OH + OH contributes to OH production, however, less at high-pressure conditions. At higher CO concentrations and leaner mixtures an important role of reaction CO + OH = CO₂ + H is observed in the oxidation of CO.     

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.     

Turbulent flame structure characteristics of hydrogen enriched natural gas with CO2 dilution

This paper investigated the hydrogen enriched methane/air flames diluted with CO₂. The turbulent premixed flame was stabilized on a Bunsen type burner and the two dimensional instantaneous OH profile was measured by Planar Laser Induced Fluorescence (PLIF). The flame front structure characteristics were obtained by extracting the flame front from OH-PLIF images. And the turbulence-flame interaction was analyzed through the statistic parameters. The role of hydrogen addition as well as CO₂ dilution on the features of turbulent flame were revealed by those parameters. In this work, hydrogen fractions of 0, 0.2 and CO₂ dilution ratios of 0, 0.05 and 0.1 were studied. Results showed that hydrogen addition can enhance turbulent burning velocity ST/S_L through decreasing the scale of the finer structure of the wrinkled flame front, caused by the smaller flame instability scale. In contrast, CO₂ dilution decreased turbulent burning velocity S_T/S_L due to its inactive response to turbulence perturbation and larger flame wrinkles. For all flames, the probability density function (PDF) profile of the local curvature radius R shows a bias to positive value, resulted from the flame intrinsic instability. The PDF profile of R decreases with CO₂ dilution, while the value of local curvature radius corresponding to the peak PDF is larger. This indicates that larger wrinkles structure was generated due to CO₂ dilution, which leads to the decrease in S_T/S_L as a consequence. Hydrogen addition increases the flame volume and results in more intense combustion. CO₂ dilution has a decrease effect on flame volume for both X_H2 = 0 and X_H2 = 0.2 while the decrease is obvious at X_H2 = 0.2, Z_CO2 = 0.1. In all, hydrogen enrichment improves the combustion while CO₂ can moderate combustion. Therefore, adding hydrogen and CO₂ in natural gas can be a potential method for adjusting the combustion intensity in combustion chamber during the combustor design.