Different chemical effect of hydrogen addition on soot formation in laminar coflow methane and ethylene diffusion flames

Previous studies showed that adding hydrogen (H₂) can have an opposite chemical effect on soot formation: its chemical effect enhances and suppresses soot formation in methane (CH₄) and ethylene (C₂H₄) diffusion flames, respectively. Such opposite chemical effect of H₂ (CE-H₂) remains unresolved. The different CE-H₂ is studied numerically in the two laminar coflow diffusion flames. A detailed chemical mechanism with the addition of a chemically inert virtual species FH₂ is used to model the gas-phase combustion chemistry in this study. Particularly, a reaction pathway analysis was performed based on the numerical results to gain insights into how H₂ addition to fuel affects the pathways leading to the formation of benzene (A₁) in CH₄ and C₂H₄ flames. The numerical results show that the CE-H₂ in CH₄ diffusion flame to prompt soot formation is ascribed that the higher mole fraction of H atom promotes the formation of A₁ and Acetylene (C₂H₂) and leads to higher nucleation rate and eventually higher soot surface growth rate. In contrast, adding H₂ to C₂H₄ diffusion flames decreases soot nucleation and surface growth rate. The lower soot nucleation rate is due to the lower mole fractions of pyrene (A₄), while the lower soot surface growth rate is due to the lower mole fractions of H atom and C₂H₂, higher mole fraction of H₂ and lower soot nucleation rate. Furthermore, the CE-H₂ in C₂H₄ diffusion flames promotes the formation of A₁, but suppresses the formation of A₄.     

Effects of hydrogen addition on the propagation of spherical methane/air flames: A computational study

A computational study is performed to investigate the effects of hydrogen addition on the fundamental characteristics of propagating spherical methane/air flames at different conditions. The emphasis is placed on the laminar flame speed and Markstein length of methane/hydrogen dual fuel. It is found that the laminar flame speed increases monotonically with hydrogen addition, while the Markstein length changes non-monotonically with hydrogen blending: it first decreases and then increases. Consequently, blending of hydrogen to methane/air and blending methane to hydrogen/air both destabilize the flame. Furthermore, the computed results are compared with measured data available in the literature. Comparison of the computed and measured laminar flame speeds shows good agreement. However, the measured Markstein length is shown to strongly depend on the flame radii range utilized for data processing and have very large uncertainty. It is found that the experimental results cannot correctly show the trend of Markstein length changing with the hydrogen blending level and pressure and hence are not reliable. Therefore, the computed Markstein length, which is accurate, should be used in combustion modeling to include the flame stretch effect on flame speed.     

Effect of hydrogen and helium addition to fuel on soot formation in an axisymmetric coflow laminar methane/air diffusion flame

A detailed numerical study was conducted to investigate the effects of hydrogen and helium addition to fuel on soot formation in atmospheric axisymmetric coflow laminar methane/air diffusion flame. Detailed gas-phase chemistry and thermal and transport properties were employed in the numerical calculations. Soot was modeled using a PAH based inception model and the HACA mechanism for surface growth and oxidation. Numerical results were compared with available experimental data. Both experimental and numerical results show that helium addition is more effective than hydrogen addition in reducing soot loading in the methane/air diffusion flame. These results are different from the previous investigations in ethylene/air diffusion flames. Hydrogen chemically enhances soot formation when added to methane. The different chemical effects of hydrogen addition to ethylene and methane on soot formation are explained in terms of the different effects of hydrogen addition on propargyl, benzene, and pyrene formation low in the flames.     

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.     

A numerical study on the effects of pressure and gravity in laminar ethylene diffusion flames

The effects of pressure and gravity on sooting characteristics and flame structure were studied numerically in coflow ethylene–air laminar diffusion flames between 0.5 and 5 atm. Computations were performed by solving the unmodified and fully-coupled equations governing reactive, compressible, gaseous mixtures which include complex chemistry, detailed radiation heat transfer, and soot formation/oxidation. Soot formation/oxidation was modeled using an acetylene-based, semi-empirical model which has been verified with previously published experimental data to correctly capture many of the observed trends at normal-gravity. Calculations for each pressure considered were performed for both normal- and zero-gravity conditions to help separate the effects of pressure and buoyancy on soot formation. Based on the numerical predictions, pressure and gravity were observed to significantly influence the flames through their effects on buoyancy and reaction rates. The zero-gravity flames have higher soot concentrations, lower temperatures and broader soot-containing zones than normal-gravity flames at the same pressure. The zero-gravity flames were also found to be longer and wider. Differences were observed between the two levels of gravity when pressure was increased. The zero-gravity flames displayed a stronger dependence of the maximum soot yield on pressure from 0.5 to 2 atm and a weaker dependence from 2 to 5 atm as compared to the normal-gravity flames. In addition, flame diameter decreased with increasing pressure under normal-gravity while it increased with pressure in the zero-gravity cases. Changing the prescribed wall boundary condition from fixed-temperature to adiabatic significantly altered the numerical predictions at 5 atm. When the walls were assumed to be adiabatic, peak soot volume fractions and temperatures increased in both the zero- and normal-gravity flames, emphasizing the importance of heat conduction to the burner rim on flame structure.     

Numerical study of laminar flame properties of diluted methane-hydrogen-air flames at high pressure and temperature using detailed chemistry

Technical limits of high pressure and temperature measurements as well as hydrodynamic and thermo-diffusive instabilities appearing in such conditions prevent the acquisition of reliable results in term of burning velocities, restraining the domain of validity of current laminar flame speed correlations to few bars and hundreds of Kelvin. These limits are even more important when the reactivity of the considered fuel is high. For example, the high-explosive nature of pure hydrogen makes measurements even more tricky and explains why only few correlations are available to describe the laminar flame velocity of high hydrogen blended fuels as CH₄-H₂ mixtures. The motivation of this study is thereby to complement experimental measurements, by extracting laminar flame speeds and thicknesses from complex chemistry one-dimensional simulations of premixed laminar flames. A wide number of conditions are investigated to cover the whole operating range of common practical combustion systems such as piston engines, gas turbines, industrial burners, etc. Equivalence ratio is then varied from 0.6 to 1.3, hydrogen content in the fuel from 0 to 100%, residual burned gas mass ratio from 0 to 30%, temperature of the fresh mixtures from 300 to 950 K, and pressure from 0.1 to 11.0 MPa. Many chemical kinetics mechanisms are available to describe premixed combustion of CH₄-H₂ blends and several of them are tested in this work against an extended database of laminar flame speed measurements from the literature. The GRI 3.0 scheme is finally chosen. New laminar flame speed and thickness correlations are proposed in order to extend the domain of validity of experimental correlations to high proportions of hydrogen in the fuel, high residual burned gas mass ratios as well as high pressures and temperatures. A study of the H₂ addition effect on combustion is also achieved to evaluate the main chemical processes governing the production of H atoms, a key contributor to the dumping of the laminar flame velocity.     

Effects of content of hydrogen on the characteristics of co-flow laminar diffusion flame of hydrogen/nitrogen mixture in various flow conditions

Effect of content of hydrogen (H₂) in fuel stream, mole fraction of H₂$\left(X_{H_2}\right)$ in fuel composition, and velocity of fuel and co-flow air $\left(V_{avg}\right)$ on the flame characteristics of a co-flow H₂/N₂ laminar diffusion flame is investigated in this paper. Co-flow burner of Toro et al. [1] is used as a model geometry in which the governing conservation transport equations for mass, momentum, energy, and species are numerically solved in a segregated manner with finite rate chemistry. GRI3 reaction mechanisms are selected along with the weight sum of grey gas radiation (WSGG) and Warnatz thermo-diffusion models. Reliability of the newly generated CFD (computational fluid dynamics) model is initially examined and validated with the experimental results of Toro et al. [1]. Then, the method of investigation is focused on a total of 12 flames with $X_{H_2}$ varying between 0.25 and 1, and $V_{avg}$ between 0.25 and 1 ms^?1. Increase of flame size, flame temperature, chemistry heat release, and NOx emission formation resulted are affected by the escalation of either $X_{H_2}$ or $V_{avg}$. Significant effect on the flame temperature and NOx emission are obtained from a higher $X_{H_2}$ in fuel whereas the flame size and heat release are the result of increasing $V_{avg}$. Along with this finding, the role of N₂ and its higher content reducing the flame temperature and NOx emission are presented.     

Unified behaviour of maximum soot yields of methane, ethane and propane laminar diffusion flames at high pressures

Soot concentration and temperature distributions within the flame envelope of laminar diffusion flames of methane and ethane at elevated pressures were measured in a high-pressure combustion chamber. Methane measurements were made with two different fuel flow rates: 0.43 mg/s (0.32 mg/s carbon flow rate) for the pressure range of 15–60 atm, and 0.83 mg/s for the pressure range of 5–20 atm (0.62 mg/s carbon flow rate). For the ethane flames, the flow rate was 0.78 mg/s (0.62 mg/s carbon flow rate) and the pressure range was 2–15 atm. From the soot concentration distribution, soot yields were calculated as a function of flame height and pressure. Maximum soot yields from the current study and the previous measurements in similar flames with methane, ethane, and propane flames were shown to display a unified behaviour. Maximum soot yields, when scaled properly, were represented by an empirical exponential function in terms of the reduced pressure, actual pressure divided by the critical pressure of the fuel. The maximum soot yield seems to reach a plateau asymptotically as the pressure exceeds the critical pressure of the fuel.     

Soot formation and temperature field structure in laminar propane-air diffusion flames at elevated pressures

The effect of pressure on soot formation and the structure of the temperature field was studied in coflow propane-air laminar diffusion flames over the pressure range of 0.1 to 0.73 MPa in a high-pressure combustion chamber. The fuel flow rate was selected so that the soot was completely oxidized within the visible flame and the flame was stable at all pressures. Spectral soot emission was used to measure radially resolved soot volume fraction and soot temperature as a function of pressure. Additional soot volume fraction measurements were made at selected heights using line-of-sight light attenuation. Soot concentration values from these two techniques agreed to within 30% and both methods exhibited similar trends in the spatial distribution of soot concentration. Maximum line-of-sight soot concentration along the flame centerline scaled with pressure; the pressure exponent was about 1.4 for pressures between 0.2 and 0.73 MPa. Peak carbon conversion to soot, defined as the percentage of fuel carbon content converted to soot, also followed a power-law dependence on pressure, where the pressure exponent was near to unity for pressures between 0.2 and 0.73 MPa. Soot temperature measurements indicated that the overall temperatures decreased with increasing pressure; however, the temperature gradients increased with increasing pressure.     

Kinetic modeling study of hydrogen addition to premixed dimethyl ether-oxygen-argon flames

The chemical composition of flames was examined systematically for a series of laminar, premixed low-pressure Dimethyl ether (DME)-oxygen-argon flames blended with hydrogen. The effects of hydrogen addition to the DME base flame were seen to result in interesting differences. The flame is analyzed with a comprehensive kinetic model that combines the chemistries of hydrogen and DME combustion. The results indicated that the reduction of CH₃OCH₃ mole fraction in the blend is the dominant factor for the reduction of CH₃OCH₃ and CO mole fractions in the flame. The rate of the primary reactions related to CH₃OCH₃ and CO increases obviously with the addition of hydrogen. When the volume fractions of H₂ to the total of DME and H₂ exceeds 40%, H₂ will change from an intermediate species to a reactant, which means the effect of H₂ on the premixed combustion will be more significant. The free radicals in the radical pool, such as H, O and OH radicals, increase as hydrogen is added, which promote the combustion process. The mole fraction of CH₂O is decreased as hydrogen is added. Less soot precursors (acetylene (C₂H₂)) were produced with the addition of H₂.     

Effects of hydrogen addition on laminar flame speeds of methane,ethane and propane: Experimental and numerical analysis

The main purpose of this study is to investigate the effects of hydrogen addition on the laminar flame speeds of methane, ethane and propane. In this work, a flat flame method was used to measure the laminar flame speed in a counter-flow configuration combined with particle image velocimetry (PIV) system. The results indicate that with the increase of hydrogen amount, the laminar flame speeds of methane, ethane and propane increase linearly approximately. In addition, as hydrogen is increased, the flame speed of methane has the maximum increasing amplitude among them, which indicates that methane is more sensitive to hydrogen addition in flame speed than the other two fuels.Simulation analysis finds that the reaction R1: H + O₂ ? OH + O can promote the flame speeds of these three kinds of gaseous fuel obviously, and with the increase of hydrogen amount, the promoting effect is more obviously. Therefore, the main reason why hydrogen addition could increase flame speed is that the increase of H radical prompts reaction R1 to proceed in the forward direction. Comparing the flames of methane, ethane and propane mixed with hydrogen, it was found that the promotion of reaction R1 to the methane/hydrogen mixtures flame speed is strongest, and its free radicals concentration in flame increase more obviously. Therefore, hydrogen addition has a greater effect on the flame speed of methane than on that of ethane and propane.     

Measurements of sooting tendency in laminar diffusion flames of n-heptane at elevated pressure

This research focuses on the effects of an increasing pressure on the soot formation during combustion of vaporized liquid fuel. Therefore soot formation is measured in a laminar diffusion flame, with n-heptane as fuel, over a range of pressures from 1.0 to 3.0 bar. The soot volume fraction in the diffusion flames has been measured using Laser-Induced Incandescence (LII) calibrated by means of the Line Of Sight Attenuation (LOSA) technique. The values of the calibration factors between LII intensities and soot volume fraction from LOSA are slightly varied for different pressure. The integral soot volume fractions show power law dependence on pressures, being proportional to pⁿ, with n being 3.4 ± 0.3 in the pressure range of 1.0–3.0 bar.     

Effects of hydrogen peroxide on combustion enhancement of premixed methane/air flames

Hydrogen peroxide is generally considered to be an effective combustion promoter for different fuels. The effects of hydrogen peroxide on the combustion enhancement of premixed methane/air flames are investigated numerically using the PREMIX code of Chemkin collection 3.5 with the GRI-Mech 3.0 chemical kinetic mechanisms and detailed transport properties. To study into the enhancement behavior, hydrogen peroxide is used for two different conditions: (1) as the oxidizer substituent by partial replacement of air and (2) as the oxidizer supplier by using different concentrations of H₂O₂. Results show that the laminar burning velocity and adiabatic flame temperature of methane flame are significantly enhanced with H₂O₂ addition. Besides, the addition of H₂O₂ increases the CH₄ consumption rate and CO production rate, but reduces CO₂ productions. Nevertheless, using a lower volumetric concentration of H₂O₂ as an oxidizer is prone to reduce CO formation. The OH concentration is increased with increasing H₂O₂ addition due to apparent shifting of major reaction pathways. The increase of OH concentration significantly enhances the reaction rate leading to enhanced laminar burning velocity and combustion. As to NO emission, using H₂O₂ as an oxidizer will never produce NO, but NO emission will increase due to enhanced flame temperature when air is partially replaced by H₂O_2.     

A model for the laminar flame speed of binary fuel blends and its application to methane/hydrogen mixtures

Recent studies have demonstrated promising performance of adding hydrogen to methane in internal combustion engines and substantial attention has been devoted to binary fuel blends. Due to the strong nonlinearity of chemical reaction process, the laminar flame speed of binary fuel blends cannot be obtained from linear combination of the laminar flame speed of each individual fuel constituent. In this study, theoretical analysis is conducted for a planar premixed flame of binary fuel blends and a model for the laminar flame speed is developed. The model shows that the laminar flame speed of binary fuel blends depends on the square of the laminar flame speed of each individual fuel component. This model can predict the laminar flame speed of binary fuel blends when three laminar flame speeds are available: two for each individual fuel component and the third one for the fuel blends at one selected blending ratio. The performance of this model as well as models reported in the literature is assessed for methane/hydrogen mixtures. It is demonstrated that good agreements with calculations or measurements can be achieved by the present model prediction. Moreover, it is found that the present model also works for other binary fuel blends besides methane/hydrogen.     

Kinetic incentive of hydrogen addition on nonpremixed laminar methane/air flames

In order to find out the respective influences of chemical reactivity and physical transport of hydrogen additive on nonpremixed flame, two fabricated hydrogen additions were introduced into nonpremixed methane/air flame modeling. Hydrogen addition was assumed as inert gas or partial reactivity fuel to respectively explore the kinetic reasons by the three aspects: the elementary reaction route, heat release, and physical diffusion of hydrogen addition. The analyses were implemented in terms of OH and H production. Results showed that, hydrogen addition can enhance OH and H production via elementary reactions, and causes flame reaction zone migration through the coupling interaction between the low-temperature heat enthalpy release and diffusion behavior of hydrogen addition. R84 (OH + H₂=H + H₂O) and R38 (H + O₂=O + OH) are the most important elementary reactions related to OH and H production. The physical incentive of hydrogen addition can hardly work without the chemical effects of hydrogen addition.     

Flame structure and global flame response to the equivalence ratios of interacting partially premixed methane and hydrogen flames

The interacting partially premixed methane and hydrogen flames established in a one-dimensional counterflow field were investigated numerically with the OPPDIF code and GRI-v3.0 was used to consider both fuels. The flame structure and response of the maximum flame temperature, heat-release rate, and flame speed to the equivalence ratios (Φ) and global strain rate (a_g) were investigated. The maximum temperature decreased with increasing a_g. The maximum temperature for cases with a stoichiometric hydrogen-side flame was higher than for other cases with the same a_g.The hydrogen-side flame played a key role in determining the maximum temperature. The maximum heat-release rates (MHRRs) for all cases show different trends. The MHRR of the methane-side flame was affected considerably by the interacting flame structure and hydrogen-side flame condition. However, the MHRRs of the hydrogen were independent of methane-side flame condition. For the cases where Φ of the methane-side flame was varied while the hydrogen-side flame was kept stoichiometric (Var-S), the MHRR and flame speed of the hydrogen-side flame were independent of the methane-side flame conditions. However, the methane-side flames had a negative flame speed except near-stoichiometric conditions. On the other hand, in the cases where Φ of the hydrogen-side flame was varied while the methane-side flame was kept stoichiometric (S-Var), the hydrogen-side flames had the MHRR and flame speed similar to those of an unstretched partially premixed hydrogen flame.     

An experimental study on soot distribution characteristics of ethanol-gasoline blends in laminar diffusion flames

In view of the potential of bio-ethanol as an alternative fuel and the particulate matter (PM) issues during gasoline combustion, the soot distribution characteristics of ethanol-gasoline blends in laminar diffusion flames were studied on a Gülder liquid burner using the two-color laser induced incandescence (TC-LII) technique. During the experiments, the ethanol ratio in the blends was varied from 20% to 80% by volume in order to investigate quantitatively the soot reduction potential of ethanol. In order to study the effect of reduction in carbon content due to ethanol addition on soot formation, the experiments were performed under a fixed fuel mass flow rate and a fixed carbon mass flow rate. It was found that both peak and average soot volume fraction in the flame reduced significantly with increasing ethanol content under both fuel supplying modes, however, this effect was progressively less pronounced as ethanol content increased. By comparing the two fueling modes, it was found that the reduction in carbon content due to ethanol addition has little impact on soot reduction. For a given ethanol blending ratio, the soot reduction under the same carbon mass flow rate was only slightly smaller than that under the same fuel mass flow rate. In terms of flame characteristics, the initial height of soot formation increases with increasing ethanol content under both fuel supply modes mainly due to the increased fuel outlet velocity. Radially, the peak soot location moves from the outside towards the center gradually as height increases. However, along the center line of the flame, the initial height of soot formation decreases with increasing ethanol content under the same fuel flow rate, whereas the trend remained similar to that in the whole flame under the same carbon flow rate.     

Counterflow diffusion flames of general fluids: Oxygen/hydrogen mixtures

A comprehensive framework has been established for studying laminar counterflow diffusion flames for general fluids over the entire regime of thermodynamic states. The model incorporates a unified treatment of fundamental thermodynamic and transport theories into an existing flow solver DMCF to treat detailed chemical kinetic mechanisms and multispecies transport. The resultant scheme can thus be applied to fluids in any state. Both subcritical and supercritical conditions are considered. As a specific example, diluted and undiluted H₂/O₂ flames are investigated at pressures of 1-25 MPa and oxygen inlet temperatures of 100 and 300 K. The effects of pressure p and strain rate ?s on the heat release rate , extinction limit, and flame structure are examined. In addition, the impact of cross-diffusion terms, such as the Soret and Dufour effects, on the flame behavior is assessed. Results indicate that the flame thickness δf and heat release rate correlate well with the square root of the pressure multiplied by the strain rate as and , respectively. The strain rate at the extinction limit exhibits a quasi-linear dependence on p. Significant real-fluid effects take place in the transcritical regimes, as evidenced by the steep property variations in the local flowfield. However, their net influence on the flame properties appears to be limited due to the ideal-gas behavior of fluids in the high-temperature zone.     

A comprehensive study of light hydrocarbon mechanisms performance in predicting methane/hydrogen/air laminar burning velocities

In order to obtain the precise predicted values of methane/hydrogen/air burning velocities from simulations, the performances of GRI mech 3.0, Aramco mech 1.3, USC mech 2.0 and San diego mech mechanisms were systematically studied under various conditions by PREMIX code and compared with experimental data from literature. The conditions where each mechanism gave their good performance are obtained and concluded. The flowrate sensitivity and rate constants of key elementary reactions were analyzed to insight the different behavior of each mechanism. The results showed that all these widely used small hydrocarbon mechanisms could gave reasonable predictions for pure methane and methane hydrogen blends. Nevertheless, they lack sensitivity for rich hydrogen at elevated pressures due to their complex reactions competitions controlled by hydrogen sub model. USC mech 2.0 was found more suitable for being used at low hydrogen contents while San diego mech gained better results at high hydrogen contents. GRI 3.0 gave good predictions for methane hydrogen blends except for high initial pressures. Generally, Aramco mech 1.3 showed the best performance for all testing conditions. Moreover, there was relatively large deviation from the predicted results and experimental data in the transition regime where the hydrogen fractions were between 60% and 80%, it may could be optimized by tuning the rate constants of reactions.