Laminar flame speeds of primary reference fuels and reformer gas mixtures

The laminar flame speeds of neat primary reference fuels (PRFs), n-heptane and iso-octane, PRF blends, reformer gas, and reformer gas/iso-octane/air mixtures are measured over a range of equivalence ratios at atmospheric pressure, using counterflow configuration and digital particle image velocimetry (DPIV). PRF blends with various octane numbers are studied. The synthetic reformer gas mixture employed herein has a composition that would be produced from the partial oxidation of rich iso-octane/air mixture into CO and H₂, namely, 28% H₂, 25% CO, and 47% N₂. Computationally, the experimentally determined laminar flame speeds are simulated using the detailed kinetic models available in the literature. Both experimental and computational results demonstrate that the flame speeds of hydrocarbon/air mixtures increase with addition of a small amount of reformer gas, and the flame speeds of reformer gas/air mixtures are dramatically reduced with addition of a small amount of hydrocarbon fuel. Furthermore, the number density effect of seeding particles on flame speed measurement is assessed, and the experimental uncertainties associated with the present DPIV setup as well as the linear extrapolation method employed herein are discussed.     

Determination of laminar flame speeds using stagnation and spherically expanding flames: Molecular transport and radiation effects

The uncertainties associated with the extraction of laminar flame speeds through extrapolations from directly measured experimental data were assessed using one-dimensional direct numerical simulations with focus on the effects of molecular transport and thermal radiation loss. The simulations were carried out for counterflow and spherically expanding flames given that both configurations are used extensively for the determination of laminar flame speeds. The spherically expanding flames were modeled by performing high fidelity time integration of the mass, species, and energy conservation equations. The simulation results were treated as “data” for stretch rate ranges that are encountered in experiments and were used to perform extrapolations using formulas that have been derived based on asymptotic analyses. The extrapolation results were compared then against the known answers of the direct numerical simulations. The fuel diffusivity was varied in order to evaluate the flame response to stretch and to address reactant differential diffusion effects that cannot be captured based on Lewis number considerations. It was found that for large molecular weight hydrocarbons at fuel-rich conditions, the flame behavior is controlled by differential diffusion and that the extrapolation formulas can result in notable errors. Analysis of the computed flame structures revealed that differential diffusion modifies the fluxes of fuel and oxygen inside the flame and thus affect the reactivity as stretch increases. Radiation loss was found to affect notably the extracted laminar flame speed from spherically expanding flame experiments especially for slower flames, in agreement with recent similar studies. The effect of radiation could be eliminated however, by determining the displacement speed relative to the unburned gas. This can be achieved in experiments using high-speed particle image velocimetry to determine the flow velocity field within the few milliseconds duration of the experiment. In general, extrapolations were found to be unreliable under certain conditions, and it is proposed that the raw experimental data in either flame configurations are compared against results of direct numerical simulations in order to avoid potential falsifications of rate constants upon validation.     

Nonlinear effects in the extraction of laminar flame speeds from expanding spherical flames

Various factors affecting the determination of laminar flames speeds from outwardly propagating spherical flames in a constant-pressure combustion chamber were considered, with emphasis on the nonlinear variation of the stretched flame speed to the flame stretch rate, and the associated need to nonlinearly extrapolate the stretched flame speed to yield an accurate determination of the laminar flame speed and Markstein length. Experiments were conducted for lean and rich n-butane/air flames at initial pressure, demonstrating the complex and nonlinear nature of the dynamics of flame evolution, and the strong influences of the ignition transient and chamber confinement during the initial and final periods of the flame propagation, respectively. These experimental data were analyzed using the nonlinear relation between the stretched flame speed and stretch rate, yielding laminar flame speeds that agree well with data determined from alternate flame configurations. It is further suggested that the fidelity in the extraction of the laminar flame speed from expanding spherical flames can be facilitated by using small ignition energy and a large combustion chamber.     

Laminar flame speeds and kinetic modeling of hydrogen/chlorine combustion

In this study, laminar flame speeds at atmospheric pressure are accurately measured for H₂/Cl₂/N₂ mixtures at different equivalence ratios and N₂ mole fractions by the counterflow flame technique. A kinetic mechanism based on recently published and evaluated rate constants is developed to model these measured laminar flame speeds as well as the literature data on the concentrations of H₂, Cl₂, and HCl species in flat-burner flames and the ignition delay times from shock tube experiments. The kinetic model yields satisfactory comparison with these experimental data, and suggests that the reactions involving excited HCl(v) species and energy branching are not of substantial significance in combustion situations, and that the use of accurate elementary rate constants is instead crucial to the accuracy of the reaction mechanism.     

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 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.     

Laminar flame speeds of moist syngas mixtures

This work experimentally investigates the effect of the presence of water vapor on the laminar flame speeds of moist syngas/air mixtures using the counterflow twin-flame configuration. The experimental results presented here are for fuel lean syngas mixtures with molar percentage of hydrogen in the hydrogen and carbon monoxide mixture varying from 5% to 100%, for an unburned mixture temperature of 323 K, and under atmospheric pressure. At a given equivalence ratio, the effect of varying amount of water vapor addition on the measured laminar flame speed is demonstrated. The experimental laminar flame speeds are also compared with computed values using chemical kinetic mechanisms reported in the literature. It is found that laminar flame speed varies non-monotonically with addition of water for the carbon monoxide rich mixtures. It first increases with increasing amount of water addition, reaches a maximum value, and then decreases. An integrated reaction path analysis is further conducted to understand the controlling mechanism responsible for the non-monotonic variation in laminar flame speed due to water addition. On the other hand, for higher values of H₂/CO ratio the laminar flame speed monotonically decreases with increasing water addition. It is shown that the competition between the chemical and thermal effects of water addition leads to the observed response. Furthermore, reaction rate sensitivity analysis as well as binary diffusion coefficient sensitivity analysis are conducted to identify the possible sources of discrepancy between the experimental and predicted values. The sensitivity results indicate that the reaction rate constant of H₂ + OH = H₂O + H is worth revisiting and refinement of binary diffusion coefficient data of N₂–H₂O, N₂–H₂, and H₂–H₂O pairs can be considered.     

Measurement of laminar burning speeds and Markstein lengths using a novel methodology

Three different methodologies used for the extraction of laminar information are compared and discussed. Starting from an asymptotic analysis assuming a linear relation between the propagation speed and the stretch acting on the flame front, temporal radius evolutions of spherically expanding laminar flames are postprocessed to obtain laminar burning velocities and Markstein lengths. The first methodology fits the temporal radius evolution with a polynomial function, while the new methodology proposed uses the exact solution of the linear relation linking the flame speed and the stretch as a fit. The last methodology consists in an analytical resolution of the problem. To test the different methodologies, experiments were carried out in a stainless steel combustion chamber with methane/air mixtures at atmospheric pressure and ambient temperature. The equivalence ratio was varied from 0.55 to 1.3. The classical shadowgraph technique was used to detect the reaction zone. The new methodology has proven to be the most robust and provides the most accurate results, while the polynomial methodology induces some errors due to the differentiation process. As original radii are used in the analytical methodology, it is more affected by the experimental radius determination. Finally, laminar burning velocity and Markstein length values determined with the new methodology are compared with results reported in the literature.     

Lift-off of jet diffusion flame in sub-atmospheric pressures: An experimental investigation and interpretation based on laminar flame speed

This paper reveals lift-off behavior of jet diffusion flames in sub-atmospheric pressures less than 100 kPa, in view of that the current knowledge on this topic is limited for normal pressure conditions. Physically, the variation of ambient pressure may have significant influence on the lift-off behavior of jet diffusion flames due to the change of some critical parameters such as laminar flame speed. In this work, experiments are conducted in a large pressure-controllable chamber of 3 m (width) × 2 m (length) × 2 m (height) at different sub-atmospheric pressures of 60 kPa, 70 kPa, 80 kPa, 90 kPa as well as at normal pressure of 100 kPa. Axisymmetric turbulent jet diffusion flames are produced by nozzles with diameters of 4 mm, 5 mm and 6 mm using propane as fuel. It is revealed that the lift-off height increases as the pressure decreases and being much higher than that in normal pressure condition. The laminar flame speed with its dependency on pressure is introduced to interpret such behavior based on classic Kalghatgi model. It is found theoretically that the lift-off height has a power law dependency on pressure by P¹⁻ⁿ, where n is overall reaction order of the fuel which is usually larger than 1 indicating a negative power law function with pressure (for example p^−0.75 for propane as n = 1.75) as well verified by the experimental correlation. Finally, a global model is proposed by including such pressure dependency function into the Kalghatgi model, which is shown to well collapse the experimental results of lift-off heights of different sub-atmospheric pressures.     

Effect of cylindrical confinement on the determination of laminar flame speeds using outwardly propagating flames

The effect of nonspherical (i.e. cylindrical) bomb geometry on the evolution of outwardly propagating flames and the determination of laminar flame speeds using the conventional constant-pressure technique is investigated experimentally and theoretically. The cylindrical chamber boundary modifies the propagation rate through the interaction of the wall with the flow induced by thermal expansion across the flame (even with constant pressure), which leads to significant distortion of the flame surface for large flame radii. These departures from the unconfined case, especially the resulting nonzero burned gas velocities, can lead to significant errors in flame speeds calculated using the conventional assumptions, especially for large flame radii. For example, at a flame radius of 0.5 times the wall radius, the flame speed calculated neglecting confinement effects can be low by ∼15% (even with constant pressure).A methodology to estimate the effect of nonzero burned gas velocities on the measured flame speed in cylindrical chambers is presented. Modeling and experiments indicate that the effect of confinement can be neglected for flame radii less than 0.3 times the wall radius while still achieving acceptable accuracy (within 3%). The methodology is applied to correct the flame speed for nonzero burned gas speeds, in order to extend the range of flame radii useful for flame speed measurements. Under the proposed scaling, the burned gas speed can be well approximated as a function of only flame radius for a given chamber geometry - i.e. the correction function need only be determined once for an apparatus and then it can be used for any mixture. Results indicate that the flow correction can be used to extract flame speeds for flame radii up to 0.5 times the wall radius with somewhat larger, yet still acceptable uncertainties for the cases studied. Flow-corrected burning velocities are measured for hydrogen and syngas mixtures at atmospheric and elevated pressures. Flow-corrected flame speeds in the small cylindrical chamber used here agree well with previously reported flame speeds from large spherical chambers. Previous papers presenting burning velocities from cylindrical chambers report performing data analysis on flame radii less than 0.5 or 0.6 times the wall radius, where the flame speed calculated neglecting confinement effects may be low by ∼15 or 20%, respectively. For cylindrical chambers, data analysis should be restricted to flame radii less than 0.3 times the wall radius or a flow correction should be employed to account for the burned gas motions.With regard to the design of future vessels, larger vessels that minimize the flow aberrations for the same flame radius are preferred. Larger vessels maximize the relatively unaffected region of data allowing for a more straightforward approach to interpret the experimental data.     

On the extraction of laminar flame speed and Markstein length from outwardly propagating spherical flames

Large discrepancies among the laminar flame speeds and Markstein lengths of methane/air mixtures measured by different researchers using the same constant-pressure spherical flame method are observed. As an effort to reduce these discrepancies, one linear model (LM, the stretched flame speed changes linearly with the stretch rate) and two non-linear models (NM I and NM II, the stretched flame speed changes non-linearly with the stretch rate) for extracting the laminar flame speed and Markstein length from propagating spherical flames are investigated. The accuracy and performance of the LM, NM I, and NM II are found to strongly depend on the Lewis number. It is demonstrated that NM I is the most accurate for mixtures with large Lewis number (positive Markstein length) while NM II is the most accurate for mixtures with small Lewis number (negative Markstein length). Therefore, in order to get accurate laminar flame speed and Markstein length from spherical flame experiments, different non-linear models should be used for different mixtures. The validity of the theoretical results is further demonstrated by numerical and experimental studies. The results of this study can be used directly in spherical flame experiments measuring the laminar flame speed and Markstein length.     

A numerical study of laminar flame speed of stratified syngas/air flames

Numerical simulations are performed to study the flame propagation of laminar stratified syngas/air flames with the San Diego mechanism. Effects of fuel stratification, CO/H₂ mole ratio and temperature stratification on flame propagation are investigated through comparing the distribution of flame temperature, heat release rate and radical concentration of stratified flame with corresponding homogeneous flame. For stratified flames with fuel rich-to-lean and temperature high-to-low, the flame speeds are faster than homogeneous flames due to more light H radical in stratified flames burned gas. The flame speed is higher for case with larger stratification gradient. Contrary to positive gradient cases, the flame speeds of stratified flames with fuel lean-to-rich as well as with temperature low-to-high are slower than homogeneous flames. The flame propagation accelerates with increasing hydrogen mole ratio due to higher H radical concentration, which indicates that chemical effect is more significant than thermal effect. Additionally, flame displacement speed does not match laminar flame speed due to the fluid continuity. Laminar flame speed is the superposition of flame displacement speed and flow velocity.     

Application of advanced correlative approaches to modeling hydrogen solubility in hydrocarbon fuels

In petroleum and petrochemical refineries, having precise knowledge regarding H₂ solubility in hydrocarbon fuels and feedstocks is critical. In this study, the hydrogen solubility in hydrocarbon fuels was estimated using genetic programming (GP) and group method of data handling (GMDH), two exemplary robust advanced models for generating correlation. To do this, 445 observations derived from labratory findings on hydrogen solubility in 17 different hydrocarbon fuels such as bitumen, atmospheric residue, heavy coking gas oil, heavy virgin gas oil, light virgin gas oil, straight run gas oil, shale fuel oil, dephenolated shale fuel oil, diesel, hydrogenated coal liquid, coal liquid, and coal oil, over a large interval of P- operating pressures and T-temperatures were collected. Temperature, pressure, as well as density at 20 °C, molecular weight, and weight percentage of carbon (C) and hydrogen (H) in hydrocarbon fuels, were used as input parameters in developing robust correlations. The outcomes showed the GMDH approach is more precise compared to the GP, with a root mean square error (RMSE) of 0.053302 and a determination coefficient (R²) of 0.9641. Additionally, sensitivity analysis showed that pressure, followed by temperature and H (wt%) of hydrocarbon fuels, has the greatest impact on hydrogen solubility in hydrocarbon fuels. Ultimately, the Leverage method's results suggested that the GMDH model could be relied on to predict hydrogen solubility in hydrocarbon fuels.     

Effects of steam dilution on laminar flame speeds of H2/air/H2O mixtures at atmospheric and elevated pressures

The laminar flame speeds of H₂/air with steam dilution (up to 33 vol%) were measured over a wide range of equivalence ratio (0.9–3.0) at atmospheric and elevated pressures (up to 5 atm) by an improved Bunsen burner method. Burke, Sun, HP (High Pressure H₂/O₂ mechanism), and Davis mechanisms were employed to calculate the laminar flame speeds and analyze different effects of steam addition. Four studied mechanisms all underestimated the laminar flame speeds of H₂/air/H₂O mixtures at medium equivalence ratios while the Burke mechanism provided the best estimates. When the steam concentration was lower than 12%, increasing pressure first increased and then decreased the laminar flame speed, the inflection point appeared at 2.5 atm. When the steam concentration was greater than 12%, increasing the pressure monotonously decrease the laminar flame speed. The chemical effect was amplified by elevated pressure and it played an important role for the inhibiting effect of the pressure on laminar flame speed. The fluctuations of the chemical effect at 1 atm were mainly caused by three-body reactions, while the turn at 5 atm was mainly caused by the direct reaction effect. Elevated pressure and steam addition amplified the influences of uncertainties in the rate constants for elementary reactions, which might leaded to the disagreement between experimental and simulation results.     

Simulations of laminar flame propagation in droplet mists

In order to clarify the conditions conducive to propagation of premixed flames in quiescent sprays, a one-dimensional code with detailed chemistry and transport was used. n-Heptane and n-decane, distinguished by their volatility, were studied under atmospheric and low temperature, low pressure conditions. The effects of initial droplet diameter, overall equivalence ratio ?₀ and droplet residence time before reaching the flame front were examined. Increasing the residence time had an effect only for n-heptane, with virtually no evaporation occurring before the flame front for n-decane. The trends were only marginally correlated with the local gaseous equivalence ratio ?_eff at the location of maximum heat release rate. ?_eff could be as low as 0.4 (beyond the lean flammability limit), but the flame speed could still be 40% of the gaseous stoichiometric flame speed S_L,0. For n-heptane, ?_eff increased towards ?₀ with smaller droplets while high flame speeds occurred when ?_eff was near 1. This implied that the highest flame speed was achieved with small droplets for ?₀ ? 1 and with relatively large droplets for ?₀ > 1. In the latter case, the oxidiser was completely consumed in the reaction zone and droplets finished evaporating behind the flame where the fuel was pyrolysed. The resulting small species, mainly C₂H₂, C₂H₄ and H₂, diffused back to the oxidation zone and enhanced the reaction rate there. Ultimately, this could result in flame speeds higher than S_L,0 even with ?₀ = 4. For n-decane, the same trends were followed but smaller droplets were needed to reach the same ?_eff due to the slow evaporation rate. Under low pressure and low temperature, the effects of pressure and temperature on ?_eff and the flame speed were competitive and resulted in values close to the ones at atmospheric conditions.     

Radiation-induced uncertainty in laminar flame speed measured from propagating spherical flames

Laminar flame speeds measured using the propagating spherical flame method are inherently affected by radiation. Under certain conditions, a substantial uncertainty in laminar flame speed measurement is caused by radiation, which results in a great concern for kinetic mechanism validation and development. In this study, numerical simulations with detailed chemistry and different radiation models are conducted to examine the effects of radiation on spherical flame propagation. The emphasis is placed on quantifying the uncertainty and corrections associated with radiation in laminar flame speed measurements using propagating spherical flames. The radiation effects on flame speeds at normal and elevated temperatures and pressures are examined for different fuel/air mixtures including methane, propane, iso-octane, syngas, hydrogen, dimethyl ether, and n-heptane. The radiative effects are conservatively evaluated without considering radation reflection on the wall. It is found that radiation-induced uncertainty in laminar flame speeds is affected in the opposite ways by the initial temperature and pressure. An empirical correlation quantifying the uncertainty associated with radiation is obtained. This correlation is shown to work for different fuels at normal and elevated temperatures and pressures. Therefore, it can be directly used in spherical flame experiments measuring the laminar flame speed. Furthermore, a method to obtain the radiation-corrected flame speed (RCFS) is presented and it can be used for laminar flame speed measurement using the propagating spherical flame method.     

Study on laminar flame speed and flame structure of syngas with varied compositions using OH-PLIF and spectrograph

Various Bunsen flame information of premixed syngas/air mixtures was systematically collected. A CCD camera was used to capture the flame images. The OH-PLIF technique was applied to obtain the flame OH distribution and overall flame radiation spectra were measured with a spectrograph. Experiments were conducted on a temperature un-controlled burner and syngas over a wide range of H₂/CO ratios (from 0.25 to 4) and equivalence ratios (from 0.5 to 1.2). Results show that increasing hydrogen fraction (XH₂$X_{{\text{H}}_2}$) extends the blow-off limit significantly. The measured laminar flame speed using cone-angle method based on CCD flame imaging and OH-PLIF images increases remarkably with the increase of XH₂$X_{{\text{H}}_2}$, and these measurements agrees well with kinetic modeling predictions through Li's mechanism when the temperature for computation is corrected. Kinetic study shows that as XH₂$X_{{\text{H}}_2}$ increases, the production of H and OH radicals is accelerated. Additionally, the main H radical production reaction (or OH radical consumption reactions) changes from R29 (CO + OH = CO₂ + H) to R3 (H₂ + OH = H₂O + H) as XH₂$X_{{\text{H}}_2}$ increases. Sensitivity analysis was conducted to access the dominant reactions when XH₂$X_{{\text{H}}_2}$ increases. The difference on flame color for different XH₂$X_{{\text{H}}_2}$ mixtures is due to their difference in radiation spectrum of the intermediate radicals produced in combustion.     

Numerical analysis on the combined transport effects of H2 and CO2 on the laminar premixed flame characteristics of biogas-hydrogen blends

The combined and respective transport effects of H₂ and CO₂ on the flame structure, laminar flame speed and radical pool of the BG40H60 blends at different equivalence ratios are investigated quantitatively with the numerical simulation in this study. The results show that H₂ transport dominates the decrease and enhancement of HRR and mole fractions of minor species at the fuel-lean and fuel-rich conditions. However, H₂ or CO₂ transport hardly affects concentrations of major species expect for H₂ and CO₂. Besides, the dominated H₂ transport contributes to the decreased/increased laminar flame speed at the fuel-lean/fuel-rich condition, while the OH radical can reflect the laminar flame speed variation caused by the H₂ and CO₂ transport. Based on the rate-of-production (ROP) analysis of OH radical, the most sensitive reactions to H₂ and CO₂ transport are OH + H₂H₂O + H/H + O₂O + OH and OH + CH₂OHCO + H₂O at the fuel-lean and fuel-rich conditions respectively. The major production reactions (H + O₂O + OH, H + HO₂ = 2OH, O + H₂H + OH, 2OH = O + H₂O) of OH radical are suppressed or improved more significantly with the H₂ and CO₂ transport at the fuel-lean or fuel-rich condition, leading to the suppressed or improved OH radical pool and the flame propagation at the fuel-lean or fuel-rich condition. Furthermore, it is demonstrated that CO₂ transport suppresses the reaction of OH + H₂H₂O + H considerably to improve the OH radical pool at the fuel-rich condition and cannot be neglected when investigating the flame propagation of biogas-hydrogen blends.     

Measurement and correlation of laminar flame speeds of CO and C2 hydrocarbons with hydrogen addition at atmospheric and elevated pressures

The laminar flame speeds of mixtures of ethane, ethylene, acetylene, and carbon monoxide with small amount of hydrogen addition at atmospheric and elevated pressures were experimentally and computationally determined. It was found that the approximate linear correlation identified previously between the laminar flame speeds and an appropriate definition of the amount of hydrogen addition for methane, propane and n-butane at atmospheric pressure also largely applies to ethane, ethylene, and acetylene at atmospheric as well as elevated pressures. The linear correlation, however, does not hold for carbon monoxide, at all pressures, due to the strong catalytic effect of hydrogen on the oxidation of carbon monoxide. A mechanistic analysis shows that both the Arrhenius and diffusive contributions to the laminar flame speed are nearly linear functions of the hydrogen addition, which explains this overall approximate linear correlation.     

Laminar flame speed correlations of ammonia/hydrogen mixtures at high pressure and temperature for combustion modeling applications

Ammonia/hydrogen mixtures are among the most promising solutions to decarbonize the transportation and energy sector. The implementation of these alternative energy carriers in practical systems requires developing suitable numerical tools, able to estimate their burning velocities as a function of both thermodynamic conditions and mixture quality. In this study, laminar flame speed correlations for ammonia/hydrogen/air mixtures are provided for high pressures (40 bar–130 bar) and elevated temperatures (720 K–1200 K), and equivalence ratios ranging from 0.4 to 1.5. Based on an extensive dataset of chemical kinetics simulations for ammonia/hydrogen blends (0-20-40-60-80-90-100 mol% of hydrogen), dedicated correlations are derived using a regression fitting. Besides these blend-specific correlations, a generalized (i.e., hydrogen-content adaptive) formulation, with hydrogen content used as additional parameter, is proposed and compared to the dedicated correlations.