Flame Speed Correlation Research Articles

A laminar flame speed correlation of hydrogen–methanol blends valid at engine-like conditions

The hydrogen addition is effective on improving the performance of spark-ignited (SI) methanol engines. However, there is still a lack of laminar flame speed data of hydrogen–methanol blends at engine-like conditions. This blocks the combustion modeling of hydrogen-enriched methanol engines. In this paper, a laminar flame speed correlation of hydrogen–methanol blends was proposed for the computational fluid dynamics (CFD) simulation. This correlation was derived through a self-developed calculation program according to the flame-temperature-based mixing rule. Wide ranges of hydrogen volume fractions (0–10%), equivalence ratios (0.6–1.5), unburned gas temperatures (400–2600 K), pressures (1–50 bar) and residual gas mass fractions (0–20%) were simultaneously considered in this correlation. The new correlation was implemented in the extended coherent flame model (ECFM) to evaluate its suitability for CFD simulation. Satisfying agreement between the experimental and calculated results was observed. This indicated that the new correlation was suitable for the CFD simulation of hydrogen-enriched methanol engines.

Measurements of laminar flame speeds and flame instability analysis of 2-methyl-1-butanol–air mixtures

Laminar flame speeds of 2-methyl-1-butanol (2MB)–air mixture at temperatures of 393, 433 and 473K, pressures of 0.1, 0.25, 0.5 and 0.75MPa, and equivalence ratios of 0.6–1.8 were measured using the spherically propagating flame. Flame instabilities were analyzed using the Lewis number, flame thickness, density ratio and Markstein length combining with the schlieren photos. Flame instability is insensitive to temperature while it increases with the increase of pressure. A correlation of laminar flame speed is provided on the basis of experimental data. Comparison was made between the laminar flame speeds of 2-methyl-1-butanol and ethanol, n-butanol and iso-octane. Results show that 2-methyl-1-butanol has the close values of laminar flame speed with those of n-butanol, but lower than those of ethanol and higher than those of iso-octane.

Laminar Flame Characteristics of iso-Octane/n-Butanol Blend–Air Mixtures at Elevated Temperatures

In this work, laminar flame speeds and Markstein lengths of iso-octane/n-butanol–air mixtures were measured via the outwardly expanding spherical flame method and high-speed schlieren photography over a wide range of equivalence ratios and blending ratios of n-butanol at elevated initial temperatures. Results show that laminar flame speeds of fuel blends slightly increase with increasing blending ratio of n-butanol. The effect of blending ratio on laminar flame speed of fuel blends was mechanistically interpreted through examining the thermodynamic and diffusive property, as well as the overall oxidation kinetics. Measurements on the burned gas Markstein length show a generalized behavior. There exists a minimum absolute Markstein length at the critical equivalence ratio ϕ*, under which the stretch effect on the flame propagating speed is minimized. At equivalence ratios less than ϕ*, Markstein length is decreased with increasing blending ratio of n-butanol, indicating that the addition of n-butanol reduces the diffusional thermal stability of the blend; while at equivalence ratios larger than ϕ*, Markstein length increases with increasing blending ratio of n-butanol. This experimental observation on the Markstein length is consistent with the theoretical investigation. A correlation of laminar flame speed of n-butanol/iso-octane blend as a function of equivalence ratio, temperature, and blending ratio of n-butanol is given on the basis of experimental data.

Development and validation of a laminar flame speed correlation for the CFD simulation of hydrogen-enriched gasoline engines

In this paper, a laminar flame speed correlation was developed and validated for the computational fluid dynamics (CFD) simulation of hydrogen-enriched gasoline engines. This correlation was derived through the tabulated data which was determined by a self-developed calculation program according to the flame temperature-based mixing rule. Wide ranges of hydrogen volume fractions (0–10%), equivalence ratios (0.6–1.5), unburned gas temperatures (300–2500 K), pressures (1–50 bar) and residual gas mass fractions (0–20%) were simultaneously considered in this correlation to cover the burning conditions encountered in SI engines. The estimated values of the new correlation were found to be in satisfying agreement with the experimental data under normal burning conditions. Moreover, the new correlation was implemented in the extended coherent flame model to evaluate its suitability for CFD simulation. Satisfying agreement between the experimental and calculated results was observed under all examined hydrogen addition levels. This indicated that the new correlation was suitable for the CFD simulation of hydrogen-enriched gasoline engines.

Laminar flame speed correlations for pure-hydrogen and high-hydrogen content syngas blends with various diluents

Hydrogen is an attractive fuel for large-scale combustion systems due to its high flame speeds and clean burning characteristics. This paper presents a new set of correlations for the laminar flame speeds of hydrogen-oxygen mixtures with nitrogen (air) and helium as diluents, using a recently updated chemical kinetics mechanism. A wide excursion of equivalence ratios (φ = 0.5–5.0), pressures (1–30 atm) and temperatures (270–620 K) was performed. Flame speed correlations were developed at five pressures, namely 1, 5, 10, 20, and 30 atm for the pure-hydrogen case. The disparities between the kinetic model predictions and the correlation estimates, commonly associated with existing correlations, were significantly reduced, and the correlation estimates are within ±13 cm/s of the model predictions. Also, a correlation for lean and high-hydrogen content (HHC) syngas blends of H2 + CO + H2O was developed from the pure-hydrogen correlations. A wide range of pressures (1–30 atm), initial temperatures (323–550 K), steam contaminant levels (5–15%), and hydrogen content in the fuel blend (15–100%) were simulated. A design of experiments approach was adopted to determine the critical mixtures necessary to develop the correlation. The developed HHC correlation agrees within ±12% of the model predictions.

Three-dimensional computational fluid dynamics simulation of hydrogen engines using a turbulent flame speed closure combustion model

The mixture formation and combustion process of a hydrogen direct-injection internal combustion engine is computed using a modified version of a commercial three-dimensional computational fluid dynamics code. The aim of the work is the evaluation of hydrogen laminar flame speed correlations and turbulent flame speed closures with respect to combustion of premixed and stratified mixtures at various levels of air-to-fuel equivalence ratio. Heat-release rates derived from in-cylinder pressure traces are used for the validation of the combustion simulations. A turbulent combustion model with closures for a turbulent flame speed is investigated. The value of the computed heat-release rates mainly depends on the quality of laminar burning velocities and standard of turbulence quantities provided to the combustion model. Combustion simulations performed with experimentally derived laminar flame speed data give better results than those using laminar flame speeds obtained from a kinetic scheme. However, experimental data of hydrogen laminar flame speeds found in the literature are limited regarding the range of pressures, temperatures and air-to-fuel equivalence ratios, and do not comply with the demand of high-pressure engine-relevant conditions.

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 CH 4–H 2 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 CH 4–H 2 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 H 2 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.

Characterization of syngas laminar flames using the Bunsen burner configuration

Laminar flame speeds of syngas mixtures (H 2/CO/Air) have been studied using the Bunsen flame configuration with both straight and nozzle burners. The flame surface area and flame cone angle methodologies, respectively based on the OH* chemiluminescence and Schlieren imaging techniques, have been performed to extract flame speeds for a wide range of equivalence ratios (0.3 < φ < 1.2) and mixture compositions (1% < %H 2 < 100%). As a result, a flame speed correlation established for lean syngas flames with 0.6 < φ < 1.0 and 10% < %H 2 < 70% is proposed. A particular attention has been devoted to the development and validation of the OH* chemiluminescence methodology with the identification of important parameters governing the measurement accuracy.

On the combination of complex chemistry with a 0-D coherent flame model to account for the fuel properties in spark ignition engines simulations: Application to methane-air-diluents mixtures

A promising way to reduce green house gases emissions of spark ignition (SI) engines is to burn alternative fuels like bio-mass-derived products, hydrogen or compressed natural gas. However, their use strongly impacts combustion processes in terms of burning velocities and emissions. Specific engine architectures as well as dedicated control strategies should then be optimized to take advantage of these fuels. Such developments are today increasingly performed using complete engine simulators running in times close to the real time and thus requiring very CPU efficient models. For this purpose, 0-dimensional models are commonly used to describe combustion processes in the cylinders. However, these models should reproduce the engine response for all possible fuels, which is not an obvious task regarding the evocated CPU constraints. This paper deals with the extension of a 0-dimensional coherent flame model (CFM), called CFM1D, to integrate chemical effects related to the fuel composition at low computational costs. Improvements have been carried out using an innovative approach based on a priori simulations of 1D premixed flames with a complex chemistry code. Databases of laminar flame speeds and thicknesses as well as burned gases compositions in engine-like conditions are generated from these simulations, all these information being required as inputs in CFM1D. The proposed approach is here applied to methane-air-diluents mixtures to simulate a large range of operating conditions of a spark ignition (SI) engine. A new laminar flame speed correlation adapted to engine thermodynamic conditions is also developed to save CPU time. Comparisons are made with experiments and with simulations performed using the original version of CFM1D based on a simple chemistry. The achieved results evidence the advantage of this new approach to account for the fuel composition effects on the engine behavior.

Derivation of burning velocities of premixed hydrogen/air flames at engine-relevant conditions using a single-cylinder compression machine with optical access

With respect to hydrogen internal combustion engines beside turbulence also flame front instabilities of high-pressure combustion provoke an acceleration of the flame. To account for this effect within engine simulations, it is suggested to include the impact of flame front instabilities directly into a “quasi-laminar” burning velocity that is an input for turbulent combustion models. Premixed hydrogen/air flames are investigated in a single-cylinder compression machine using OH-chemiluminescence and in-cylinder pressure analysis. Values of burning velocities are calculated from flame front velocities considering thermal expansion effects. A flame speed correlation is derived which covers temperatures and pressures of the unburned mixture, relevant for internal combustion engines, ranging from 350 K to 700 K and 5 bar to 45 bar. Values of air/fuel equivalence ratio cover lean and rich regimes between 0.4 ≤ λ ≤ 2.8. For an evaluation of stretch and instability effects a comparison to fundamental laminar burning velocities of a one-dimensional flame computed with a detailed chemical kinetic-mechanism is given. At high-pressure conditions flame speed measurements demonstrate that flame front instabilities have an accelerating effect on the value of laminar burning velocities, which cannot be reproduced by computations with a chemical model. A linear stability analysis is applied in order to estimate the magnitude of instabilities. The proposed “quasi-laminar” burning velocity does not account for interaction between turbulence and instability effects. Consequently, at increasing turbulence levels partially counter-balancing of instabilities by turbulence is not followed which may allegorize a possible limitation of the suggested approach.

Prediction and Measurement of the Product Gas Composition of the Ultra Rich Premixed Combustion of Natural Gas: Effects of Equivalence Ratio, Residence Time, Pressure, and Oxygen Concentration

The ultra rich combustion (partial oxidation) of natural gas to hydrogen and carbon monoxide is theoretically and experimentally investigated. The effect of the process parameters equivalence ratio, residence time, pressure, and composition of the oxidizer is explored. Computations are performed with the use of the chemical kinetics simulation package CHEMKIN. First, the ultra rich combustion process is modeled as a freely propagating flame in order to establish the rich flame propagation properties. An Arrhenius correlation of the laminar flame speed with the adiabatic flame temperature is found with activation temperature 20,000 K. Subsequently, perfectly stirred reactor (PSR) computations were performed. From these, it is concluded that optimal natural gas conversion to hydrogen and carbon monoxide requires a residence time of at least 50 ms and, depending on residence time, an equivalence ratio between 2 and 4. To reach chemical equilibrium in ultra rich mixtures, the residence time is very long (> 1000 ms). The model predictions are validated by experiments on ultra rich combustion of natural gas by means of air enriched to 40% oxygen concentration at up to 3 bar and 300 kW. The effect of equivalence ratio at residence time 50 ms was investigated. Good comparison was found between measurements and model predictions on carbon monoxide, hydrogen, and the soot precursor acetylene. It can be concluded that the model provides reliable information on product gas concentrations as a result of ultra rich combustion of natural gas.

Modelling of combustion and nitrogen oxide formation in hydrogen-fuelled internal combustion engines within a 3D CFD code

The concerns about global warming and long-term lack of fossil fuels are strong incentives for alternative fuel research and adaptation of the internal combustion engines (ICE) to these fuels. Because it is free of any carbon compounds and can be produced from alternative sources, hydrogen is an interesting candidate for future ICE-based powertrains. However, the peculiar properties of hydrogen, among those its low density and its very high laminar flame speed, impose specific operating strategies and the adaptation of the conventional research tools. In this context, the 3D CFD models dedicated to combustion and pollutant prediction have to be modified. In the present work, the ECFM (Extended Coherent Flame Model) is adapted to hydrogen combustion through the addition of a new laminar flame speed correlation and a new laminar flame thickness expression. Furthermore, the prediction of the NO x emissions is performed with a modified version of the extended Zeldovitch model.

Laboratory Study of Premixed H2-Air and H2–N2-Air Flames in a Low-Swirl Injector for Ultralow Emissions Gas Turbines

The objective of this study is to conduct laboratory experiments on low-swirl injectors (LSIs) to obtain the basic information for adapting LSI to burn H2 and diluted H2 fuels that will be utilized in the gas turbines of the integrated gasification combined cycle coal power plants. The LSI is a novel ultralow emission dry-low NOx combustion method that has been developed for gas turbines operating on natural gas. It is being developed for fuel-flexible turbines burning a variety of hydrocarbon fuels, biomass gases, and refinery gases. The adaptation of the LSI to accept H2 flames is guided by an analytical expression derived from the flow field characteristics and the turbulent flame speed correlation. The evaluation of the operating regimes of nine LSI configurations for H2 shows an optimum swirl number of 0.51, which is slightly lower than the swirl number of 0.54 for the hydrocarbon LSI. Using particle image velocimetry (PIV), the flow fields of 32 premixed H2-air and H2–N2-air flames were measured. The turbulent flame speeds deduced from PIV show a linear correlation with turbulence intensity. The correlation constant for H2 is 3.1 and is higher than the 2.14 value for hydrocarbons. The analysis of velocity profiles confirms that the near field flow features of the H2 flames are self-similar. These results demonstrate that the basic LSI mechanism is not affected by the differences in the properties of H2 and hydrocarbon flames and support the feasibility of the LSI concept for hydrogen fueled gas turbines.

Correlation of flame speed with stretch in turbulent premixed methane/air flames

Direct numerical simulations of two-dimensional unsteady premixed methane/air flames are performed to determine the correlation of flame-speed with stretch over a wide range of curvatures, and strain rates generated by intense two-dimensional turbulence. Lean and stoichiometric premixtures are considered with a detailed C1-mechanism for methane oxidation. The computed correlation shows the existence of two distinct stable branches. It further shows that exceedingly large negative values of stretch can be obtained solely through curvature effects, which give rise to an overall nonlinear correlation of the flame speed with stretch. Over a narrower stretch range, −1≤Ka≤1, which includes 90% of the sample, the correlation is approximately linear, and, hence, the asymptotic theory for stretch is practically applicable. Overall, one-third of the sample has negative stretch. In this linear range, the Markstein number associated with the positive branch is determined for different initial turbulence intensities. For high turbulence intensity, the large eddy turnover time becomes shorter than a flame time, and the flame propagation becomes less responsive to unsteady straining. Reductions in strain Markstein numbers by as much as 37% from comparable steady counterflow computations are reported. In addition to the conventional positive branch, a negative-branch is identified. This negative branch occurs when a flame cusp, with a center of curvature in the burnt gases is subjected to intense compressive strain, resulting in a negative displacement speed. Negative flame speeds are also encountered for extensive tangential strain rates exceeding a Karlovitz number of unity, a value consistent with steady counterflow computations. In both situations, consistent with earlier findings, the source of the reduction in flame speed is attributed to an imbalance between diffusion and reaction.

Turbulence Effects on Flame Speed and Flame Structure

Turbulence effects on methane-air flames stabilized in grid turbulence were investigated through measurements of flame speed and mean and fluctuating flame temperature profiles. Published turbulent flame speed correlations were able to correlate the experimental flame speed data but were contradictory in indicating flame structure and combustion mechanisms. A simple, one-dimensional, wrinkled laminar flame model was used to predict characteristic flame temperature fluctuation levels. Comparison of these predictions with measured temperature fluctuations indicated that the majority of the flames studied were wrinkled laminar flames. However, a wrinkled laminar flame structure was inappropriate for the most intensely turbulent flames examined.

On laminar flame quenching and its application to spark ignition engines

Analyses are made of flame quenching in flows both perpendicular and parallel to a heat sink. A fundamental question is raised about the characteristic length over which a flame in a tube loses heat. Two possibilities are examined, the radius and the flame thickness. By using a functional form prescribed by the theoretical analyses a correlation of laminar flame speed, quenching diameter, and lean limit flame temperature is developed. The correlation suggests that the characteristic length for heat loss is the flame thickness. Laminar flame speeds of isooctane and propane have been correlated and used to compute quench distances in an engine. The agreement is reasonable and leads to speculation that quenching in an engine is basically laminar and dependent upon the free stream unburned gas temperature rather than the wall temperature.

A rational correlation of combustion noise results from open turbulent premixed flames

The results are presented of 103 tests on the noise generation capability of bunsen burner type premixed hydrocarbon-air turbulent flames. The fuels used have been propane, propylene, acetylene and ethylene, all burning at 1 atmosphere in an anechoic chamber.The results for sound power output, reacting volume and spectral content are correlated with the burner diameter, flow velocity, fuel mass fraction and laminar flame speed; these variables are sufficient to produce correlations with respect to the Reynolds number, Mach number, fuel mass fraction and the first Damköhler similarity group. Introducing a new simplified version of the theory of combustion noise, known turbulent flame speed correlations, and the theory of isotropic turbulence decay downstream of the burner lip, it is shown that the correlations for frequency content and radiated sound power have a rational basis.