Determination of the laminar burning velocities for mixtures of ethanol and air at elevated temperatures

Experimental test for premixed laminar combustion of ethanol–air mixtures has been conducted in a constant volume combustion bomb. The laminar burning velocities of ethanol–air mixtures are determined over a wide range of equivalence ratio at elevated temperatures, by means of the measurements of spherically expanding flames using schlieren photography technique. The effect of flame stretch imposed at the flame front has been discussed and the Markstein lengths are deduced to characterize the stretch effect on flame propagation. Following a linear relation between flame speed and flame stretch, the unstretched laminar burning velocities of ethanol–air flames have been derived. Over the ranges studied, a power law correlation has been suggested for the unstretched laminar burning velocities as a function of initial temperature and equivalence ratio. The empirical correlation is also compared with those data available in the literature, and it is found that the discrepancies are acceptable.     

Correlations for laminar burning velocities of liquefied petroleum gas–air mixtures

Spherically expanding flames have been employed to determine the laminar flame speeds of liquefied petroleum gas–air mixtures, diluted or not by the combustion exhaust gas, over equivalence ratios from 0.7 to 1.4. The effect of the stretch imposed at the flame front has been explored experimentally, and Markstein lengths are estimated to characterize the flame stretch. After omitting the stretch effect, one has obtained the unstretched laminar burning velocities of liquefied petroleum gas–air flames with or without diluent. Explicit formulas have been obtained to express the laminar burning velocity dependencies on the equivalence ratio and diluent rate.     

Measurement of laminar burning velocity and Markstein length relative to fresh gases using a new postprocessing procedure: Application to laminar spherical flames for methane,ethanol and isooctane/air mixtures

The purpose of this study is to present a new tool for extracting the laminar burning velocity in the case of spherically outward expanding flames. This new procedure makes it possible to determine the laminar burning velocity directly based on the flame displacement speed and the global fresh gas velocity near the preheat zone of the flame front. It therefore presents a very interesting alternative to the standard method (commonly used in the literature), which is based on the flame front displacement and the ratio of unburned and burned gas densities. The influence of external flame stretching on the burning velocity can be characterized and the Markstein length relative to the unburned gases (i.e., fresh gases) can be deduced by using this new tool. Contrary to the standard procedure, the unstretched laminar burning velocity is determined directly without using the fuel mixture properties. The temporal evolution of the flame front is visualized by high-speed laser tomography and the algorithm, based on a tomographic image correlation method, makes it possible to accurately measure the fresh gas velocity near the preheat zone of the flame front. The measurements of laminar flame speeds are carried out in a high-pressure and high-temperature constant-volume vessel over a wide range of equivalence ratios for methane, ethanol, and isooctane/air mixtures. To validate the experimental facility and the postprocessing of the flame images, fresh gas velocities and unstretched laminar burning velocities, as well as Markstein lengths relative to burned and unburned gases, are presented and compared with experimental and numerical results of the literature for methane/air flames. New results concerning ethanol/air and isooctane/air flames are presented for various experimental conditions (373 K, equivalence ratios range 0.7–1.5, pressure range 0.1–5 MPa).     

Turbulent burning velocity of hydrogen–air premixed propagating flames at elevated pressures

Lewis number represents the thermo-diffusive effects on laminar flames. That of hydrogen–air mixture varies extensively with the equivalence ratio due to the high molecular diffusivity of hydrogen. In this study, the influences of pressure and thermo-diffusive effects on spherically propagating premixed hydrogen–air turbulent flames were studied using a constant volume fan-stirred combustion vessel. It was noted that the ratio of the turbulent to unstretched laminar burning velocity increased with decreasing equivalence ratio and increasing mixture pressure. Turbulent burning velocity was dominated by three factors: (1) purely hydrodynamic factor, turbulence Reynolds number, (2) relative turbulence intensity to reaction speed, the ratio of turbulence intensity to unstretched laminar burning velocity, and (3) sensitivity of the flame to the stretch due to the thermo-diffusive effects, Lewis and Markstein numbers. A turbulent burning velocity correlation in terms of Reynolds and Lewis numbers is presented.     

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.     

Nonlinear effects of stretch on the flame front propagation

In all experimental configurations, the flames are affected by stretch (curvature and/or strain rate). To obtain the unstretched flame speed, independent of the experimental configuration, the measured flame speed needs to be corrected. Usually, a linear relationship linking the flame speed to stretch is used. However, this linear relation is the result of several assumptions, which may be incorrected. The present study aims at evaluating the error in the laminar burning speed evaluation induced by using the traditional linear methodology. Experiments were performed in a closed vessel at atmospheric pressure for two different mixtures: methane/air and iso-octane/air. The initial temperatures were respectively 300 K and 400 K for methane and iso-octane. Both methodologies (linear and nonlinear) are applied and results in terms of laminar speed and burned gas Markstein length are compared. Methane and iso-octane were chosen because they present opposite evolutions in their Markstein length when the equivalence ratio is increased.The error induced by the linear methodology is evaluated, taking the nonlinear methodology as the reference. It is observed that the use of the linear methodology starts to induce substantial errors after an equivalence ratio of 1.1 for methane/air mixtures and before an equivalence ratio of 1 for iso-octane/air mixtures. One solution to increase the accuracy of the linear methodology for these critical cases consists in reducing the number of points used in the linear methodology by increasing the initial flame radius used.     

Effects of hydrogen addition on the premixed laminar-flames of ethanol–air gaseous mixtures: An experimental study

The effects of hydrogen addition on the laminar premixed-flame characteristics of ethanol–air gaseous mixtures were investigated experimentally by using outwardly propagating spherical flames. The experiments were conducted in a constant-volume combustion vessel with a central ignition at an initial temperature of 383 K, a pressure of 0.1 MPa, a hydrogen fraction from 0% to 100%, and an equivalence ratio from 0.6 to 1.6, and the flame images were obtained by a high-speed schlieren camera system. The results show that the unstretched flame propagation speeds and burning velocities increase exponentially with the increase in hydrogen fraction for a constant equivalence ratio. When the hydrogen fraction is equal to or less than 60%, the burned gas Markstein length reduces with the increase of equivalence ratio, indicating a positive correlation between the flame instability and hydrogen fraction, while the opposite effect is observed when the hydrogen fraction is greater than 60%. At an equivalence ratio below 1.4, the Markstein length decreases with increased hydrogen fraction, indicating that the flame instability is exacerbated with hydrogen addition, while the reverse holds in the case of equivalence ratio above 1.4. Finally, an empirical formula is developed to estimate the laminar burning velocity of ethanol–hydrogen–air flames on the basis of present experimental data.     

Measurements of laminar burning velocities and flame stability analysis for dissociated methanol–air–diluent mixtures at elevated temperatures and pressures

The laminar burning velocities and Markstein lengths for the dissociated methanol–air–diluent mixtures were measured at different equivalence ratios, initial temperatures and pressures, diluents (N₂ and CO₂) and dilution ratios by using the spherically outward expanding flame. The influences of these parameters on the laminar burning velocity and Markstein length were analyzed. The results show that the laminar burning velocity of dissociated methanol–air mixture increases with an increase in initial temperature and decreases with an increase in initial pressure. The peak laminar burning velocity occurs at equivalence ratio of 1.8. The Markstein length decreases with an increase in initial temperature and initial pressure. Cellular flame structures are presented at early flame propagation stage with the decrease of equivalence ratio or dilution ratio. The transition positions can be observed in the curve of flame propagation speed to stretch rate, indicating the occurrence of cellular structure at flame fronts. Mixture diluents (N₂ and CO₂) will decrease the laminar burning velocities of mixtures and increase the sensitivity of flame front to flame stretch rate. Markstein length increases with an increase in dilution ratio except for very lean mixture (equivalence ratio less than 0.8). CO₂ dilution has a greater impact on laminar flame speed and flame front stability compared to N₂. It is also demonstrated that the normalized unstretched laminar burning velocity is only related to dilution ratio and is not influenced by equivalence ratio.     

Effects of hydrogen concentration on premixed laminar flames of hydrogen–methane–air

The unstretched laminar burning velocities and Markstein numbers of spherically propagating hydrogen–methane–air flames were studied at a mixture pressure of 0.10 MPa and a mixture temperature of 350 K. The fraction of hydrogen in the binary fuel was varied from 0 to 1.0 at equivalence ratios of 0.8, 1.0 and 1.2. The unstretched laminar burning velocity increased non-linearly with hydrogen fraction for all the equivalence ratios. The Markstein number varied non-monotonically at equivalence ratios of 0.8 and 1.0 and increased monotonically at equivalence ratio of 1.2 with increasing hydrogen fraction. Analytical evaluation of the Markstein number suggested that the trends could be due to the effective Lewis number, which varied non-monotonically with hydrogen fraction at equivalence ratios of 0.8 and 1.0 and increased monotonically at 1.2. The propensity of flame instability varied non-monotonically with hydrogen fraction at equivalence ratios of 0.8 and 1.0.     

Experimental study on cellular instabilities in hydrocarbon/hydrogen/carbon monoxide–air premixed flames

To investigate cell formation in methane (or propane)/hydrogen/carbon monoxide-air premixed flames, the outward propagation and development of surface cellular instabilities of centrally ignited spherical premixed flames were experimentally studied in a constant pressure combustion chamber at room temperature and elevated pressures. Additionally, unstretched laminar burning velocities and Markstein lengths of the mixtures were obtained by analyzing high-speed schlieren images. In this study, hydrodynamic and diffusional-thermal instabilities were evaluated to examine their effects on flame instabilities. The experimentally-measured unstretched laminar burning velocities were compared to numerical predictions using the PREMIX code with a H₂/CO/C₁-C₄ mechanism, USC Mech II, from Wang et al. [22]. The results indicate a significant increase in the unstretched laminar burning velocities with hydrogen enrichment and a decrease with the addition of hydrocarbons, whereas the opposite effects for Markstein lengths were observed. Furthermore, effective Lewis numbers of premixed flames with methane addition decreased for all of the cases; meanwhile, effective Lewis numbers with propane addition increase for lean and stoichiometric conditions and increase for rich and stoichiometric cases for hydrogen-enriched flames. With the addition of propane, the propensity for cell formation significantly diminishes, whereas cellular instabilities for hydrogen-enriched flames are promoted. However, similar behavior of cellularity was obtained with the addition of methane, which indicates that methane is not a candidate for suppressing cell formation in methane/hydrogen/carbon monoxide-air premixed flames.     

Experimental and numerical study of the accuracy of flame-speed measurements for methane/air combustion in a slot burner

Measuring the velocities of premixed laminar flames with precision remains a controversial issue in the combustion community. This paper studies the accuracy of such measurements in two-dimensional slot burners and shows that while methane/air flame speeds can be measured with reasonable accuracy, the method may lack precision for other mixtures such as hydrogen/air. Curvature at the flame tip, strain on the flame sides and local quenching at the flame base can modify local flame speeds and require corrections which are studied using two-dimensional DNS. Numerical simulations also provide stretch, displacement and consumption flame speeds along the flame front. For methane/air flames, DNS show that the local stretch remains small so that the local consumption speed is very close to the unstretched premixed flame speed. The only correction needed to correctly predict flame speeds in this case is due to the finite aspect ratio of the slot used to inject the premixed gases which induces a flow acceleration in the measurement region (this correction can be evaluated from velocity measurement in the slot section or from an analytical solution). The method is applied to methane/air flames with and without water addition and results are compared to experimental data found in the literature. The paper then discusses the limitations of the slot-burner method to measure flame speeds for other mixtures and shows that it is not well adapted to mixtures with a Lewis number far from unity, such as hydrogen/air flames.     

Experimental and numerical study on laminar burning characteristics of premixed methane–hydrogen–air flames

An experimental and numerical study on laminar burning characteristics of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity and the Markstein length were obtained over a wide range of equivalence ratios and hydrogen fractions. Moreover, for further understanding of the effect of hydrogen addition on the laminar burning velocity, the sensitivity analysis and flame structure were performed. The results show that the unstretched laminar burning velocity is increased, and the peak value of the unstretched laminar burning velocity shifts to the richer mixture side with the increase of hydrogen fraction. Three regimes are identified depending on the hydrogen fraction in the fuel blend. They are: the methane-dominated combustion regime where hydrogen fraction is less than 60%; the transition regime where hydrogen fraction is between 60% and 80%; and the methane-inhibited hydrogen combustion regime where hydrogen fraction is larger than 80%. In both the methane-dominated combustion regime and the methane-inhibited hydrogen combustion regime, the laminar burning velocity increases linearly with the increase of hydrogen fraction. However, in the transition regime, the laminar burning velocity increases exponentially with the increase of hydrogen fraction in the fuel blends. The Markstein length is increased with the increase of equivalence ratio and is decreased with the increase of hydrogen fraction. Enhancement of chemical reaction with hydrogen addition is regarded as the increase of H, O and OH radical mole fractions in the flame. Strong correlation is found between the burning velocity and the maximum radical concentrations of H and OH in the reaction zone of the premixed flames.     

异丁醇/辛酸甲酯混合燃料预混层流火焰速度的研究

针对生物柴油与醇类混合燃料燃烧机理研究的需求,采用高速纹影光学诊断方法和定容燃烧弹系统试验研究了异丁醇/辛酸甲酯混合燃料的预混层流燃烧特性。测量了不同当量比和初始压力条件下的不同配比混合燃料—空气预混合气的层流燃烧火焰速度,火焰拉伸率以及马克斯坦长度。分析了燃烧初始条件及异丁醇掺混比例对混合燃料的无拉伸层流燃烧速度及火焰不稳定性的影响规律。结果表明:异丁醇/辛酸甲酯混合燃料的拉伸层流火焰传播速度和层流火焰燃烧速度随着当量比的增加先增加后减少,随着初始压力的增加而减小;马克斯坦长度随着当量比和初始压力的增加而减小;异丁醇掺混比例的增加加快了层流火焰燃烧速度,但使得火焰的不稳定性倾向增加。     

Asymptotic analysis of transport properties and burning velocities for premixed hydrocarbon flames

IntroductionThe fundamental meChedsm Of a p~xed flamewith the flow near the front stagnation point of a platewall has receiVed considerable attention in the field ofcombushon, which helps us to realize the behavior offlame Propagation. The CO~thew teChnique,inboduced by Law and coworkers["n, has produced theIndnar flame speed data that are ~ntiy usedextensively fof validation Of chemical ldnetics and themodeling of turbulent combustion. The laminar flamespeed is an important Property of a …     

Studies on properties of laminar premixed hydrogen-added ammonia/air flames for hydrogen production

In order to evaluate the potential of burning and reforming ammonia as a carbon-free fuel in production of hydrogen, fundamental unstretched laminar burning velocities, and flame response to stretch (represented by the Markstein number) for laminar premixed hydrogen-added ammonia/air flames were studied both experimentally and computationally. Freely (outwardly)-propagating spherical laminar premixed flames at normal temperature and pressure were considered for a wide range of global fuel-equivalence ratios, flame stretch rates (represented by the Karlovitz number) and the extent of hydrogen substitution. Results show the substantial increase of laminar burning velocities with hydrogen substitution, particularly under fuel-rich conditions. Also, predicted flame structures show that the hydrogen substitution enhances nitrogen oxide (NO_x) and nitrous oxide (N₂O) formation. At fuel-rich conditions, however, the amount of NO_x and N₂O emissions and the extent of the increase with the hydrogen substitution are much lower than those under fuel-lean conditions. These observations support the potential of hydrogen as an additive for improving the burning performance with low NO_x and N₂O emissions in fuel-rich ammonia/air flames and hence the potential of using ammonia as a clean fuel. Increasing the amount of added hydrogen tends to enhance flame sensitivity to stretch.     

Laminar burning velocities and flame instabilities of 2,5-dimethylfuran–air mixtures at elevated pressures

An experimental investigation on laminar burning velocities and onset of flame instabilities on spherically expanding flames in 2,5-dimethylfuran–air mixtures at elevated pressures was conducted over a wide range of equivalence ratios. The laminar burning velocities, laminar burning fluxes and Markstein lengths at different equivalence ratios and initial pressures were obtained. Furthermore, the diffusional–thermal and hydrodynamic effects on flame front instabilities were specified, and the onset of cellularity was reported. Results show that laminar burning velocities are decreased with increasing initial pressure due to the increase of the free-stream density and the progressively more important three-body termination reactions. With increasing initial pressure, Markstein length decrease, while the laminar burning flux increases. Onsets of flame instabilities, expressed in terms of critical radius or Peclet number, were found to be promoted with increasing equivalence ratio and initial pressures, due to the combined effects of diffusional–thermal and hydrodynamic instabilities.     

Effects of diluents on cellular instabilities in outwardly propagating spherical syngas–air premixed flames

Experiments were conducted in a constant pressure combustion chamber using schlieren system to investigate the effects of carbon dioxide–nitrogen–helium diluents on cellular instabilities of syngas–air premixed flames at room temperature and elevated pressures. The cellular instabilities for the diluted syngas–air flames were interpreted and evaluated in the viewpoint of the hydrodynamic and diffusional-thermal instabilities. Laminar burning velocities and Markstein lengths were calculated by analyzing high-speed schlieren images at various diluent concentrations and equivalence ratios. The measured unstretched laminar burning velocities were compared with the predicted results computed using the PREMIX code with the kinetic mechanism developed by Sun et al. Also, experimentally measured Peclet numbers were compared with the predicted results for fuel–lean flames. Experimental results showed substantial reduction of the laminar burning velocities and of the Markstein lengths with the diluent additions in the fuel blends. Effective Lewis numbers of helium-diluted syngas–air flames increased but those of carbon dioxide- and nitrogen-diluted syngas–air flames decreased in increase of diluents in the reactant mixtures. With helium diluent, the propensity for cells formation was significantly diminished, whereas the cellular instabilities for carbon dioxide- and nitrogen-diluted syngas–air flames were not suppressed.     

Numerical study on laminar burning velocity and NO formation of premixed methane–hydrogen–air flames

Numerical study on laminar burning velocity and NO formation of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity, adiabatic flame temperature, and radical mole fractions of H, OH and NO are obtained at various equivalence ratios and hydrogen fractions. The results show that the unstretched laminar burning velocity is increased with the increase of hydrogen fraction. Methane-dominated combustion is presented when hydrogen fraction is less than 40%, where laminar burning velocity is slightly increased with the increase of hydrogen addition. When hydrogen fraction is larger than 40%, laminar burning velocity is exponentially increased with the increase of hydrogen fraction. A strong correlation exists between burning velocity and maximum radical concentration of H + OH radicals in the reaction zone of premixed flames. High burning velocity corresponds to high radical concentration in the reaction zone. With the increase of hydrogen fraction, the overall activation energy of methane–hydrogen mixture is decreased, and the inner layer temperature and Zeldovich number are also decreased. All these factors contribute to the enhancement of combustion as hydrogen is added. The curve of NO versus equivalence ratio shows two peaks, where they occur at the stoichiometric mixture due to Zeldovich thermal-NO mechanism and at the rich mixture with equivalence ratio of 1.3 due to the Fenimore prompt-NO mechanism. In the stoichiometric flames, hydrogen addition has little influence on NO formation, while in rich flames, NO concentration is significantly decreased. Different NO formation responses to stretched and unstretched flames by hydrogen addition are discussed.     

Laminar burning velocities and transition to unstable flames in H2/O2/N2 and C3H8/O2/N2 mixtures

Effects of positive flame stretch on laminar burning velocities, and conditions for transition to unstable flames, were studied experimentally for freely propagating spherical flames at both stable and unstable preferential-diffusion conditions. The data base involved new measurements for H₂/O₂/N₂ mixtures at values of flame stretch up to 7600 s⁻¹, and existing measurements for C₃H₈/O₂/N₂ mixtures at values of flame stretch up to 900 s⁻¹. Laminar burning velocities varied linearly with increasing Karlovitz numbers—either decreasing or increasing at stable or unstable preferential-diffusion conditions—yielding Markstein numbers that primarily varied with the fuel-equivalence ratio. Neutral preferential-diffusion conditions, however, were shifted toward the unstable side of the maximum laminar burning velocity condition that the simplest preferential-diffusion theories associate with neutral stability. All flames exhibited transition to unstable flames: unstable preferential-diffusion coditions yielded early transition to irregular flame surfaces, and stable preferential-diffusion conditions yielded delayed transition to cellular flames by hydrodynamic instability. Conditions for hydrodynamic instability transitions for H₂/O₂/N₂ mixtures were consistent with an earlier correlation due to Groff for propane/air flames, based on the predictions of Istratov and Librovich.     

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.