Unstretched Laminar Burning Velocity Research Articles

Laminar burning velocities and Markstein lengths of aromatics at elevated temperature and pressure

Laminar burning velocities are presented for benzene, toluene, ethylbenzene, m-xylene, and n-propylbenzene. The data have been acquired at elevated temperature (450 K) and pressure (304 kPa) over the equivalence ratio range 0.80 ⩽ ϕ ⩽ 1.4. High speed schlieren visualization, used to monitor flame growth following ignition, provides a direct determination of the laminar flame speed. The data are corrected for flame stretch to yield the unstretched laminar burning velocities and burned gas Markstein lengths. The data show benzene to be the fastest, followed by ethylbenzene > n-propylbenzene > toluene > m-xylene. Iso-octane results are also presented for comparison to the literature. Flame front perturbations are observed, particularly under fuel-rich conditions, due to hydrodynamic and preferential mass diffusion instabilities. The kinetic factors responsible for the changes in the laminar burning velocities upon alkyl substitution of the aromatic ring are discussed.

04/02491 Extension of no. 3 coke oven service life at JFE-Fukuyama works: Maruoka, M. et al. ISSTech Conference Proceedings, 1st, Indianapolis, IN, United States, 27–30 April, 2003, 1197–1206

Using a high pressure constant volume combustion vessel, the propagation and morphology of spark-ignited outwardly expanding nitrogen diluted propane–air flames were imaged and recorded by schlieren photography and high-speed digital camera. The unstretched laminar burning velocities and Markstein lengths were subsequently determined over wide range of initial temperatures, initial pressures and nitrogen dilution ratios. Two recently developed mechanisms were used to predict the reference laminar burning velocity. The results show that the measured unstretched laminar burning velocities agree well with those in the literature and the computationally predicted results. The flame images show that the diffusional–thermal instability is promoted as the mixture becomes richer, and the hydrodynamic instability is increased with the increase of the initial pressure and it is decreased with the increase of dilution ratio. The normalized laminar burning velocities show a linear correlation with respect to the dilution ratio, indicating that the effect of nitrogen dilution is more obvious at higher pressures.

Dynamic properties of outwardly propagating spherical hydrogen-air flames at high temperatures and pressures

Computational experiments on fundamental unstretched laminar burning velocities and flame response to stretch (represented by the Markstein number) of hydrogen-air flames at high temperatures and pressures were conducted in order to understand the dynamics of the flames including hydrogen as an attractive energy carrier in conditions encountered in practical applications such as internal combustion engines. Outwardly propagating spherical premixed flames were considered for a fuel-equivalence ratio of 0.6, pressures of 5 to 50 atm, and temperatures of 298 to 1000 K. For these conditions, ratios of unstretched-to-stretched laminar burning velocities varied linearly with flame stretch (represented by the Karlovitz number), similar to the flames at normal temperature and normal to moderately elevated pressures, implying that the “local conditions” hypothesis can be extended to the practical conditions. Increasing temperatures tended to reduce tendencies toward preferential-diffusion instability behavior (increasing the Markstein number) whereas increasing pressures tended to increase tendencies toward preferential-diffusion instability behavior (decreasing the Markstein number).

Measurements of Markstein numbers and laminar burning velocities for liquefied petroleum gas–air mixtures

Experimental test for premixed laminar combustion of liquefied petroleum gas–air mixtures is conducted in a constant volume combustion bomb. Spherically expanding flames have been employed to measure laminar flame speeds over wide equivalence ratios, at the initial pressures of 0.05, 0.1 and 0.15 MPa, and preheat temperatures from 300 to 400 K. To study the effects of stretch on burning velocity, various Markstein numbers for both strain and curvature have been measured and the effects of initial temperature and pressure on these parameters have been discussed. Following the linear relation between flame speeds and flame stretches, one has then obtained the corresponding unstretched laminar burning velocity after omitting the effect of stretches imposed on these flames. Over the ranges studied, laminar burning velocities are fit by a functional form u l= u l0( T u/ T u0) α T ( P u/ P u0) β P , and the dependencies of α T and β P upon the equivalence ratio of mixture are also discussed.

Determination of laminar burning velocities for natural gas

Spherically expanding flames of natural gas–air mixtures have been employed to measure the laminar flame speeds, at the equivalence ratios from 0.6 to 1.4, initial pressures of 0.05, 0.1 and 0.15 MPa, and preheat temperatures from 300 to 400 K. Following Markstein theory, one then obtains the corresponding unstretched laminar burning velocity after omitting the effect of stretch imposed at the flame front. Over the ranges studied, the burning velocities are fit by a functional form u l= u l0( T u/ T u0) α T ( P u/ P u0) β P , and the dependencies of α T and β P upon the equivalence ratio of mixture are also given. The effects of dilute gas on burning velocities have been studied at the equivalence ratios from 0.7 to 1.2, and the explicit formula of laminar burning velocities for dilute mixtures is achieved.

Effects of Pressure on Unstretched Laminar Burning Velocity, Markstein Length and Cellularity of Propagating Spherical Laminar Flames

Outwardly propagating spherical laminar flames in the constant volume bomb were studied. Markstein length was employed to quantify the effects of the flame stretch on the burning velocity. The effects of the pressure on the unstretched laminar burning velocity and the Markstein length were investigated using methane and propane-air mixtures at the equivalence ratios from 0.8 to 1.4 varying the initial pressure from 0.10 to 0.50 MPa. The Markstein length increased for the methane mixture and decreased for the propane mixture with increasing the equivalence ratio. The Markstein length decreased with increasing the initial pressure at all the equivalence ratios irrespective of fuel. It was negative for the lean methane and the rich propane mixtures at high pressures. Flame was unstable and covered with cells in such cases.

The Effects of Pressure on Unstretched Laminar Burning Velocity, Markstein Length and Cellularity of Spherically Propagating Laminar Flames(S.I. Engines, Flame Propagation)

The Effects of Pressure on Unstretched Laminar Burning Velocity, Markstein Length and Cellularity of Spherically Propagating Laminar Flames(S.I. Engines, Flame Propagation)

Study of Burning Velocities of Dimethyl Ether-Air Flames(S.I. Engines, Flame Propagation)

Combustion properties of dimethyl ether (DME) and air mixtures have been elucidated in a spherical bomb by using schlieren images with high-speed camera, for equivalence ratios ranging from 0.8 to 2.0 at an initial pressure of 1 atm and a temperature of 298 K. The unstretched laminar burning velocities were deduced and compared with other study and calculation. Good and partial agreements were obtained with the other study and calculation, respectively. Flame response to stretch (represented by the Markstein number) were also considered both experimentally and computationally and were compared with methane, ethane, propane-air mixture. For our experimental conditions, stretched laminar burning velocities were found to vary linearly with flame stretch, yielding Markstein numbers for particular reactant conditions. In addition, turbulent burning velocities were measured for the above mixtures. In a given explosion, the burning velocity increased with time and radius, as a consequence of the continual broadening of the effective spectrum of turbulence and decreasing of the flame stretch to which the flame was subjected. A decrease in the Markstein number of the mixture increased the turbulent burning velocity.

Measurements of Markstein Numbers and Laminar Burning Velocities for Natural Gas−Air Mixtures

Spherically expanding flames of Chinese natural gas−air mixtures have been used to measure the laminar flame speeds, at equivalence ratios of φ = 0.6−1.4, initial pressures of p = 0.05, 0.1, and 0.15 MPa, and preheat temperatures of T = 300−400 K. Following Markstein theory, one then obtains the corresponding unstretched laminar burning velocity after omitting the effect of stretch imposed on these flames. To study the effects of stretch on burning velocity, various Markstein numbers for both strain and curvature have been derived and the effects of initial temperature and pressure on these parameters have been discussed. Over the ranges studied, the laminar burning velocities are comparable with those previously reported for pure methane−air mixtures and fit by a functional form ul = ul0(Tu/Tu0)αT(pu/pu0)βp, where the dependencies of αT and βp on the φ value of the mixture are also deduced. Furthermore, it is presented that the extrapolation results are still in good agreement with the previous work at relatively high pressure. The effects of the dilution gases on the burning velocity have been studied at φ = 0.7−1.2, and the variations in burning velocities are plotted as functions of the dilution ratio and the equivalence ratio of the mixture.

Effects of Halons and Halon Replacements on Hydrogen-Fueled Laminar Premixed Flames

Fundamental unstretched laminar burning velocities and flame response to stretch, as characterized by Markstein numbers, were considered experimentally and computationally. The investigation was limited to laminar premixed flames involving hydrogen burning in air with Halon 1301 (CF 3 Br), nitrogen, and carbon dioxide considered as flame suppressing diluents. Outwardly propagating spherical laminar premixed flames were observed for fuel equivalence ratios of 0.6-1.8, concentrations of oxygen in oxygen/nitrogen mixtures of 21% by volume before dilution with a suppressant, and normal temperature and pressure. Suppressants involved concentrations of the chemically active suppressant, Halon 1301, of 0-2% by volume and 'concentrations of the chemically passive (chemically inert) suppressants, nitrogen and carbon dioxide, of 0-50% by volume. The present flames were sensitive to flame stretch, yielding values of unstretched-to-stretched laminar burning velocities in the range of 0.6-1.2 for levels of flame stretch well below quenching conditions, for example, for Karlovitz numbers less than 0.15. The agreement between measured and predicted unstretched laminar burning velocities was good based on the mechanisms of Mueller et al. for H 2 /O 2 reactions and Babushok et al. for halogen reactions. The resulting flame structure predictions suggest that H and OH radical production and transport are important for flame suppression and preferential-diffusion/stretch interactions; this reflects the strong correlation between laminar burning velocities and the maximum concentrations of H and OH in the reaction zone of the present flames. It also was found that effects of flame suppression that reduced concentrations of H and OH in the reaction zone made the present flames more unstable to preferential-diffusion/stretch interactions, an effect that tends to counteract the beneficial effects of flame suppressants to some extent.

Determination of the laminar burning velocity and the Markstein length of powder–air flames

This work deals with the determination of the laminar burning velocity and introduces the Markstein length of powder–air mixtures. A powder burner was used to stabilize laminar cornstarch–air dust flames and the laminar burning velocity was determined by means of laser Doppler anemometry. The dust concentration was varied from 0.26 to 0.38 kg m −3. The measured laminar burning velocities were found to be sensitive to the shape of the flame. With the same dust concentration, parabolic flames were found to have a laminar burning velocity, which was almost twice that of a planar flame (ca. 30 cm s −1 for the latter as compared with ca. 54 cm s −1 for the former). From this discrepancy and the flame curvature, the Markstein length could be determined. It was found to have a value of 11.0 mm. This Markstein length was subsequently used to correct the measured laminar burning velocities at various dust concentrations in order to obtain the unstretched laminar burning velocity. The unstretched laminar burning velocity lies between 15 and 30 cm s −1 and is thought to be a property of the dust and of the concentration.

Flame/stretch interactions of premixed hydrogen-fueled flames: measurements and predictions

Fundamental unstretched laminar burning velocities, and flame response to stretch (represented by the Markstein number) were considered both experimentally and computationally for laminar premixed flames. Mixtures of hydrogen and oxygen with nitrogen, argon and helium as diluents were considered to modify flame transport properties for computationally tractable reactant mixtures. Freely (outwardly)-propagating spherical laminar premixed flames were considered for fuel-equivalence ratios of 0.6 to 4.5, pressures of 0.3 to 3.0 atm, volumetric oxygen concentrations in the nonfuel gases of 0.21 to 0.36, and Karlovitz numbers of 0 to 0.5, at normal temperatures. For these conditions, both measured and predicted ratios of unstretched-to-stretched laminar burning velocities varied linearly with flame stretch (represented by the Karlovitz number), yielding constant Markstein numbers for particular reactant conditions. The present flames were very sensitive to flame stretch, exhibiting ratios of unstretched-to-stretched laminar burning velocities in the range 0.6 to 3.0 for levels of flame stretch well below quenching conditions. At fuel-lean conditions, increasing flame temperatures (by dilution with argon rather than nitrogen) tended to reduce flame sensitivity to stretch whereas increasing pressures tended to increase tendencies toward preferential-diffusion instability behavior. At low pressures, helium-diluted flames had reduced tendencies toward preferential-diffusion instability behavior compared to nitrogen- and argon-diluted flames due to stabilization of flame properties by strong effects of preferential diffusion of heat. Predicted and measured flame properties exhibited encouraging agreement using contemporary reaction mechanisms. Finally, flame structure predictions suggest that H and OH radical production and transport are important aspects of preferential-diffusion/stretch interactions, reflecting the strong correlation between laminar burning velocities and H+OH radical concentrations for present test conditions.

(2-12) A Study on Turbulent Combustion Properties of Spherically Propagating Methane Flames((SI-4)S. I. Engine Combustion 4-Flame Propagation)

Natural gas is now considered to be one of the most promising alternative fuels for automotive engines because of the low exhaust emission characteristics and very large deposits spread around the world, much of which is not exploited. So it is important to clarify the nature of this energy source. The purpose of this paper is to examine the turbulent combustion properties of methane, which is the main component of natural gas, considering the application to spark-ignited engines. The below left figure shows the experimental result on turbulent burning velocities, S_T, as a function of turbulence intensity, u', where both values are non-dimensionalized by the corresponding laminar burning velocities, S^o_u, using a constant-volume combustion bomb. From the figure, it is found that for methane mixtures the leaner mixtures become, the larger the turbulent burning velocities even at the same turbulent intensity. We have inferred that this effect is caused by the change in local burning velocity due to the flame stretch to which each local flamelet in a turbulent flame is subjected. In this case, the local burning velocity of each flamelet changes from the unstretched laminar burning velocity with increasing stretch near-linearly. According to the asymptotic analysis, general relation can be expressed as the below equation. [numerical formula] Where S_u is stretched laminar burning velocity, Ka is non-dimensional flame stretch, characterized by Karlovitz number. So it is found that Ma, which is referred to as Markstein number, represents the sensitivity of local burning velocity to flame stretch. At the same value of Ka, mixture with the smaller value of Ma has the larger value of S_u/S^o_u. So in turn we have investigated the correlation between Markstein numbers and turbulent burning velocities. The below right figure shows the variations of Markstein numbers with equivalence ratio, which were obtained in this study and cited from other studies. From the figure, for methane mixtures Ma is found to decrease with decreasing equivalence ratio, indicating that the stretched burning velocity is estimated to relatively increase on fuel leaner region. These trends quantitatively agree with those of turbulent burning velocities, as seen in left figure. From the above discussion, it can be concluded that methane intrinsically favors lean burn combustion. And this can be explained by the difference in sensitivity to flame stretch.

Local scalar flame properties of freely propagating premixed turbulent flames at various Lewis numbers

Local scalar flame properties of freely propagating turbulent premixed flames including the flame curvature h, and local displacement speed relative to the fresh gases S d u , have been measured simultaneously for methane, propane, and hydrogen/air flames at Lewis numbers varying from 0.33 to 1.4 and ratios of rms turbulent velocity to unstretched laminar burning velocity u′/ S L,0 u varying from 0 to 3.1. Three different mixtures were separately spark-ignited in a vertical wind tunnel. The expanding flame freely propagated in a grid-generated decaying turbulent flow. An advanced field-imaging technique based on high-speed laser tomography measured the temporal evolution of local flame properties. Local flame curvature and local displacement speed were calculated from flame-front contours. Curvature probability density functions (pdfs) were negatively skewed, especially for nonunity Lewis numbers, and displacement speed distributions underlined the influence of local stretch and thermodiffusive effects on flame speed variations. The temporal evolution of the mean flamelet radius of curvature converges towards half of the integral length scale measured in the cold flow. Flame response in terms of displacement speed to curvature is found to be statistically dependent, and a linear relationship is observed. For propane/air flames, the displacement speed can be assumed to be independent of local flame curvature at each stage of flame propagation, whereas a very strong increase of displacement speed with positive curvatures can be observed along the wrinkled flame contour for hydrogen/air flames.

Flame/Stretch Interactions of Premixed Fuel-Vapor/O/N Flames

Unstretched laminar burning velocities and e ame response to stretch (Markstein numbers ) were measured for outwardly propagating spherical laminar premixed e ames involving mixtures of hydrocarbon vapors, oxygen, and nitrogen. Experimental conditions consisted of vapors of several liquid fuels (n-hexane, n-heptane, iso-octane, methyl-alcohol, and ethyl-alcohol ), concentrations of oxygen in the nonfuel gases of 19 ‐33% by volume, pressures of 0.5‐2.0 atm, fuel-equivalence ratios of 0.80 ‐1.60, and reactant mixture temperatures of 298 § 5 K. The present e ames were very sensitive to e ame stretch, yielding ratios of unstretched to stretched laminar burning velocities in the range 0.4 ‐4.0 for levels of e ame stretch well below quenching conditions (Karlovitz numbers less than 0.2 ). At low pressures, the present hydrocarbon vapor e ames had positive Markstein numbers at fuel-lean conditions, which is consistent with classical preferential-diffusion ideas. Increasing pressures, however, reduced Markstein numbers and progressively decreased the fuel-equivalence ratio range where Markstein numbers were positive. Negative Markstein numbers were associated with the presence of preferential-diffusion instability as evidenced by the appearance of chaotically distorted (wrinkled) e ame surfaces early during the e ame propagation process.

Laminar burning velocity and Markstein lengths of methane–air mixtures

Spherically expanding flames propagating at constant pressure are employed to determine the unstretched laminar burning velocity and the effect of flame stretch as quantified by the associated Markstein lengths. Methane–air mixtures at initial temperatures between 300 and 400 K, and pressures between 0.1 and 1.0 MPa are studied at equivalence ratios of 0.8, 1.0, and 1.2. This is accomplished by photographic observation of flames in a spherical vessel. Power law correlations are suggested for the unstretched laminar burning velocity as a function of pressure, temperature, and equivalence ratio. Zeldovich numbers are derived to express the effect of temperature on the mass burning rate and from this, a more general correlation of burning velocity, based on theoretical arguments, is presented for methane–air mixtures. Flame instability is observed for mixtures at high pressure, and the critical radius for the onset of cellularity is correlated with Markstein number. Experimental results are compared with two sets of modeled predictions; one model considers the propagation of a spherically expanding flame using a reduced mechanism, and the second considers a one-dimensional flame using a full kinetic scheme. The results are compared with those of other researchers. Comparison also is made with iso-octane–air mixtures, reported elsewhere, to emphasize the contrast in the burning of lighter and heavier hydrocarbon fuels.

Measured and predicted properties of laminar premixed methane/air flames at various pressures

Effects of positive flame stretch on the laminar burning velocities of methane/air flames were studied both experimentally and computationally, considering freely (outwardly) propagating spherical laminar premixed flames. Measurements based on motion picture shadowgraphs, and numerical simulations based on typical contemporary chemical reaction mechanisms, were used to find the sensitivities of the laminar burning velocities to flame stretch, characterized as Markstein numbers, and the fundamental laminar burning velocities of unstretched flames. Reactant conditions included methane/air mixtures having fuel-equivalence ratios of 0.60–1.35 and pressures of 0.5–4.0 atm, at normal temperatures. Both measured and predicted ratios of unstretched-to-stretched laminar burning velocities varied significantly from unity (in the range 0.6–2.3) even though present stretch levels did not approach quenching conditions. Absolute values of Markstein numbers increased with increasing pressure, while the transition from unstable to stable preferential-diffusion conditions with increasing fuel-equivalence ratio shifted from an equivalence ratio of 0.6 at 0.5 atm to 1.2 at 4.0 atm, suggesting increased unstable flame behavior due to preferential-diffusion effects at the elevated pressures of interest for many practical applications. Finally, predictions using two contemporary chemical reaction mechanisms were in reasonably good agreement with present measurements of both Markstein numbers and unstretched laminar burning velocities.

Flame stretch interactions of laminar premixed hydrogen/air flames at normal temperature and pressure

Effects of positive flame stretch on the laminar burning velocities of hydrogen/air flames were studied both experimentally and computationally, considering freely (outwardly) propagating spherical laminar premixed flames. Measurements were based on motion picture shadowgraphy, while numerical simulations were based on typical contemporary chemical reaction mechanisms. Flame conditions studied included hydrogen/air flames having fuel-equivalence ratios in the range 0.3–5.0 at normal temperature and pressure. Both measured and predicted ratios of unstretched (plane flames) to stretched laminar burning velocities varied linearly with Karlovitz numbers over the test range (Karlovitz numbers up to 0.4), yielding Markstein numbers that were independent of Karlovitz numbers for a particular reactant mixture. Markstein numbers were in the range −1 to 6, with unstable (stable) preferential-diffusion conditions observed at fuel-equivalence ratios below (above) roughly 0.7. Present stretch-corrected laminar burning velocities were in reasonably good agreement with other determinations of laminar burning velocities at fuel-lean conditions where Markstein numbers, and thus effects of stretch, are small. In contrast, the stretch-corrected laminar burning velocities generally were smaller than other measurements in the literature at fuel-rich conditions, where Markstein numbers, and thus effects of stretch, are large. Finally, predicted unstretched laminar burning velocities and Markstein numbers were in reasonably good agreement with measurements, although additional study to improve the comparison between predictions and measurements at fuel-rich conditions should be considered.

Experimental study of premixed turbulent combustion in opposed streams. Part II—Reacting flow field and extinction

Turbulent combustion in opposed streams of premixed reactants was studied experimentally. In this geometry two flames are stabilized in the vicinity of the free stagnation point and separated by a region of products. In the mean the flames appear flat and perpendicular to the axis of the jets. Both the mean flow field and the distribution of the mean progress variable is found to be symmetric about the stagnation point. Buoyancy forces do not appear to affect the symmetry of the flow. The flows upstream of the flame zones has the same form of velocity field as nonreacting opposed flow, but displaced away from the stagnation point by the volume source due to combustion. Under certain conditions these flames can be forced to extinction, and this limit is studied for lean propane-air mixtures. This extinction depends only on the aerothermochemistry of the flow because there are no surfaces near the flames. For near extinct flames the flow field approaches that of nonreacting opposed streams. A physical mechanism is proposed for the extinction and results are compared with an extinction model. There is good qualitative agreement with the extinction model, and good quantitative agreement for the dependency of unstretched laminar burning velocity and the heat release parameter on extinction.