Laminar burning velocities,Markstein lengths,and flame thickness of liquefied petroleum gas with hydrogen enrichment

In this paper, experimental data of laminar burning velocity, Markstein length, and flame thickness of LPG flames with various percentages of hydrogen (H₂) enrichments have been presented. The experiments were conducted under the conditions of 0.1 MPa, 300 K in a constant volume chamber. The tested equivalence ratios of air/fuel mixture range from 0.6 to 1.5, and the examined LPG contains 10%–90% of hydrogen in volume. Experimental results show that hydrogen addition significantly increase the laminar burning velocity of LPG, and the accelerating effectiveness is substantial when the percentage of hydrogen is larger than 60%. Effect of hydrogen addition on diffusion thermal instability, as indicated by Markstein length, was analyzed at various equivalence ratios. Hydrogen addition decreases the flame thickness. Equivalence ratio has more dominating effect on flame thickness than hydrogen does. For the fuel with 10% LPG and 90% hydrogen, the flame thickness values are close for all equivalence ratios.     

Laminar burning velocity and Markstein length of nitrogen diluted natural gas/hydrogen/air mixtures at normal,reduced and elevated pressures

Flame propagation of premixed nitrogen diluted natural gas/hydrogen/air mixtures was studied in a constant volume combustion bomb under various initial pressures. Laminar burning velocities and Markstein lengths were obtained for the diluted stoichiometric fuel/air mixtures with different hydrogen fractions and diluent ratios under various initial pressures. The results showed that both unstretched flame speed and unstretched burning velocity are reduced with the increase in initial pressure (except when the hydrogen fraction is 80%) as well as diluent ratio. The velocity reduction rate due to diluent addition is determined mainly by hydrogen fraction and diluent ratio, and the effect of initial pressure is negligible. Flame stability was studied by analyzing Markstein length. It was found that the increase of initial pressure and hydrogen fraction decreases flame stability and the flame tends to be more stable with the addition of diluent gas. Generally speaking, Markstein length of a fuel with low hydrogen fraction is more sensitive to the change of initial pressure than that of a one with high hydrogen fraction.     

Measurements of laminar burning velocities and Markstein lengths for methanol-air-nitrogen mixtures at elevated pressures and temperatures

The laminar burning velocities and Markstein lengths for the methanol-air mixtures were measured at different equivalence ratios, elevated initial pressures and temperatures, and dilution ratios by using a constant volume combustion chamber and high-speed schlieren photography system. The influences of these parameters on the laminar burning velocity and Markstein length were analyzed. The results show that the laminar burning velocity of the methanol-air mixture decreases with an increase in initial pressure and increases with an increase in initial temperature. The Markstein length decreases with an increase in initial pressure and initial temperature, and increases with an increase in the dilution ratio. A cellular flame structure is observed at an early stage of flame propagation. The transition point is identified on the curve of flame propagation speed against stretch rate. The reasons for the cellular structure development are also analyzed.     

Effect of ignition position and inert gas on hydrogen/air explosions

The effects of inert gas (i.e., He, Ar, and N₂) and ignition position on flame dynamics in a half-open duct with an aspect-ratio of 10 are analyzed for hydrogen/air mixtures with constant laminar burning velocity S_L. The results indicate that hydrodynamic and thermo-diffusive instabilities dominate flame propagations with ignition at the right-half part of the duct, while Rayleigh–Taylor instability dominates with ignition at the left-half part of the duct. The flame-sound interaction results in the periodic pressure oscillations. Due to decreased instability, the He-diluted flame exhibits a weaker sensitivity of explosion parameters to the ignition position. The maximum pressure P_max is dominated by different mechanisms depending on the ignition position. Although constant S_L is used, P_max for the worst case with N₂ dilution is two times that with He dilution, demonstrating the considerable effect of flame instabilities. Finally, a chemical kinetic calculation is performed to clarify the flame stabilities.     

Laminar burning velocities and Markstein lengths of premixed methane/air flames near the lean flammability limit in microgravity

Effects of flame stretch on the laminar burning velocities of near-limit fuel-lean methane/air flames have been studied experimentally using a microgravity environment to minimize the complications of buoyancy. Outwardly propagating spherical flames were employed to assess the sensitivities of the laminar burning velocity to flame stretch, represented by Markstein lengths, and the fundamental laminar burning velocities of unstretched flames. Resulting data were reported for methane/air mixtures at ambient temperature and pressure, over the specific range of equivalence ratio that extended from 0.512 (the microgravity flammability limit found in the combustion chamber) to 0.601. Present measurements of unstretched laminar burning velocities were in good agreement with the unique existing microgravity data set at all measured equivalence ratios. Most of previous 1-g experiments using a variety of experimental techniques, however, appeared to give significantly higher burning velocities than the microgravity results. Furthermore, the burning velocities predicted by three chemical reaction mechanisms, which have been tuned primarily under off-limit conditions, were also considerably higher than the present experimental data. Additional results of the present investigation were derived for the overall activation energy and corresponding Zeldovich numbers, and the variation of the global flame Lewis numbers with equivalence ratio. The implications of these results were discussed.     

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.     

Flame front evolution and laminar flame parameter evaluation of buoyancy-affected ammonia/air flames

For flames with very low burning speed, the flame propagation is affected by buoyancy. Flame front evolution and laminar flame parameter evaluation methods of buoyancy-affected flame have been proposed. The evolution and propagation process of a center ignited expanding ammonia/air flame has been analyzed by using the methods. The laminar flame parameters of ammonia/air mixture under different equivalence ratio (ER) and initial pressure have been studied. At barometric pressure, with the increase of ER, the laminar burning velocity (LBV) of ammonia/air mixture undergoes a first increase and then decrease process and reaches its maximum value of 7.17 cm/s at the ER of 1.1, while the Markstein length increases monotonously. For ammonia/air flames with ER less than unity, the flame velocity shows a decreasing trend with stretch rate, resulting in the propensity to flame instability, but no cellular structure was observed in the process of flame propagation. As the initial pressure increases, the LBV decreases monotonously as well as the Markstein length. The flame thicknesses of ammonia/air mixtures decrease with initial pressure and are much thicker than those of hydrogen flames, which makes a stronger stabilizing effect of curvature on the flame front. The most enhancement of LBV is contributed by the dehydrogenation reaction of NH₃ with OH. The NO concentration decreases significantly with the increase of ER.     

Measurement of laminar burning velocities and Markstein lengths of diluted hydrogen-enriched natural gas

The laminar flame characteristics of natural gas–hydrogen–air–diluent gas (nitrogen/CO₂) mixtures were studied in a constant volume combustion bomb at various diluent ratios, hydrogen fractions and equivalence ratios. Both unstretched laminar burning velocity and Markstein length were obtained. The results showed that hydrogen fraction, diluent ratio and equivalence ratio have combined influence on laminar burning velocity and flame instability. The unstretched laminar burning velocity is reduced at a rate that is increased with the increase of the diluent ratio. The reduction effect of CO₂ diluent gas is stronger than that of nitrogen diluent gas. Hydrogen-enriched natural gas with high hydrogen fraction can tolerate more diluent gas than that with low hydrogen fraction. Markstein length can either increase or decrease with the increase of the diluent ratio, depending on the hydrogen fraction of the fuel.     

Parameter extraction from spherically expanding flames propagated in hydrogen/air mixtures

The characteristics of hydrogen/air flame were studied by using the spherical expanding flame propagated in a constant volume chamber. The influence of ignition induced blast wave and the flame instability on flame propagation was investigated. The nonlinear evaluation method for laminar flame parameter evaluation was established. By using the nonlinear evaluation method and the experimental results of flame propagation, the laminar flame speed and Markstein length were extracted and the difference between the nonlinearly evaluated laminar flame speed and the linearly evaluated one was analyzed. The influence of initial pressure and equivalence ratio on laminar flame speed and flame thickness was investigated. The laminar flame speed varies with equivalence ratio and initial pressure. There exists an equivalence ratio at which the laminar flame speed gets its maximum value. And there also exists an initial pressure at which the laminar flame speed gets its maximum value. The critical radius, Markstein length and flame instability of hydrogen/air flame with different equivalence ratio at different initial pressure had been studied. In hydrogen/air flame the flame stability decreases with the increase of initial pressure, while it increases with the increase of equivalence ratio. The global stability of flame is determined by the combination of the stabilizing effect of stretch effect, thermodiffusive instability mechanism and hydrodynamic instability mechanism.     

Experimental and kinetic study of laminar flame characteristics of H2/O2/diluent flame under elevated pressure

Laminar burning velocity, Markstein length, and critical flame radius of an H₂/O₂ flame with different diluents, He, Ar, N₂ and CO₂, were measured under elevated pressure with different diluent concentrations. The effects of pressures, diluents, and dilution and equivalence ratios were studied by comparing calculated and experimental results. The laminar burning velocity showed non-monotonic behavior with pressure when the dilution ratio was low. The reason is the radical pool reduced with increasing pressure and leads to the decrease of overall reaction order from larger than 2 to smaller than 2, and further leads to this non-monotonic phenomenon. A modified empirical equation was presented to capture the relationship between active radicals and laminar burning velocity. Critical radii and Markstein lengths both decrease with initial pressure and increase with equivalence ratio and dilution ratio. The calculated critical radii indicate that the Peclet number and flame thickness control the change of R_cr. It can be found that Le_eff has a significant influence on Peclet number and leads to the decrease of critical flame radii of Ar, N_2, and CO₂ diluted mixture. Interestingly, the CO₂ diluted mixture has the lowest Markstein length under stoichiometric conditions and a high value under fuel-rich conditions, consistent as the flame instability observed on the flame images. The reason is that the Le_eff of CO₂ diluted mixture increased rapidly with the equivalence ratio.     

Experimental determination of laminar burning velocities and Markstein lengths for 75% hydrous-ethanol, hydrogen and air gaseous mixtures

Hydrogen-rich mixtures generated by the on-board reforming of biomass-derived hydrous-ethanol can be used as a potential alternative fuel (i.e., reformed ethanol fuel, RE fuel). In this paper, outwardly propagating spherical flames were employed to observe the laminar flame characteristics of the gaseous mixtures composed of simulated RE fuel (mixture of 75% hydrous-ethanol and hydrogen) and air in a constant-volume combustion vessel at an initial temperature of 383 K, a pressure of 0.1 MPa, a hydrogen fraction from 0% to 80%, and an equivalence ratio from 0.6 to 1.6. The results show that the unstretched flame propagation speeds and burning velocities increase with increasing hydrogen fraction, especially when the fraction is above 40%. When the hydrogen fraction is less than 40%, the Markstein length and flame instability decrease and increase with the equivalence ratio, respectively, while the reverse holds when the hydrogen fraction is greater than 40%. At an equivalence ratio below 1.4, the Markstein length decreases with increasing hydrogen fraction, indicating a positive correlation between the flame instability and hydrogen fraction. At an equivalence ratio above 1.4, a negative relationship is observed. Finally, it is concluded that a hydrogen fraction of approximately 40% in simulated RE fuel is feasible for spark ignition engines by comparing the laminar burning characteristics of ethanol-air mixtures.     

An experimental and mechanistic study on the laminar flame speed,Markstein length and flame chemistry of the butanol isomers

Laminar flame speeds and Markstein lengths for n-butanol, s-butanol, i-butanol and t-butanol at pressures from 1 to 5 atm were experimentally measured in a heated, dual-chamber vessel. Results at all pressures show that n-butanol has the highest flame speeds, followed by s-butanol and i-butanol, and then t-butanol. Results further show that the reduced Markstein length measured for n-butanol as compared to other isomers is a flame thickness effect, and that all four isomers have similar Markstein numbers, which is the appropriate nondimensional parameter to quantify flame stretch. Computation and flame chemistry analysis were performed using two recently published kinetic models on butanol isomers by Sarathy et al. and Ranzi et al., respectively. Comparison shows the former model satisfactorily agrees with the present results while agreement of the latter is less satisfactory. Based on reaction path analysis the major differences of the two models on fuel cracking pathway were identified. It is concluded that the primary reason for the lowered flame speed of s-butanol, i-butanol and t-butanol is that they crack into more branched intermediate species which are relatively stable, such as iso-butene, iso-propenol and acetone. This indicates that the general rule that fuel branching reduces flame speed for hydrocarbons can also be applied to alcohols, and that the fundamental reason for this generality is that in alcohols CO has similar bond energy to the CC bond while OH has similar bond energy to the CH bond.     

Laminar burning velocities and flame stability analysis of hydrogen/air premixed flames at low pressure

An experimental and numerical study on laminar burning velocities of hydrogen/air flames was performed at low pressure, room temperature, and different equivalence ratios. Flames were generated using a small contoured slot-type nozzle burner (5 mm × 13.8  mm). Measurements of laminar burning velocity were conducted using the angle method combined with Schlieren photography. Numerical calculations were also conducted using existing detailed reaction mechanisms and transport properties. Additionally, an analysis of the intrinsic flame instabilities of hydrogen/air flames at low pressure was performed. Results show that the behavior of the laminar burning velocity is not regular when decreasing pressure and that it depends on the equivalence ratio range. The behavior of the laminar burning velocity with decreasing pressure can be reasonably predicted using existing reaction mechanisms; however changes in the magnitude of the laminar burning velocity are underestimated. Finally, it has been found experimentally and proved analytically that the intrinsic flame instabilities are reduced when decreasing the pressure at sub-atmospheric conditions.     

Sensitivity to change in laminar burning velocity and Markstein length resulting from variable hydrogen fraction in blast furnace gas for changing ambient conditions

The sensitivity to changes in fuel characteristics has been investigated for combustion of Blast Furnace Gas resulting from small volumetric increases in H₂ concentration. A nonlinear methodology has been employed to quantify unstretched flame speeds and the effect of flame stretch from outwardly propagating spherical flames. Following benchmarking work with CH₄, results were obtained under ambient conditions of 303 K and 0.1 MPa, with small absolute change in hydrogen concentration shown to at least triple the laminar burning velocity for all tested mixtures. Fuel composition and equivalence ratio were shown to independently influence mixture diffusivity and Lewis number, quantified by change in the obtained values of Markstein length. Temperature and pressure were increased to respective values of 393 K and 0.2 MPa to investigate influence of ambient conditions, with a power law correlation presented. Finally the performance of several published chemical reaction mechanisms has been evaluated through comparison of 1-D flame models.     

Experimental and numerical studies of laminar flame characteristics of ethyl acetate with or without hydrogen addition

Hydrogen (H₂) is an effective additive to improve the issue of low laminar burning velocity of some biofuels. In order to better understand the laminar burning characteristics of ethyl acetate (EA) with or without H₂ addition, experimental investigations of laminar burning characteristics were carried out by using the high-speed Schlieren photography technique in a constant volume combustion chamber. Tests were conducted under various equivalence ratios ranging from 0.5 to 1.4 with an initial temperature of 358 K, an initial pressure of 0.1 MPa and a H₂/air proportion of 0%, 4%, 8% and 12% by volume. Laminar burning velocities, together with other parameters such as laminar burning flux, flame thickness, Markstein length and Markstein number, were calculated and discussed. In addition, the experimental data were compared with numerical simulations based on the Dayma model. Results showed that the laminar burning velocity of EA was enhanced with the increase of H₂ addition, and the maximum value reached 95.09 cm/s at φ = 0.6 with 12% H₂, a value more than twice as fast as that of pure EA (39.3 cm/s). Moreover, H₂ was found to extend the lower flammability limit of EA. The laminar burning velocities simulated with the Dayma model agreed well with the experimental results of EA at various H₂ additions.     

Laminar burning velocities and flame instabilities of diluted H2/CO/air mixtures under different hydrogen fractions

An experimental study was conducted using outwardly propagating flame to evaluate the laminar burning velocity and flame intrinsic instability of diluted H₂/CO/air mixtures. The laminar burning velocity of H₂/CO/air mixtures diluted with CO₂ and N₂ was measured at lean equivalence ratios with different dilution fractions and hydrogen fractions at 0.1 MPa; two fitting formulas are proposed to express the laminar burning velocity in our experimental scope. The flame instability was evaluated for diluted H₂/CO/air mixtures under different hydrogen fractions at 0.3 MPa and room temperature. As the H₂ fraction in H₂/CO mixtures was more than 50%, the flame became more unstable with the decrease in equivalence ratio; however, the flame became more stable with the decrease in equivalence ratio when the hydrogen fraction was low. The flame instability of 70%H₂/30%CO premixed flames hardly changed with increasing dilution fraction. However, the flames became more stable with increasing dilution fraction for 30%H₂/70%CO premixed flames. The variation in cellular instability was analyzed, and the effects of hydrogen fraction, equivalence ratio, and dilution fraction on diffusive-thermal and hydrodynamic instabilities were discussed.     

Effects of hydrogen addition and nitrogen dilution on the laminar flame characteristics of premixed methane–air flames

The effect of hydrogen addition and nitrogen dilution on laminar flame characteristics was investigated. The spherical expanding flame technique, in a constant volume bomb, was employed to extract laminar flame characteristics. The mole fraction of hydrogen in the methane–hydrogen mixture was varied from 0 to 1 and the mole fraction of nitrogen in the total mixture (methane–hydrogen–air–diluent) from 0 to 0.35. Measurements were performed at an initial pressure of 0.1 MPa and an initial temperature of 300 K. The mixtures investigated were under stoichiometric conditions. Based on experimental measurements, a new correlation for calculating the laminar burning velocity of methane–hydrogen–air–nitrogen mixtures is proposed. The laminar burning velocity was found to increase linearly with hydrogen mass fraction for all dilution ratios while the burned gas Markstein length decreases with the increase in hydrogen amount in the mixture except for high hydrogen mole fractions (>0.6). Nitrogen dilution has a nonlinear reducing effect on the laminar burning velocity and an increasing effect on the burned gas Markstein length. The experimental results and the proposed correlation obtained are in good agreement with literature values.     

The characteristics of flame propagation in hydrogen/oxygen mixtures

The extreme explosiveness and high flame velocity of hydrogen challenge its application. Overcoming these challenges requires improving the fundamental flame characteristics of H₂/O₂ mixtures. In this study, the propagation characteristics of H₂/O₂ flames are investigated. The laminar burning velocity (LBV) is evaluated using nonlinear extrapolation. The empirical relations of LBV are given with the equivalence ratio (ER) and initial mixture pressure (IMP). The LBV increases first and then decreases as the ER increases and reaches its maximum value at the ER slightly higher than 1.0 (φ = 1.1–1.2). The LBV increases monotonically with increasing IMP. The critical instability radius and Markstein length increase as the ER increases, while decreasing with the IMP increase. The flame thickness decreases significantly with increasing IMP. The flame remains stable and smooth throughout the propagation process for all examined ERs only at the lower IMPs of 0.1 atm and 0.3 atm.     

Spherical expanding flames in H2–N2O–Ar mixtures: flame speed measurements and kinetic modeling

Although ignition of hydrogen–nitrous oxide mixtures is a serious issue for nuclear waste storage and semi-conductor manufacturing, available flame speed data have not been recently updated and thermodiffusive stability is not known. In order to palliate this, the flame speed of a hydrogen–nitrous oxide mixture diluted in Ar (60% mol) was measured in a spherical bomb as a function of equivalence ratio. The initial pressure and temperature were held constant around ambient conditions. It is shown that the unstretched flame speed of the hydrogen–nitrous oxide mixture is relatively low for a hydrogen-based mixture, with a maximum of 56 cm/s for the stoichiometric condition. Further, hydrogen–nitrous oxide–argon flames appear unstable with respect to thermodiffusive effects at an equivalence ratio of 1. The downward flammability limit of hydrogen–nitrous oxide–argon was observed for hydrogen content of 8 mol%. The modeling of these experimental data has been performed with three recently developed models. All kinetic schemes give satisfactory predictions of the experimentally observed data. Sensitivity and reaction pathway analysis have demonstrated that the dynamic of the system is dominated by the reaction N₂O + H = N₂ + OH which governs the rate of energy release.