Laminar Flame Speeds Of Mixtures Research Articles

Laminar flame speed of H2/CH4/air mixtures with CO2 and N2 dilution

The laminar flame speeds of H2/CH4/air mixtures with CO2 and N2 dilution were systematic investigated experimentally and numerically over a wide range of H2 blending ratios (0–75 vol%) with CO2 (0–67 vol%) and N2 (0–67 vol%) dilution in the fuels. The experimental measurements were conducted via the Bunsen flame method incorporating the Schlieren technique under the condition of equivalence ratios from 0.8 to 2.0. To gain an insightful understanding of the experimental observations, detailed numerical simulation was carried out using Chemkin-Pro with GRI3.0-Mech. The experimental measurements were also used to validate the corresponding performance of a semiempirical correlation derived through asymptotic analysis method coupled with the reduced chemistry mechanism. The results showed that at lower H2 fraction (xH2 ≤ 0.5), the laminar flame speeds of H2/CH4/air mixtures displayed great linearly increase with the growth of H2 fractions. The combustion of mixtures with low H2 contents was dominated by CH4 conversion which was mainly controlled by the increasing OH radicals produced from the key oxidation reactions of H + O2 = O + OH. With the further increase of H2 fractions, the methane-dominated combustion gradually transformed into the methane-inhibited hydrogen combustion, resulting to the growth of laminar flame speeds was dramatical and non-linear. Due to the larger heat capacity and chemical kinetic effect, CO2 presented a stronger dilution effect on reducing the laminar flame speeds than N2. With the addition of CO2, the increasing stronger competition for H radical through CO + OH = CO2 + H with H + O2 = O + OH due to the significant reduction of H mole fractions, leading to the larger decrease of laminar flame speeds of mixtures. Besides, although the contribution of thermal effect of CO2 decreased near the equivalence ratio, the thermal effect of CO2 still preformed the dominated contribution to the total dilution effect. A comparison between the experimental data and estimated results using the semiempirical correlation showed that, the correlation using new modified coefficients provided the satisfactorily accuracy predictions on the laminar flame speeds of diluted H2/CH4/air mixtures at lower xH2 (xH2 ≤ 0.5) and lower xdilution (xdilution = 0.25). The estimated results were generally located within a deviation range of ±20% errors except for two unsatisfactory eases occurred at conditions of xH2 = 0.75 and xCO2 = 0.67. The considerably poor predictions were attributed to the significantly variation of the chemical kinetics under high H2 content and large CO2 dilution conditions.

Flame propagation of mixtures of air with high molecular weight neat hydrocarbons and practical jet and diesel fuels

Laminar flame speeds of mixtures of air with n-C14H30, n-C16H34, a petroleum-derived JP-5 jet fuel, a camelina-derived hydrotreated renewable JP-5 jet fuel, a petroleum-derived F-76 diesel fuel, and an algae-derived hydrotreated renewable F-76 diesel fuel, were measured in the counterflow configuration at atmospheric pressure and elevated unburned mixture temperatures. Digital particle image velocimetry was used to measure the axial flow velocities along the stagnation streamline. The experiments for n-C14H30/air and n-C16H34/air mixtures were modeled using recently developed kinetic models, and the experimental data were predicted satisfactorily. Both experiments and simulations revealed that the laminar flame speeds of n-C14H30/air and n-C16H34/air mixtures are very close to each other, as expected. On the other hand, the laminar flame speeds for the four practical fuels were found to be lower than n-C14H30 and n-C16H34, due to the presence of aromatics and branched hydrocarbons. Similarly, the laminar flame speeds for the alternative fuels were found to be higher than the petroleum-derived ones, again due to the presence of aromatic compounds in the latter. Further insight into the effects of kinetics and molecular transport was obtained through sensitivity analysis.

Experimental and numerical investigation on diluted DME flames: Thermal and chemical kinetic effects on laminar flame speeds

Experiments and computational simulations were used to study effects of diluents on laminar flame speeds of stoichiometric, laminar, premixed dimethyl ether (DME)/air flames. The experiments were conducted in a constant volume bomb under initial temperature of 298K and initial pressure of 0.1MPa. Outwardly propagating spherical flames were used to measure the laminar flame speeds of mixtures with varying concentration of Ar, N2 or CO2 addition. Laminar flame speeds were also computed using the steady, one-dimensional laminar premixed flame code PREMIX with detailed chemical kinetics. Predictions showed a good agreement with experiments. For all the three diluting agents invested in this work, laminar flame speed decreases with the increase of dilution ratio, while for a given dilution ratio, the significance of suppression effect on laminar flame speed is in the order of CO2, N2 and Ar. The suppression effects of N2 and Ar on flame propagation are mainly caused by reduced reactant concentration and modified heat capacity, both of which result in a decreased flame temperature, while besides that, CO2 further influences the chemical kinetics because it is a major product of combustion. Further experimental study on flame speed and simulation study on flame structure at fixed adiabatic flame temperature were conducted to evidence this speculation and the possible reaction pathways which contribute to the chemical retarding effect by CO2 addition were recognized through sensitivity analysis.

Effects of fuel branching on the propagation of octane isomers flames

Laminar flame speeds of mixtures of air with 3-methylheptane/air, 2,5-dimethylhexane/air, and iso-octane/air, were determined for a wide range of equivalence ratios in the counterflow configuration at atmospheric pressure, and an unburned mixture temperature of 353K. The results were compared against those obtained in recent investigations for n-octane/air and 2-methylheptane/air flames. It was determined that n-octane/air flames propagate the fastest, followed by 2- and 3-methylheptane/air, 2,5-dimethylhexane/air, and iso-octane/air flames, confirming that the overall reactivity decreases as the extent of fuel branching increases. The experimental data were modeled using a combination of recently developed kinetic models. Detailed sensitivity and reaction path analyses were performed in order to provide further insight into the high temperature oxidation chemistry of the octane isomers. It was determined that for all octane isomers, flame propagation is largely sensitive to H2/CO and C1C4 kinetics, while the effects of fuel-related reactions were found to be minor. Additionally, the analysis of the computed flame structures revealed that the low reactivity of branched isomers could be attributed to the production of unreactive, H-scavenging, resonantly stabilized intermediates such as propene, allyl, and iso-butene. Although fuel specific reactions do not exert a first order effect on flame propagation, the products of the initial fuel decomposition affect the concentrations of C1C4 intermediates, whose subsequent reactions are responsible for the observed differences in the overall flame reactivity among all isomers.

Measurement and correlation of laminar flame speeds of CO and C 2 hydrocarbons with hydrogen addition at atmospheric and elevated pressures

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

Non-premixed ignition, laminar flame propagation, and mechanism reduction of n-butanol, iso-butanol, and methyl butanoate

The non-premixed ignition temperature of n-butanol (CH3CH2CH2CH2OH), iso-butanol ((CH3)2CHCH2OH) and methyl butanoate (CH3CH2CH2COOCH3) was measured in a liquid pool assembly by heated oxidizer in a stagnation flow for system pressures of 1 and 3 atm. In addition, the stretch-corrected laminar flame speeds of mixtures of air–n-butanol/iso-butanol/methyl butanoate were determined from the outwardly propagating spherical flame at initial pressures of up to 2 atm, for an extensive range of equivalence ratio. The ignition temperature and laminar flame speeds of n-butanol and methyl butanoate were computationally simulated with three recently developed kinetic mechanisms in the literature. Dominant reaction pathways to ignition and flame propagation were identified and discussed through a chemical explosive mode analysis (CEMA) and sensitivity analysis. The detailed models were further reduced through a series of systematic strategies. The reduced mechanisms provided excellent agreement in both homogeneous and diffusive combustion environments and greatly improved the computation efficiency.

Flame propagation of mixtures of air with binary liquid fuel mixtures

The laminar flame speeds of mixtures of air with 80% n-dodecane+20% methylcyclohexane and 80% n-dodecane+20% toluene, on a per volume basis, were determined in the counterflow configuration over a wide range of equivalence ratio, at atmospheric pressure and 403K unburned mixture temperature. The choice of the fuel blends was dictated by their anticipated compositions in jet fuels surrogates. Phenomenological analysis shows that the laminar flame speeds of binary fuels mixtures can be estimated using the laminar flame speeds and adiabatic flame temperatures of the neat components. The propagation rates of various binary fuels blends were computed using detailed descriptions of chemical kinetics and molecular transport and were found to be in good agreement with the estimations. Although the fuel initial consumption pathways and the resulting intermediates and radicals may be different for each neat component, the propagation of flames of binary fuels is mostly sensitive to the flame temperature through its influence on the main branching reaction H+O2→OH+O. Thus, kinetic couplings resulting from the presence of two different fuels, appear to have minor effect on flame propagation.

Flame propagation and counterflow nonpremixed ignition of mixtures of methane and ethylene

The ignition temperature of nitrogen-diluted mixtures of methane and ethylene counterflowing against heated air was measured up to five atmospheres. In addition, the stretch-corrected laminar flame speeds of mixtures of air, methane and ethylene were determined from outwardly-propagating spherical flames up to 10 atmospheres, for extensive range of the lean-to-rich equivalence ratio. These experimental data, relevant to low- to moderately-high-temperature ignition chemistry and high-temperature flame chemistry, respectively, were subsequently compared with calculations using two detailed kinetic mechanisms. A chemical explosive mode analysis (CEMA) was then conducted to identify the dominant ignition chemistry and the role of ethylene addition in facilitating nonpremixed ignition. Furthermore, the hierarchical structure of the associated oxidation kinetics was examined by comparing the sizes and constituents of the skeletal mechanisms of the pure fuels and their mixtures, derived using the method of directed relation graph (DRG). The skeletal mechanism was further reduced by time-scale analysis, leading to a 24-species reduced mechanism from the detailed mechanism of USC Mech II, validated within the parameter space of the conducted experiments.

Experimental determination of counterflow ignition temperatures and laminar flame speeds of C 2–C 3 hydrocarbons at atmospheric and elevated pressures

Experimental data were acquired for: (1) the ignition temperatures of nitrogen–diluted ethylene and propylene by counterflowing heated air for various strain rates and system pressures up to 7 atm; (2) the laminar flame speeds of mixtures of air with acetylene, ethylene, ethane, propylene, and propane, deduced from an outwardly propagating spherical flame in a constant-pressure chamber, for extensive ranges of lean-to-rich equivalence ratio and system pressure up to 5 atm. These data, respectively, relevant for low- to intermediate-temperature ignition chemistry and high-temperature flame chemistry, were subsequently compared with calculated results using a literature C 1–C 3 mechanism and an ethylene mechanism. Noticeable differences were observed in the comparison for both mechanisms, and sensitivity analyses were conducted to identify the reactions of importance.

Determination of laminar flame speeds using digital particle image velocimetry: Binary Fuel blends of ethylene, n-Butane, and toluene

The atmospheric laminar flame speeds of mixtures of air with ethylene, n-butane, toluene, ethylene-n-butane, ethylene-toluene, and n-butane-toluene were experimentally and computationally investigated over an extended range of equivalence ratios. Binary fuel blends with 1:1, 1:2, and 2:1 molar ratios were examined. Experimentally, the laminar flame speeds were determined using digital particle image velocimetry (DPIV). Since the use of DPIV enables the mapping of the two-dimensional flow field adhead of the flame, the reference speed based on the minimum axial velocity point as well as the imposed strain rate can be identified simultaneously. The latter can now be unambiguously determined by the radial velocity gradient at the minimum velocity point. By systematically varying the imposed strain rate, the corresponding laminar flame speed was obtained through nonlinear extrapolation to zero strain rate. The associated experimental accuracy of the DPIV measurements was also assessed and discussed. Computationally, the laminar flame speeds were simulated for all single-component fuel/air and binary fuel blend/air mixtures with a detailed kinetic model. Comparison of experimental and computed flame speeds shows generally good agreement. A semiempirical mixing rule was developed. The mixing rule which requires only the knowledge of the flame speeds and flame temperatures of the individual fuel constituents, is shown to provide acurate estimates for the laminar flame speeds of binary fuel blends under the conditions tested.

Experimental and numerical determination of laminar flame speeds: Mixtures of C 2-hydrocarbons with oxygen and nitrogen

Using the counterflow flame technique, laminar flame speeds of mixtures of ethane, ethylene, acetylene and propane with oxygen and nitrogen have been accurately determined over extensive lean-to-rich fuel concentration ranges and over the pressure range of 0.25 to 3 atm. These data are then compared with the numerically calculated values obtained by using the various kinetic schemes in the literature as well as one compiled in the present study. The present scheme yields close agrrement with all of the experimental flame speeds except for diluted, rich acetylene flames, for which the calculated values are higher. The relative importance and influence of the individual reactions on the flame speed and reaction mechanism are assessed and discussed with the aid of sensitivity analysis. The study also demonstrates that C2 schemes validated through comparisons based on methane flame speeds may not be accurate enough for flame speed predictions of the C2 fuels, and that the C2 schemes developed through comparisons with the flame speeds of the C2 fuels are rather insensitive to the details of the C3 sub-mechanism. The importance of having accurate values of the thermophysical properties of radicals for flame simulation is also emphasized.

A study of burning velocity

Using the counterflow flame technique, laminar flame speeds of mixtures of ethane, ethylene, acetylene and propane with oxygen and nitrogen have been accurately determined over extensive lean-to-rich fuel concentration ranges and over the pressure range of 0.25 to 3 atm. These data are then compared with the numerically calculated values obtained by using the various kinetic schemes in the literature as well as one compiled in the present study. The present scheme yields close agrrement with all of the experimental flame speeds except for diluted, rich acetylene flames, for which the calculated values are higher. The relative importance and influence of the individual reactions on the flame speed and reaction mechanism are assessed and discussed with the aid of sensitivity analysis. The study also demonstrates that C2 schemes validated through comparisons based on methane flame speeds may not be accurate enough for flame speed predictions of the C2 fuels, and that the C2 schemes developed through comparisons with the flame speeds of the C2 fuels are rather insensitive to the details of the C3 sub-mechanism. The importance of having accurate values of the thermophysical properties of radicals for flame simulation is also emphasized.