Fokion N. Egolfopoulos

Direct experimental determination of laminar flame speeds

The stability of premixed flames at ultralow strain rates was assessed experimentally and numerically in the stagnation flow configuration. Results indicate that there are inherent limitations in establishing weakly strained planar flames, and that the accuracy of the laminar flame speeds obtained through linear extrapolations can, thus, be compromised. In view of these limitations, a new methodology is proposed for the direct experimental determination of laminar flame speeds. It includes the use of the stagnation flow configuration and large separation distances betwenn the nozzle and the stagnation plane, which allow for the establishment of Bunsen-type flames as the flow rate is reduced. The flow velocities are measured by using laser Doppler velocimetry. The proposed technique is based on the principle that whereas the planar, strained flames are positively stretched, the Bunsen flames are negatively stretched. Thus, by achieving a smooth, quasi-steady transition between planar and Bunsen flames, the flames pass through a near-zero strain-rate state. Real-time LDV measurements were obtained at numerous fixed spatial locations in the region within which transition occurs. The minimum velocity obtained in these measurements corresponds to the flame speed at the limit of near-zero stretch and is proposed as a representative value of the true laminar flame speed, S H o . Laminar flame speeds were obtained for atmospheric CH 4 /air, C 2 H 6 /air, and C 3 H 8 /air mixtures and for a wide range of equivalence ratios. The new S n o values were found to be systematically lower than the values that have been determined by using the traditional stagnation flow technique and linear extrapolations to zero strain rate.

Further considerations on the determination of laminar flame speeds with the counterflow twin-flame technique

The accuracy of the laminar flame speed determination by using the counterflow twin-flame techniquehas been computationally and experimentally examined in light of the recent understanding that linear extrapolation of the reference upstream velocity to zero strain rate would yield a value higher than that of the laminar flame speed, and that such an overestimate can be reduced by using either lower strain rates and/or larger nozzle separation distances. A systematic evaluation of the above concepts has been conducted and verified for the ultralean hydrogen/air flames, which have relatively large Karlovitz numbers, even for small strain rates, because of their very small laminar flame speeds. Consequently, the significantly higher values of the previous experimentally measured flame speeds, as compared with the independently calculated laminar flame speeds, can now be attributed to the use of nozzle separation distances that were not sufficiently large and/or strain rates that were not sufficiently small. Thus, by using lower strain rates and larger nozzle separation distances, the experimentally and computationally redetermined values of these ultralean hydrogen/air flames agree well with the calculated laminar flame speeds. The laminar flame speeds of methane/air and propane/air mixtures have also been experimentally redetermined over extensive ranges of the equivalence ratio and are found to be slightly lower than the previously reported experimental values.

Laminar flame speeds of methane-air mixtures under reduced and elevated pressures☆

Using the counterflow methodology, the laminar flame speeds of methane-air mixtures have been accurately determined over the pressure range of 0.25-3 atm and over extensive lean-to-rich concentration ranges. These flame speeds are then compared with the numerically calculated values obtained by using various published kinetic schemes of either the C1 mechanism or the full C2 mechanism. Two such schemes show very close agreement with the experimental data. However, available information cannot further differentiate the relative superiority between them for flame speed calculations, especially the importance of C2 reactions for moderately rich situations. Two reduced mechanisms are also deduced through sensitivity analysis and are expected to be useful for flame speed calculations and approximate flame structure studies.

Development of an Experimental Database and Chemical Kinetic Models for Surrogate Gasoline Fuels

The development of surrogate mixtures that represent gasoline combustion behavior is reviewed. Combustion chemistry behavioral targets that a surrogate should accurately reproduce, particularly for emulating homogeneous charge compression ignition (HCCI) operation, are carefully identified. Both short and long term research needs to support development of more robust surrogate fuel compositions are described. Candidate component species are identified and the status of present chemical kinetic models for these components and their interactions are discussed. Recommendations are made for the initial components to be included in gasoline surrogates for near term development. Components that can be added to refine predictions and to include additional behavioral targets are identified as well. Thermodynamic, thermochemical and transport properties that require further investigation are discussed.

Experimental and numerical determination of laminar flame speeds: Mixtures of C2-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.

Laminar flame speeds and extinction strain rates of mixtures of carbon monoxide with hydrogen, methane, and air

The effect of hydrogen and methane addition on the propagation and extinction of atmospheric CO/airflames was investigated experimentally and numerically. Experiments were conducted by using the counterflow, twin-flame technique and laser-Doppler velocimetry for the determination of laminar flame speeds and extinction strain rates. The simulation was conducted by using the one-dimensional flame code, and by solving the conservation equations of mass, momentum, energy, and species along the stagnation stream-line of the counterflow. In both cases, detailed description of the chemistry and transport was used. Results indicate that the addition of small amounts of hydrogen and methane to CO flames increases the laminar flame speeds and extinction strain rates by accelerating the main CO oxidation reaction. The sensitivity of the mass burning rate to this reaction is particularly high when trace amounts of hydrogen and methane are added. For large amounts of additives, the chemistry shifts toward that of the additive, and the advantages of the CO kinetic simplicity are lost. The experimental data were closely predicted by the numerical calculations for both propagation and extinction, indicating that existing CO, hydrogen, and methane kinetics can be used with confidence for similar studies. Detailed analysis of the flame structure revealed that, when CO and methane are both supplied as reactants, the CO oxidation follows that of methane, and that, for methane-rich mixtures, the supplied CO remains unreacted until the intermediate CO has been completely formed.

Chain mechanisms in the overall reaction orders in laminar flame propagation

Abstract The laminar flame speeds of methane-oxygen-nitrogen mixtures as functions of the flame temperature Tad and system pressure p have been experimentally determined by using the counterflow, twin-flame technique. These data are then compared with numerically-calculated values obtained by using an independently validated kinetic scheme. Results show that the overall reaction order n and activation energy Ea are far from being constants, that n decreases with increasing p and decreasing Tad whereas Ea increases with increasing p, and that n can actually assume negative values; the last result implies that the mass burning rate of some weakly burning flames may decrease with increasing pressure. By further identifying the crucial reaction steps through sensitivity analysis, the present results are interpreted on the basis of the influence of chain branching-termination mechanisms on the overall reaction rate.

CO2* Chemiluminescence in Premixed Flames

ABSTRACT Chemiluminescence from species such as CH*, C2*, OH* and CO2* often are used as a quantitative diagnostic in experimental studies of premixed combustion. This paper reports results from a numerical investigation of CO2* chemiluminescence as a quantitative diagnostic in laminar and turbulent premixed flames. Calculations are carried out using a complex reaction mechanism for methane and propane and a model for CO2* chemiluminescence. Relationships between chemiluminescent intensity and both H-atom concentration and heat release rate are quantified as functions of dilution, equivalence ratio, steady and unsteady strain-rate. These relationships are monotonic, but not unique; they depend on which flame parameter is varied. However, the effect of unsteadiness on the relationship for a strained flame is negligible, and this allows the use of chemiluminescence-based diagnostics for measurement of relative H-atom concentration and relative heat release rates in unsteady laminar and turbulent premixed fl...

A study on ethanol oxidation kinetics in laminar premixed flames, flow reactors, and shock tubes

A comprehensive experimental and numerical study on ethanol oxidation kinetics has been conducted. The laminar flame speeds of ethanol/air mixtures were determined by using the counterflow twin-flame technique at 1 atm pressure and for initial mixture temperatures between 363 and 453 K. A detailed kinetic scheme was subsequently compiled by grafting the latest information on ethanol kinetics onto a previously developed methanol scheme, and was found to be self-consistent in that it closely predicts not only the experimental laminar flame speeds of ethanol, but also those of methane, methanol, and all the C2-hydrocarbons. Further recognizing that prediction of the laminar flame speeds is not sufficient for the satisfactory validation of a kinetic mechanism, the present scheme has also been tested against experimental data in the literature on the species and temperature profiles in flow reactors and on the ignition delay times in shock tubes. Such studies demonstrate the importance of the CH3 and HO2 radical chemistry, and the present results suggest that the rate of CH3+HO2→ CH3O+OH may be slower while that of CH3+HO2→CH4+O2 may be faster than values frequently used in recent literature.

A comprehensive study of methanol kinetics in freely-propagating and burner-stabilized flames, flow and static reactors, and shock tubes

Abstract An experimental and numerical study of methanol kinetics has been conducted. A detailed kinetic scheme was compiled which closely predicts properties of mixtures of methanol, oxygen, and inert for a variety of experimental configurations and conditions. The scheme incorporates the most recent kinetic information and was tested against experimental data for the propagation speeds and structure of laminar flames as well as the species concentration evolutions in flow reactors, static reactors, and shock tubes. The laminar flame speeds of atmospheric methanol/air mixtures were determined using the counter-flow flame technique over extensive lean-to-rich fuel concentration ranges and for initial mixture temperatures ranging from 318 to 368 K., while the experimental data on the laminar flame structure and from reactors and shock tubes were obtained from the literature. The scheme compiled herein includes the detailed C1, C2, and methanol submechanisms and yields close agreement with all of the experi...