Autoignited laminar lifted flames of propane in coflow jets with tribrachial edge and mild combustion

Characteristics of laminar lifted flames have been investigated experimentally by varying the initial temperature of coflow air over 800 K in the non-premixed jets of propane diluted with nitrogen. The result showed that the lifted flame with the initial temperature below 860 K maintained the typical tribrachial structure at the leading edge, which was stabilized by the balance mechanism between the propagation speed of tribrachial flame and the local flow velocity. For the temperature above 860 K, the flame was autoignited without having any external ignition source. The autoignited lifted flames were categorized in two regimes. In the case with tribrachial edge structure, the liftoff height increased nonlinearly with jet velocity. Especially, for the critical condition near blowout, the lifted flame showed a repetitive behavior of extinction and reignition. In such a case, the autoignition was controlled by the non-adiabatic ignition delay time considering heat loss such that the autoignition height was correlated with the square of the adiabatic ignition delay time. In the case with mild combustion regime at excessively diluted conditions, the liftoff height increased linearly with jet velocity and was correlated well with the square of the adiabatic ignition delay time.     

Evaluation of the laminar diffusion flamelet model in the calculation of an axisymmetric coflow laminar ethylene-air diffusion flame

A numerical study of an axisymmetric coflow laminar ethylene-air diffusion flame at atmospheric pressure was conducted using detailed chemistry and complex thermal and transport properties and two different methodologies: (1) the direct simulation method of solving the two-dimensional axisymmetric elliptic governing equations, and (2) the steady-state stretched diffusion flamelet model. Soot formation and radiative heat transfer were not taken into account in these calculations, both for simplicity and to avoid the complications associated with the issues of how to incorporate these chemical and physical processes into the flamelet model. The same reaction mechanism and thermal and transport properties were used in the 2D direct simulation and the generation of the flamelet library. The flamelet library was generated from the solutions of counterflow ethylene-air diffusion flames at a series of stretch rates. Results of the 2D direct simulation and the flamelet model are compared in physical space. Although the overall results of the flamelet model are qualitatively similar to those of the direct simulation, significant differences exist between the results of the two methods even for temperature and major species. The direct simulation method predicts that the peak concentrations of CO₂ and H₂O occur in different regions in the flame, while the flamelet model results show that the peak concentrations of CO₂ and H₂O occur in the same region. The flamelet model predicts an overly rapid approach to the equilibrium structure in the downstream region, leading to significantly higher flame temperatures. The main reason for the failure of the flamelet model in the downstream region is due to the neglect of the effects of multidimensional convection and diffusion and the fundamental difference in the chemical structure between a coflow diffusion flame and a counterflow diffusion flame. The findings of this paper are highly relevant to understanding the flamelet model results in the calculations of multidimensional turbulent diffusion flames.     

Effect of CO2/N2/CH4 dilution on NO formation in laminar coflow syngas diffusion flames

The effect of CO₂/N_2/CH₄ dilution on NO formation in laminar coflow H₂/CO syngas diffusion flames was experimentally and numerically investigated. The results reveal that the NO emission index increases with H₂/CO mole ratio. In all cases, CO₂/N₂/CH₄ dilution can reduce the peak temperature of syngas flame and have the ability to reduce peak flame temperature is decreased in the following order: CO₂>N₂>CH₄. CO₂/N₂ dilution reduces the NO formation in syngas flame while CH₄ dilution promotes the NO formation. Besides, the dilution of CO₂/N₂/CH₄ can reduce the peak mole fraction of OH and its variations with H₂/CO mole ratio and dilution ratio show the same trend as the peak flame temperature variations. The height of the flame with CO₂ and N₂ dilution increases with dilution ratio. The flame with CH₄ dilution becomes higher and wider with the increase of dilution ratio.     

Numerical study of laminar flame properties of diluted methane-hydrogen-air flames at high pressure and temperature using detailed chemistry

Technical limits of high pressure and temperature measurements as well as hydrodynamic and thermo-diffusive instabilities appearing in such conditions prevent the acquisition of reliable results in term of burning velocities, restraining the domain of validity of current laminar flame speed correlations to few bars and hundreds of Kelvin. These limits are even more important when the reactivity of the considered fuel is high. For example, the high-explosive nature of pure hydrogen makes measurements even more tricky and explains why only few correlations are available to describe the laminar flame velocity of high hydrogen blended fuels as CH₄-H₂ mixtures. The motivation of this study is thereby to complement experimental measurements, by extracting laminar flame speeds and thicknesses from complex chemistry one-dimensional simulations of premixed laminar flames. A wide number of conditions are investigated to cover the whole operating range of common practical combustion systems such as piston engines, gas turbines, industrial burners, etc. Equivalence ratio is then varied from 0.6 to 1.3, hydrogen content in the fuel from 0 to 100%, residual burned gas mass ratio from 0 to 30%, temperature of the fresh mixtures from 300 to 950 K, and pressure from 0.1 to 11.0 MPa. Many chemical kinetics mechanisms are available to describe premixed combustion of CH₄-H₂ blends and several of them are tested in this work against an extended database of laminar flame speed measurements from the literature. The GRI 3.0 scheme is finally chosen. New laminar flame speed and thickness correlations are proposed in order to extend the domain of validity of experimental correlations to high proportions of hydrogen in the fuel, high residual burned gas mass ratios as well as high pressures and temperatures. A study of the H₂ addition effect on combustion is also achieved to evaluate the main chemical processes governing the production of H atoms, a key contributor to the dumping of the laminar flame velocity.     

Autoignited laminar lifted flames of methane, ethylene, ethane, and n-butane jets in coflow air with elevated temperature

The autoignition characteristics of laminar lifted flames of methane, ethylene, ethane, and n-butane fuels have been investigated experimentally in coflow air with elevated temperature over 800 K. The lifted flames were categorized into three regimes depending on the initial temperature and fuel mole fraction: (1) non-autoignited lifted flame, (2) autoignited lifted flame with tribrachial (or triple) edge, and (3) autoignited lifted flame with mild combustion.For the non-autoignited lifted flames at relatively low temperature, the existence of lifted flame depended on the Schmidt number of fuel, such that only the fuels with Sc > 1 exhibited stationary lifted flames. The balance mechanism between the propagation speed of tribrachial flame and local flow velocity stabilized the lifted flames. At relatively high initial temperatures, either autoignited lifted flames having tribrachial edge or autoignited lifted flames with mild combustion existed regardless of the Schmidt number of fuel. The adiabatic ignition delay time played a crucial role for the stabilization of autoignited flames. Especially, heat loss during the ignition process should be accounted for, such that the characteristic convection time, defined by the autoignition height divided by jet velocity was correlated well with the square of the adiabatic ignition delay time for the critical autoignition conditions. The liftoff height was also correlated well with the square of the adiabatic ignition delay time.     

Numerical and experimental study of an axisymmetric coflow laminar methane-air diffusion flame at pressures between 5 and 40 atmospheres

A numerical and experimental study of an axisymmetric coflow laminar methane-air diffusion flame at pressures between 5 and 40 atm was conducted to investigate the effect of pressure on the flame structure and soot formation characteristics. Experimental work was carried out in a new high-pressure combustion chamber described in a recent study [K.A. Thomson, Ö.L. Gülder, E.J. Weckman, R.A. Fraser, G.J. Smallwood, D.R. Snelling, Combust. Flame 140 (2005) 222-232]. Radially resolved soot volume fraction was experimentally measured using both spectral soot emission and line-of-sight attenuation techniques. Numerically, the elliptic governing equations were solved in axisymmetric cylindrical coordinates using the finite volume method. Detailed gas-phase chemistry and complex thermal and transport properties were employed in the numerical calculations. The soot model employed in this study accounts for soot nucleation and surface growth using a semiempirical acetylene-based global soot model with oxidation of soot by O₂, OH, and O taken into account. Radiative heat transfer was calculated using the discrete-ordinates method and a nine-band nongray radiative property model. Two soot surface growth submodels were investigated and the predicted pressure dependence of soot yield was compared with available experimental data. The experiment, the numerical model, and a simplified theoretical analysis found that the visible flame diameter decreases with pressure as . The flame-diameter-integrated soot volume fraction increases with pressure as between 5 and 20 atm. The assumption of a square root dependence of the soot surface growth rate on the soot particle surface area predicts the pressure dependence of soot yield in good agreement with the experimental observation. On the other hand, the assumption of linear dependence of the soot surface growth rate on the soot surface area predicts a much faster increase in the soot yield with pressure than that observed experimentally. Although pressure affects the gas-phase chemistry, the increased soot production with increasing pressure seems primarily due to enhanced mixture density and species concentrations in the pressure range investigated. The increased pressure causes enhanced air entrainment into the fuel stream around the burner rim, leading to accelerated fuel pyrolysis. In the pressure range of 20 to 40 atm both the model and experiment show a diminishing sensitivity of sooting propensity to pressure with a greater decrease in the predicted sensitivity of soot propensity to pressure than the experimental results.     

Different chemical effect of hydrogen addition on soot formation in laminar coflow methane and ethylene diffusion flames

Previous studies showed that adding hydrogen (H₂) can have an opposite chemical effect on soot formation: its chemical effect enhances and suppresses soot formation in methane (CH₄) and ethylene (C₂H₄) diffusion flames, respectively. Such opposite chemical effect of H₂ (CE-H₂) remains unresolved. The different CE-H₂ is studied numerically in the two laminar coflow diffusion flames. A detailed chemical mechanism with the addition of a chemically inert virtual species FH₂ is used to model the gas-phase combustion chemistry in this study. Particularly, a reaction pathway analysis was performed based on the numerical results to gain insights into how H₂ addition to fuel affects the pathways leading to the formation of benzene (A₁) in CH₄ and C₂H₄ flames. The numerical results show that the CE-H₂ in CH₄ diffusion flame to prompt soot formation is ascribed that the higher mole fraction of H atom promotes the formation of A₁ and Acetylene (C₂H₂) and leads to higher nucleation rate and eventually higher soot surface growth rate. In contrast, adding H₂ to C₂H₄ diffusion flames decreases soot nucleation and surface growth rate. The lower soot nucleation rate is due to the lower mole fractions of pyrene (A₄), while the lower soot surface growth rate is due to the lower mole fractions of H atom and C₂H₂, higher mole fraction of H₂ and lower soot nucleation rate. Furthermore, the CE-H₂ in C₂H₄ diffusion flames promotes the formation of A₁, but suppresses the formation of A₄.     

An experimental and numerical study of the effects of dimethyl ether addition to fuel on polycyclic aromatic hydrocarbon and soot formation in laminar coflow ethylene/air diffusion flames

The effects of dimethyl ether addition to fuel on the formation of polycyclic aromatic hydrocarbons and soot were investigated experimentally and numerically in a laminar coflow ethylene diffusion flame at atmospheric pressure. The relative concentrations of polycyclic aromatic hydrocarbon species and the relative soot volume fractions were measured using planar laser-induced fluorescence and two-dimensional laser-induced incandescence techniques, respectively. Experiments were conducted over the entire range of dimethyl ether addition from pure ethylene to pure dimethyl ether in the fuel stream. The total carbon mass flow rate was maintained constant when the fraction of DME in the fuel stream was varied. Numerical calculations of nine diffusion flames of different dimethyl ether fractions in the fuel stream were performed using a detailed reaction mechanism consisting of 151 species and 785 reactions and a sectional soot model including soot radiation, inception of nascent soot particle due to collision of two pyrene molecules, heterogeneous surface growth and oxidation following the hydrogen abstraction acetylene addition mechanism, soot particle coagulation, and PAH surface condensation. The addition of a relatively small amount of dimethyl ether to ethylene was found experimentally to increase the concentrations of both polycyclic aromatic hydrocarbons and soot. The synergistic effect on polycyclic aromatic hydrocarbons persists over a wider range of dimethyl ether addition. The numerical results reproduce the synergistic effects of dimethyl ether addition to ethylene on both polycyclic aromatic hydrocarbons and soot, though the magnitude of soot volume fraction overshoot and the range of dimethyl ether addition associated with the synergistic effect of soot are less than those observed in the experiment. The synergistic effects of dimethyl ether addition to ethylene on many hydrocarbon species, including polycyclic aromatic ones, and soot can be fundamentally traced to the enhanced methyl concentration with the addition of dimethyl ether to ethylene. Contrary to previous findings, the pathways responsible for the synergistic effects of benzene, polycyclic aromatic hydrocarbons, and soot in the ethylene/dimethyl ether system are found to be primarily due to the cyclization of l-C₆H₆ and n-C₆H₇ and to a much lesser degree due to the interaction between C2 and C4 species for benzene formation, rather than the propargyl self-combination reaction route, though it is indeed the most important reaction for the formation of benzene.     

An experimental study on soot distribution characteristics of ethanol-gasoline blends in laminar diffusion flames

In view of the potential of bio-ethanol as an alternative fuel and the particulate matter (PM) issues during gasoline combustion, the soot distribution characteristics of ethanol-gasoline blends in laminar diffusion flames were studied on a Gülder liquid burner using the two-color laser induced incandescence (TC-LII) technique. During the experiments, the ethanol ratio in the blends was varied from 20% to 80% by volume in order to investigate quantitatively the soot reduction potential of ethanol. In order to study the effect of reduction in carbon content due to ethanol addition on soot formation, the experiments were performed under a fixed fuel mass flow rate and a fixed carbon mass flow rate. It was found that both peak and average soot volume fraction in the flame reduced significantly with increasing ethanol content under both fuel supplying modes, however, this effect was progressively less pronounced as ethanol content increased. By comparing the two fueling modes, it was found that the reduction in carbon content due to ethanol addition has little impact on soot reduction. For a given ethanol blending ratio, the soot reduction under the same carbon mass flow rate was only slightly smaller than that under the same fuel mass flow rate. In terms of flame characteristics, the initial height of soot formation increases with increasing ethanol content under both fuel supply modes mainly due to the increased fuel outlet velocity. Radially, the peak soot location moves from the outside towards the center gradually as height increases. However, along the center line of the flame, the initial height of soot formation decreases with increasing ethanol content under the same fuel flow rate, whereas the trend remained similar to that in the whole flame under the same carbon flow rate.     

Effects of gravity and pressure on laminar coflow methane–air diffusion flames at pressures from 1 to 60 atmospheres

The effects of pressure and gravity on the sooting characteristics and flame structure of coflow methane–air laminar diffusion flames between 1 and 60 atm were studied numerically. Computations were performed by solving the unmodified and fully-coupled equations governing reactive, compressible flows which include complex chemistry, detailed radiation heat transfer and soot formation/oxidation. Soot formation/oxidation was modeled using an acetylene-based, semi-empirical model which was verified with previously published experimental data to correctly capture many of the observed trends at normal-gravity. Calculations for each pressure considered were performed under both normal- and zero-gravity conditions to help separate and identify the effects of pressure and buoyancy on soot formation. Based on the numerical predictions, pressure and gravity were observed to significantly influence the sooting behavior and structure of the flames through their effects on buoyancy and temperature. Zero-gravity flames generally have lower temperatures, broader soot-containing zones, and higher soot volume fractions than normal-gravity flames at the same pressure. Buoyancy forces caused the normal-gravity flames to narrow with increasing pressure while the increased soot concentrations and radiation at high pressures caused the zero-gravity flames to lengthen. Low-pressure flames at both gravity levels exhibited a similar power-law dependence of the maximum carbon conversion on pressure that weakened as pressure was increased. In the zero-gravity flames, increasing pressure beyond 20 atm caused the maximum carbon conversion factor to decrease.     

Effect of hydrogen and helium addition to fuel on soot formation in an axisymmetric coflow laminar methane/air diffusion flame

A detailed numerical study was conducted to investigate the effects of hydrogen and helium addition to fuel on soot formation in atmospheric axisymmetric coflow laminar methane/air diffusion flame. Detailed gas-phase chemistry and thermal and transport properties were employed in the numerical calculations. Soot was modeled using a PAH based inception model and the HACA mechanism for surface growth and oxidation. Numerical results were compared with available experimental data. Both experimental and numerical results show that helium addition is more effective than hydrogen addition in reducing soot loading in the methane/air diffusion flame. These results are different from the previous investigations in ethylene/air diffusion flames. Hydrogen chemically enhances soot formation when added to methane. The different chemical effects of hydrogen addition to ethylene and methane on soot formation are explained in terms of the different effects of hydrogen addition on propargyl, benzene, and pyrene formation low in the flames.     

Numerical study on CH4 laminar premixed flames for combustion characteristics in the oxidant atmospheres of N2/CO2/H2O/Ar-O2

The freely-propagating laminar premixed flames of CH₄–N₂/CO₂/H₂O/Ar-O₂ mixtures were conducted with the PREMIX code. The effects of the equivalence ratio and various oxidant atmospheres on the basic combustion characteristics were analyzed with the initial pressure and temperature of 1 atm and 398 K, respectively, O₂ content in the oxidant of 21%. The chemical reaction mechanism GRI-Mech 3.0 was chosen to determine the effects of the oxidant atmospheres of N₂/O₂, CO₂/O₂, H₂O/O₂, and Ar/O₂ on the adiabatic flame temperature, laminar burning velocity, flame structure, free radicals, intermediate species, net heat release rate and specific heat of the fuel/oxidant mixtures. The numerical results show that the maximum adiabatic flame temperatures and laminar burning velocities are at Ar/O₂ atmosphere. The mole fractions of CO and H₂ increased fastest at CO₂/O₂ atmosphere and H₂O/O₂, respectively. The mole fractions of CH₃ and H follow the order Ar/O₂> N₂/O₂>H₂O/O₂>CO₂/O₂. In addition, for 4 oxidant atmospheres, the peak mole fraction of C₂H₂ is following the order H₂O/O₂>Ar/O₂>N₂/O₂>CO₂/O₂ and the net heat release rate is following the order Ar/O₂>N₂/O₂>H₂O/O₂>CO₂/O₂ for all equivalence ratios.     

Effects of water vapor addition to the air stream on soot formation and flame properties in a laminar coflow ethylene/air diffusion flame

The effects of adding water vapor to the air stream on flame properties and soot volume fraction were investigated numerically in a laminar coflow ethylene/air diffusion flame at atmospheric pressure by solving the fully elliptic conservation equations and using a detailed C₂ reaction mechanism including PAH up to pyrene and detailed thermal and transport properties. Thermal radiation was calculated using the discrete-ordinates method and a statistical narrow-band correlated-k based wide band model for the absorption coefficients of CO₂ and H₂O. Soot formation was modeled using a PAH based inception model and the HACA mechanism for surface growth and oxidation. Addition of water vapor significantly reduces radiation heat loss from the flame primarily through reduced soot loading and flame temperature. The added water vapor affects soot formation and flame properties through not only dilution and thermal effects, but also through chemical effect. The chemical effect is as significant as the dilution and thermal effects. The primary pathway for the chemical effect of water vapor is the reverse reaction of OH + H₂ ↔ H + H₂O. Our numerical results confirm that the reduced H radical concentration leads to lower PAH concentrations and consequently lower soot inception rates. In contrast, the radiation effect due to the added water vapor was found to have a minor influence on both flame structure and soot formation in the laminar diffusion flame investigated.     

Conditional analysis of lifted hydrogen jet diffusion flame experimental data and comparison to laminar flame solutions

Simultaneous point measurements of temperature, mixture fraction, major species, and OH concentrations in a lifted turbulent hydrogen jet flame are reprocessed to obtain the Favre average and conditional mean profiles. Large discrepancies between the Favre average and the ensemble average temperature, H₂O, and OH mole fractions are found at the lifted flame base, due to density weighting of fairly large samples of unreacted mixtures. Conditional statistics are used to reveal the reaction zone structure in mixture fraction coordinates. The cross-stream dependence of conditional reactive scalars, which is most notable at the lifted flame base and decreases to negligible levels with increasing streamwise positions, could be attributed to radial differences in both the Damköhler number and the level of partial premixing. Conditional results indicate that the lifted flame is stabilized at the outer region of the jet characterized by low strain rates and lean mixtures. Comparison of the measured conditional mean OH vs H₂O with a series of stretched laminar partially premixed flame and diffusion flame calculations reveals that strong partial premixing takes place at the lifted flame base and the strain rates vary from a=14,000 to 100 s⁻¹. The level of partial premixing and the strain rate decrease with increasing downstream locations. The range of estimated scalar dissipation rates (χ≈1–0.13 s⁻¹) at a further downstream location (x/D=33.3) is in agreement with reported values and the flame composition reaches an equilibrium condition at x/D=194.4. These results combined with previously reported data provide a benchmark data set for evaluation and refinement of turbulent combustion models for lifted hydrogen jet flame predictions.     

Experimental and kinetic investigation on the effects of hydrogen additive on laminar premixed methanol–air flames

The effects of hydrogen addition on laminar premixed methanol–air flames were studied both experimentally and numerically. To achieve this, a constant volume chamber (CVC) and the premix code in CHEMKIN were used. During the experiments, the equivalence ratios (ϕ) and hydrogen mole fractions ($X_h$) were set to 0.6 to 1.8 and 0%–100%, respectively. In addition, initial environmental conditions were set to 375 K and 1 atm. The results indicate that the laminar flame speed (LFS) and burning velocity (LBV) both increase when more hydrogen is added into the methanol–air mixtures. For premixed methanol–air flames, the Markstein length ($L_b$) decreases monotonically with an increase in the equivalence ratio; however, when the hydrogen fraction is greater than 40%, an increasing trend in the Markstein length is presented as the mixtures move toward the fuel-rich side. The variation in Markstein length is non-monotonic with the hydrogen fraction. A kinetics analysis indicates that methanol is mainly consumed by the dehydrogenation reaction caused from the impact of the active free radicals (OH and H). Reactions involving active free radicals and light intermediate species have the highest sensitivity and contribute the most to the propagation of a laminar flame. Therefore, the promotion effect of hydrogen additive is due to an enhancement in the radical pooling of H, OH, and O. The chain branching reaction R5 (O₂ + H = O + OH) is essential for the geometric growth of free radicals. In addition, the amount of formaldehyde decreases owing to the hydrogen blending.     

Lift-off of jet diffusion flame in sub-atmospheric pressures: An experimental investigation and interpretation based on laminar flame speed

This paper reveals lift-off behavior of jet diffusion flames in sub-atmospheric pressures less than 100 kPa, in view of that the current knowledge on this topic is limited for normal pressure conditions. Physically, the variation of ambient pressure may have significant influence on the lift-off behavior of jet diffusion flames due to the change of some critical parameters such as laminar flame speed. In this work, experiments are conducted in a large pressure-controllable chamber of 3 m (width) × 2 m (length) × 2 m (height) at different sub-atmospheric pressures of 60 kPa, 70 kPa, 80 kPa, 90 kPa as well as at normal pressure of 100 kPa. Axisymmetric turbulent jet diffusion flames are produced by nozzles with diameters of 4 mm, 5 mm and 6 mm using propane as fuel. It is revealed that the lift-off height increases as the pressure decreases and being much higher than that in normal pressure condition. The laminar flame speed with its dependency on pressure is introduced to interpret such behavior based on classic Kalghatgi model. It is found theoretically that the lift-off height has a power law dependency on pressure by P¹⁻ⁿ, where n is overall reaction order of the fuel which is usually larger than 1 indicating a negative power law function with pressure (for example p^−0.75 for propane as n = 1.75) as well verified by the experimental correlation. Finally, a global model is proposed by including such pressure dependency function into the Kalghatgi model, which is shown to well collapse the experimental results of lift-off heights of different sub-atmospheric pressures.     

Investigation of the effect of molecular structure on sooting tendency in laminar diffusion flames at elevated pressure

A specially designed High Pressure Vessel and Burner and fueling system (called “doped flame”) are presented in this paper. This setup allows for soot measurements in laminar diffusion flames of liquid fuel blends at elevated pressure. Fuels with two typical molecular structures, namely linear and saturated cyclic hydrocarbons, are examined in both non-oxygenated (n-hexane (C₆H₁₄) and cyclohexane (C₆H₁₂)) and oxygenated form (1-hexanol (C₆H₁₄O) and cyclohexanol (C₆H₁₂O)). All compounds are blended into n-heptane. Focus of the research is on soot volume fraction at elevated pressure in the range of 1.5–2.0 bar. Sooting tendency is evaluated by means of Laser Induced Incandescence (LII) with Line of Sight Attenuation calibration (LOSA), and the data suggests that soot is more prevalent for cyclic structures relative to their linear counterparts.     

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.     

Numerical study of the physical and chemical effects of hydrogen addition on laminar premixed combustion characteristics of methane and ethane

Methane and ethane are taken as the research objects. Using H₂ as diluent, based on Chemkin II/Premix Code and modified detailed chemical reaction mechanism: GRI 3.0*-Mech (introducing three hypothetical substances of FH₂, FO₂ and FN₂), the physical and chemical effects of hydrogen on laminar burning velocities (LBVs), adiabatic flame temperatures (AFTs), net heat release rates (NHRRs) and elementary reactions responsible for temperature changes of two alkanes under different equivalence ratios were analyzed and determined. Results showed that the chemical effect of H₂ promotes the LBVs and ATFs of methane and ethane, while the physical effect decreases the two parameters. In addition, the physical effects of H₂ inhibit the chemical reactions of methane and ethane, resulting in the decrease of NHRRs. The chemical effect of H₂ accelerates the process of chemical reaction and obviously increases the NHRRs. The two most vital elementary reactions that promote the temperature rise of methane and ethane are H + O₂ <=> OH + O and CO + OH <=> H + CO₂. The important reactions responsible for inhibiting the temperature rise are H + CH₃(+M) <=> CH₄(+M) and H + O₂ + H₂O <=> HO₂ + H₂O.