Application of an enhanced PAH growth model to soot formation in a laminar coflow ethylene/air diffusion flame

A recently developed chemical kinetic scheme for C₂ fuel combustion with PAH growth has been implemented in a parallelized coflow flame solver. The reaction mechanism has been developed to include almost all reasonably well-established reaction classes for aromatic ring formation and soot particle precursor molecular weight growth. The model has recently been validated for zero- and one-dimensional premixed flame systems [N.A. Slavinskaya, P. Frank, Combust. Flame 156 (2009) 1705–1722] and has now been updated and extended to a sooting ethylene/air diffusion flame in the coflow geometry. Updates to the mechanism reflect the latest advances in the literature and address numerical stiffness that was present in diffusion flame systems. The chemical kinetic mechanism has been coupled to a sectional aerosol dynamics model for soot growth, considering PAH-based inception and surface condensation, surface chemistry (growth and oxidation), coagulation, and fragmentation. The sectional model predicts the soot aggregate number density and the number of primary particles per aggregate in each section, so as to yield information on particle size distribution and structure. Flame simulation data for the present mechanism is compared to data computed using two other reaction schemes [J. Appel, H. Bockhorn, M. Frenklach, Combust. Flame 121 (2000) 122–136; N.M. Marinov, W.J. Pitz, C.K. Westbrook, A.M. Vincitore, M.J. Castaldi, S.M. Senkan, Combust. Flame 114 (1998) 192–213]. The computed data are also compared to numerous experimental data sets. Whereas the fuel oxidation chemistry in all three mechanisms are essentially the same, the PAH growth pathways vary considerably. It is shown that soot concentrations on the wings of the flame (where soot formation is dominated by surface chemistry) can be predicted with two of the three mechanisms. However, only the present mechanism with its enhanced PAH growth routes can also predict the correct order of magnitude of soot volume fraction in the low-sooting, inception-dominated, central region of the flame. In applying this chemical mechanism, the parameter α, which describes the portion of soot surface sites that are available for chemical reaction, has been reduced to a theoretically acceptable range, thus improving the quality of the model.     

Studies of C4 and C10 methyl ester flames

The oxidation of three model biodiesel fuels, namely methyl butanoate (C₅H₁₀O₂, CAS No. 623-42-7), methyl crotonate (C₅H₈O₂, CAS No. 623-43-8), and methyl decanoate (C₁₁H₂₂O₂, CAS No. 110-42-9) was investigated in laminar premixed and non-premixed flames. The experiments were conducted in the counterflow configuration at atmospheric pressure, for a wide range of equivalence or inert-dilution ratios, and elevated reactant temperatures. Laminar flame speeds and local extinction strain rates were determined by measuring the flow velocities using digital particle image velocimetry. The experimental data were compared against those derived for flames of n-alkanes of similar carbon number, in order to assess the effects of saturation, the length of carbon chain, and the presence of the ester group. Several recent chemical kinetic models were tested against the experimental data, and major differences were identified and assessed. The accuracy of the Lennard–Jones potential parameters assigned to the methyl esters in the transport databases of the different models was evaluated and new values were estimated. Insight was provided into the high-temperature kinetic pathways of methyl esters in flame environments. Additionally, the reduced sooting propensity of methyl ester flames compared to n-alkane flames was investigated computationally.     

Isomer-specific combustion chemistry in allene and propyne flames

A combined experimental and modeling study is performed to clarify the isomer-specific combustion chemistry in flames fueled by the C₃H₄ isomers allene and propyne. To this end, mole fraction profiles of several flame species in stoichiometric allene (propyne)/O₂/Ar flames are analyzed by means of a chemical kinetic model. The premixed flames are stabilized on a flat-flame burner under a reduced pressure of 25 Torr (=33.3 mbar). Quantitative species profiles are determined by flame-sampling molecular-beam mass spectrometry, and the isomer-specific flame compositions are unraveled by employing photoionization with tunable vacuum-ultraviolet synchrotron radiation. The temperature profiles are measured by OH laser-induced fluorescence. Experimental and modeled mole fraction profiles of selected flame species are discussed with respect to the isomer-specific combustion chemistry in both flames. The emphasis is put on main reaction pathways of fuel consumption, of allene and propyne isomerization, and of isomer-specific formation of C₆ aromatic species. The present model includes the latest theoretical rate coefficients for reactions on a C₃H₅ potential [J.A. Miller, J.P. Senosiain, S.J. Klippenstein, Y. Georgievskii, J. Phys. Chem. A 112 (2008) 9429-9438] and for the propargyl recombination reactions [Y. Georgievskii, S.J. Klippenstein, J.A. Miller, Phys. Chem. Chem. Phys. 9 (2007) 4259-4268]. Larger peak mole fractions of propargyl, allyl, and benzene are observed in the allene flame than in the propyne flame. In these flames virtually all of the benzene is formed by the propargyl recombination reaction.     

Ignition of C3 oxygenated hydrocarbons and chemical kinetic modeling of propanal oxidation

The relative high temperature ignition behavior of selected C3 oxygenated hydrocarbons, propanal (propionaldehyde, PAL or CH₃CH₂CHO), acetone (propanone or AC), isopropanol (iPOH), and ethyl formate (EF), is studied behind reflected shock waves. An ignition delay time correlation for methyl acetate (MA) from a previous study is also employed in the comparison. This study reveals the influence of different functional groups on the oxidation of the hydrocarbons. Isomer effects are also revealed for the ketone, acetone, and the aldehyde, propanal, with propanal portraying shorter ignition delay times than acetone. In the same manner, using the correlation for methyl acetate, the ester isomers, methyl acetate and ethyl formate, are compared. In this case, ethyl formate shows shorter ignition delay times than methyl acetate. Generally, methyl acetate, isopropanol (iPOH) and acetone (AC) portray comparable ignition behavior. This is thought to be owing to the fact that they are characterized by non-terminally bonded oxygen atoms. They all have terminal methyl groups, though the number of oxygen atoms and the types of carbon–oxygen bonds differ in these three fuels. Propanal and ethyl formate have similar ignition delays that are shorter than those of the other three fuels, due to their ability to form reactive ethyl radicals. The measured ignition delay times are compared to simulated delay times using existing mechanisms for acetone, isopropanol and small alkyl esters. Whereas there is reasonable agreement at high pressures between experiments and modeling results for the small alkyl esters, methyl acetate and ethyl formate, there are deviations for acetone and isopropanol. However, the mechanisms for the latter molecules perform better at lower pressures. The ignition data in this study could be useful for further optimization of the existing models. Furthermore, a chemical kinetic mechanism for propanal oxidation is proposed and good agreement between the proposed model and experiment is observed. However, further validation against a wider set of combustion experiments is recommended. This study contributes towards better understanding of the relative oxidation behavior of C3 oxygenated hydrocarbons which are relevant in combustion processes as fuel components, important intermediate species and, in lower concentrations, as exhaust products.     

Methyl butanoate inhibition of n-heptane diffusion flames through an evaluation of transport and chemical kinetics

The extinction limits of methyl butanoate, n-heptane, and methyl butanoate/n-heptane diffusion flames have been measured as a function of fuel mole fraction with nitrogen dilution in counterflow with air. On a mole fraction basis, methyl butanoate diffusion flames are observed to have a much lower extinction strain rate than n-heptane diffusion flames and the extinction strain rate of n-heptane/methyl butanoate diffusion flames is observed to increase significantly as the n-heptane fraction is increased.Based on previous works, detailed chemical kinetic models to describe the high temperature oxidation of these fuel mixtures are assembled, tested and reduced. When the transport properties of ester species are re-evaluated by means of a thorough literature review, numerical computations of extinction generally reproduce experimental results for the pure fuels as well as for mixtures. An in-depth analysis of the kinetic model computations reveals that the extinction behaviour of both fuels is due to (1) fuel energy content affects and (2) the chemical kinetic potential of each fuel to produce the hydroperoxy radical. Comparatively, in n-heptane flames reactive ethyl radicals and ethylene are the major intermediates formed, but in methyl butanoate flames the major intermediates are formyl radicals and formaldehyde. In all flames studied, increased strain rates affect an increased interaction of formyl and/or vinyl radicals with molecular oxygen leading to a transition from hydrogen atom production at low strain rates, to the production of large quantities of the hydroperoxy radical at higher strain rates. The formation of the hydroperoxy radical induces extinction in each flame by directly interfering with the important radical chain branching and exothermic elementary reactions of H atoms and OH radicals that are dominant in weakly strained flames.It is postulated that the similar inhibitive effect of methyl butanoate fuelled flames will also be observed for more biodiesel like, larger n-alkyl esters when compared to equivalent n-alkanes. The diffusive extinction limits of methyl decanoate diffusion flames are also measured and show reactivity comparable to n-heptane diffusion flames by a molar comparison.     

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.     

Computational and experimental study of the effects of adding dimethyl ether and ethanol to nonpremixed ethylene/air flames

Two sets of axisymmetric laminar coflow flames, each consisting of ethylene/air nonpremixed flames with various amounts (up to 10%) of either dimethyl ether (CH₃-O-CH₃) or ethanol (CH₃-CH₂-OH) added to the fuel stream, have been examined both computationally and experimentally. Computationally, the local rectangular refinement method, which incorporates Newton's method, is used to solve the fully coupled nonlinear conservation equations on solution-adaptive grids for each flame in two spatial dimensions. The numerical model includes C6 chemical kinetic mechanisms with up to 59 species, detailed transport, and an optically thin radiation submodel. Experimentally, thermocouples are used to measure gas temperatures, and mass spectrometry is used to determine concentrations of over 35 species along the flame centerline. Computational results are examined throughout each flame, and validation of the model occurs through comparison with centerline measurements. Very good agreement is observed for temperature, major species, and several minor species. As the level of additive is increased, temperatures, some major species (CO₂, C₂H₂), flame lengths, and residence times are essentially unchanged. However, peak centerline concentrations of benzene (C₆H₆) increase, and this increase is largest when dimethyl ether is the additive. Computational and experimental results support the hypothesis that the dominant pathway to C₆H₆ formation begins with the oxygenates decomposing into methyl radical (CH₃), which combines with C2 species to form propargyl (C₃H₃), which reacts with itself to form C₆H₆.     

Formic acid–Formate blended solution: A new fuel system with high oxidation activity

Formic acid and formate have been proven to be among the most promising fuels for direct liquid fuel cells. To take advantage of both formic acid and formate, the oxidation activity of a formic acid–formate blended solution was studied using cyclic voltammetry, Tafel polarization measurements, and chromoamperometry. With increased concentration of formate, the oxidation potential of formic acid shifted to the negative direction, and the peak current significantly increased, suggesting a greatly enhanced oxidation activity. The transition of formic acid from an indirect to a direct oxidation route was also observed. The enhanced oxidation activity indicated that a formic acid–formate blended solution is a promising fuel system for direct liquid fuel cells. Hence, a new concept of fuel cell, i.e., direct formic acid–formate blended fuel cell, was proposed.     

Experimental and modeling studies of C2H4/O2/Ar, C2H4/methylal/O2/Ar and C2H4/ethylal/O2/Ar rich flames and the effect of oxygenated additives

The addition of dimethoxymethane (DMM or methylal) and diethoxymethane (DEM or ethylal) to a rich ethylene/oxygen/argon flame has been investigated by measuring the depletion of soot precursors. Three rich premixed ethylene/oxygen/argon (with and without added methylal or ethylal) flat flames have been stabilized at low-pressure (50 mbar) on a Spalding–Botha type burner with the same equivalence ratio of 2.50. Identification and monitoring of signal intensity profiles of species within the flames have been carried out by using molecular beam mass spectrometry (M.B.M.S.). The replacement of some C₂H₄ by C₃H₈O₂ or C₅H₁₂O₂ is responsible for a decrease of the maximum mole fractions of the detected intermediate species. This phenomenon is noticeable for C₂–C₄ intermediates and becomes more effective for C₅–C₁₀ species, mainly when C₃H₈O₂ added.A new kinetic model has been elaborated and contains 546 reactions and 107 chemical species in order to simulate the three investigated flames: C₂H₄/O₂/Ar, C₂H₄/DMM/O₂/Ar and C₂H₄/DEM/O₂/Ar. The reaction mechanism well reproduces experimental mole fraction profiles of major and intermediate species, and underlines the effect of methylal and ethylal addition on species concentration profiles for these flames.     

Effect of hydrogen on hydrogen–methane turbulent non-premixed flame under MILD condition

Energy crises and the preservation of the global environment are placed man in a dilemma. To deal with these problems, finding new sources of fuel and developing efficient and environmentally friendly energy utilization technologies are essential. Hydrogen containing fuels and combustion under condition of the moderate or intense low-oxygen dilution (MILD) are good choices to replace the traditional ones. In this numerical study, the turbulent non-premixed CH₄+H₂ jet flame issuing into a hot and diluted co-flow air is considered to emulate the combustion of hydrogen containing fuels under MILD conditions. This flame is related to the experimental condition of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147–1154]. In general, the modelling is carried out using the EDC model, to describe turbulence–chemistry interaction, and the DRM-22 reduced mechanism and the GRI2.11 full mechanism to represent the chemical reactions of H₂/methane jet flame. The effect of hydrogen content of fuel on flame structure for two co-flow oxygen levels is studied by considering three fuel mixtures, 5%H₂+95%CH₄, 10%H₂+90%CH₄ and 20% H₂+80%CH₄(by mass). In this study, distribution of species concentrations, mixture fraction, strain rate, flame entrainment, turbulent kinetic energy decay and temperature are investigated. Results show that the hydrogen addition to methane leads to improve mixing, increase in turbulent kinetic energy decay along the flame axis, increase in flame entrainment, higher reaction intensities and increase in mixture ignitability and rate of heat release.     

Modeling fuel NOx formation from combustion of biomass-derived producer gas in a large-scale burner

This study investigates the characteristics of fuel NO_x formation resulting from the combustion of producer gas derived from biomass gasification using different feedstocks. Common industrial burners are optimized for using natural gas or coal-derived syngas. With the increasing demand in using biomass for power generation, it is important to develop burners that can mitigate fuel NO_x emissions due to the combustion of ammonia, which is the major nitrogen-containing species in biomass-derived gas. In this study, the combustion process inside the burner is modeled using computational fluid dynamics (CFD) with detailed chemistry. A reduced mechanism (36 species and 198 reactions) is developed from GRI 3.0 in order to reduce the computation time. Combustion simulations are performed for producer gas arising from different feedstocks such as wood gas, wood + 13% DDGS (dried distiller grain soluble) gas and wood + 40% DDGS gas and also at different air equivalence ratios ranging from 1.2 to 2.5. The predicted NO_x emissions are compared with the experimental data and good levels of agreement are obtained. It is found out that NO_x is very sensitive to the ammonia content in the producer gas. Results show that although NO–NO₂ interchanges are the most prominent reactions involving NO, the major NO producing reactions are the oxidation of NH and N at slightly fuel rich conditions and high temperature. Further analysis of results is conducted to determine the conditions favorable for NO_x reduction. The results indicate that NO_x can be reduced by designing combustion conditions which have fuel rich zones in most of the regions. The results of this study can be used to design low NO_x burners for combustion of gas mixtures derived from gasification of biomass. One suggestion to reduce NO_x is to produce a diverging flame using a bluff body in the flame region such that NO generated upstream will pass through the fuel rich flame and be reduced.     

Kinetic modeling study of ethanol and dimethyl ether addition to premixed low-pressure propene–oxygen–argon flames

The chemical composition of flames of mixed hydrocarbon–oxygenate fuels was examined systematically for a series of laminar, premixed low-pressure propene–oxygen–argon flames blended with ethanol or dimethyl ether (DME). All flames were established at a carbon-to-oxygen ratio of C/O = 0.5 at 40 mbar. Propene was replaced incrementally by either additive, so that the entire range from pure propene to pure ethanol or pure DME was accessible. Experimental results have been reported previously (J. Wang et al., J. Chem. Phys. A 112 (2008) 9255–9265), including temperature profiles measured with laser-induced fluorescence (LIF) and quantitative mole fraction profiles for a large number of species obtained from molecular-beam mass spectrometry (MBMS), using electron ionization (EI) and vacuum-ultraviolet (VUV) photoionization (PI). The effects of oxygenate addition to the propene base flame were seen to result in interesting differences, especially regarding trends to form aldehydes. The entire flame series is now analyzed with a comprehensive kinetic model that combines the chemistries of propene, ethanol, and DME combustion. The flames of pure fuels are also compared with the predictions of different detailed mechanisms taken from the literature. Quantitative comparison of C₁- to C₆-species from this model with the measurements is provided. Major trends of propene replacement by the oxygenates are reproduced in quantitative agreement with the experiments, enabling a more detailed understanding of the combined reaction sequences in such fuel blends.     

A chemical mechanism for low to high temperature oxidation of n-dodecane as a component of transportation fuel surrogates

Using surrogate fuels in lieu of real fuels is an appealing concept for combustion studies. A major limitation however, is the capability to design compact and reliable kinetic models that capture all the specificities of the simpler, but still multi-component surrogates. This task is further complicated by the fairly large nature of the hydrocarbons commonly considered as potential surrogate components, since they typically result in large detailed reaction schemes. Towards addressing this challenge, the present work proposes a single, compact, and reliable chemical mechanism, that can accurately describe the oxidation of a wide range of fuels, which are important components of surrogate fuels. A well-characterized mechanism appropriate for the oxidation of smaller hydrocarbon species [G. Blanquart, P. Pepiot-Desjardins, H. Pitsch, Chemical mechanism for high temperature combustion of engine relevant fuels with emphasis on soot precursors, Combust. Flame 156 (2009) 588–607], and several substituted aromatic species [K. Narayanaswamy, G. Blanquart, H. Pitsch, A consistent chemical mechanism for the oxidation of substituted aromatic species, Combust. Flame 157 (10) (2010) 1879–1898], ideally suited as a base to model surrogates, has now been extended to describe the oxidation of n-dodecane, a representative of the paraffin class, which is often used in diesel and jet fuel surrogates. To ensure compactness of the kinetic scheme, a short mechanism for the low to high temperature oxidation of n-dodecane is extracted from the detailed scheme of Sarathy et al. [S. M. Sarathy, C. K.Westbrook, M. Mehl, W. J. Pitz, C. Togbe, P. Dagaut, H. Wang, M. A. Oehlschlaeger, U. Niemann, K. Seshadri, Comprehensive chemical kinetic modeling of the oxidation of 2-methylalkanes from C₇ to C₂₀, Combust. Flame 158 (12) (2011) 2338–2357] and integrated in a systematic way into the base model. Rate changes based on recent rate recommendations from literature are introduced to the resulting chemical mechanism in a consistent manner, which improve the model predictions. Extensive validation of the revised kinetic model is performed using a wide range of experimental conditions and data sets.     

Experimental and chemical kinetic modeling study of small methyl esters oxidation: Methyl (E)-2-butenoate and methyl butanoate

This study examines the effect of unsaturation on the combustion of fatty acid methyl esters (FAME). New experimental results were obtained for the oxidation of methyl (E)-2-butenoate (MC, unsaturated C₄ FAME) and methyl butanoate (MB, saturated C₄ FAME) in a jet-stirred reactor (JSR) at atmospheric pressure under dilute conditions over the temperature range 850-1400 K, and two equivalence ratios (Φ=0.375,0.75) with a residence time of 0.07 s. The results consist of concentration profiles of the reactants, stable intermediates, and final products, measured by probe sampling followed by on-line and off-line gas chromatography analyses. The oxidation of MC and MB in the JSR and under counterflow diffusion flame conditions was modeled using a new detailed chemical kinetic reaction mechanism (301 species and 1516 reactions) derived from previous schemes proposed in the literature. The laminar counterflow flame and JSR (for ?=1.13) experimental results used were from a previous study on the comparison of the combustion of both compounds. Sensitivity analyses and reaction path analyses, based on rates of reaction, were used to interpret the results. The data and the model show that MC has reaction pathways analogous to that of MB under the present conditions. The model of MC oxidation provides a better understanding of the effect of the ester function on combustion, and the effect of unsaturation on the combustion of fatty acid methyl ester compounds typically found in biodiesel.     

Shock tube study of methanol,methyl formate pyrolysis: CH3OH and CO time-history measurements

Methanol and methyl formate pyrolysis were studied by measuring CH₃OH and CO concentration time-histories behind reflected shock waves. In the study of methanol pyrolysis, experimental conditions covered temperatures of 1266–1707 K, pressures of 1.1–2.5 atm, and initial fuel concentrations of 1% and 0.2% with argon as the bath gas. Detailed comparisons of CH₃OH and CO concentration profiles with the predictions of the detailed kinetic mechanism of Li et al. (2007) [8] were made. Such comparisons combined with sensitivity analysis identified the need to include an additional methanol decomposition channel, CH₃OH ? CH₂(S) + H₂O, into the mechanism. Pathway and sensitivity analyses for methanol decomposition were performed, leading to rate constant recommendations both for CH₃OH unimolecular decomposition and H-abstraction reactions with improved model performance. In the study of methyl formate pyrolysis, methanol concentration time-histories were measured at temperatures over the range of 1261–1524 K, pressures near 1.5 atm, and initial fuel concentrations of 1% with argon as the bath gas. Our current work, and CO time-histories from previous work, indicates that the Dooley et al. (2010) [3] model is able to accurately simulate most species concentrations in shock tube experiments at early times. However, model improvement is still needed to match the CH₃OH and CO time-histories at later times. Incorporation of the modified rate constants in the methanol sub-mechanism leads to good predictions of the full methanol time-histories at all temperatures. The kinetic implications of some aspects of the CO time-histories and suggestions for further improving the predictive capabilities of these mechanisms are discussed. The current results are the first quantitative measurements of CH₃OH time-histories in shock tube experiments, and hence are a critical step toward understanding of the chemical kinetics of oxygenates.     

Numerical investigations on tri-fuel chemical kinetics of hydrogen + Methane +LPG/air mixtures using reduced skeletal mechanism

The combustion and ignition characteristics of three fuels with different reactivities have been investigated by a reduced chemical kinetic model. In the present work, the chemical kinetics of conventional single fuel and binary fuel, relevant to gas-turbine engines, are extended and attempted to explore in the tri-fuel (TF) context, with the help of TF blends of LPG + CH₄+H₂ at the pressure and temperature range of 1–20 atm and 900–2000 K, respectively. The blending of hydrogen with hydrocarbon fuels improves flame propagation, reduces emissions, and increases the combustion performance of the engine. A detailed study is conducted to explore the characteristics of TF mixture over a wide range of operating conditions by considering eight different test mixtures (M1-M8). The test mixtures (M2 to M4) contain higher hydrogen content and thus hydrogen kinetics will tend to dominate, while test mixtures (M6 to M8) contain a higher concentration of hydrocarbons, thus the methyl radical chemistry plays a prominent role in the oxidation process. Such contrasting trends were further explored by extensive chemical kinetic modeling with the help of the reduced USC Mech_50 species model from our previous work [1] to analyze the ignition delay time, laminar flame speed, flame temperature, and heat release rate characteristics. In addition, the reaction pathway analysis through sensitivity analysis of OH and CO radical, and flow rate sensitivity analysis has also been conducted to highlight the essential chemical reactions which play a crucial role in auto-ignition, combustion, and emissions characteristics of TF blends.     

A numerical study on the effects of H2 addition in non-premixed turbulent combustion of C3H8–H2–N2 mixture using a steady flamelet approach

This study has been implemented in two sections. At first, the turbulent jet flame of DLR-B is simulated by combining the k–ε turbulence model and a steady flamelet approach. The DLR-B flame under consideration has been experimentally investigated by Meier et al. who obtained velocity and scalar statistics. The fuel jet composition is 33.2% H₂, 22.1% CH₄ and 44.7% N₂ by volume. The jet exit velocity is 63.2 m/s resulting in a Reynolds number of 22,800. Our focus in the first part is to validate the developed numerical code. Comparison with experiments showed good agreement for temperature and species distribution. At the second part, we exchanged methane with propane in the fuel composition whilst maintaining all other operating conditions unchanged. We investigated the effect of hydrogen concentration on C₃H₈–H₂–N₂ mixtures so that propane mole fraction extent is fixed. The hydrogen volume concentration rose from 33.2% up to 73.2%. The achieved consequences revealed that hydrogen addition produces elongated flame with increased levels of radiative heat flux and CO pollutant emission. The latter behavior might be due to quenching of CO oxidation process in the light of excessive cold air downstream of reaction zone.     

A reduced mechanism for ethylene/methane mixtures with excessive NO enrichment

A detailed mechanism for methane–ethylene mixtures enriched with excessive amount of NO was systematically reduced for efficient numerical simulations of flames in arc-heated co-flowing air. Methane and ethylene were selected as the surrogate fuel in the present study due to their drastically different features of ignition and extinction properties and flame propagation speeds, such that the mixtures of them may be utilized to mimic practical hydrocarbon fuels with various kinetic properties in experiments. The recently released USC Mech-II for C1–C4 was grafted with the NO_x sub-mechanism in GRI-Mech 3.0 with updated reaction parameters for prompt NO formation. The resulting detailed mechanism with 129 species and 900 reactions was first validated against experiments involving NO_x enrichment and reasonably good agreements were observed. The detailed mechanism was then employed as the starting mechanism for the reduction. A skeletal mechanism with 44 species and 269 reactions was derived using the methods of directed relation graph (DRG) and DRG-aided sensitivity analysis (DRGASA); a 39-species reduced mechanism with 35 semi-global reaction steps was further obtained using the linearized quasi steady state approximations (LQSSA). Five species related to prompt NO were retained in the reduced mechanism because of their significant impacts on the fuel oxidation. The reduced mechanism closely agrees with the detailed mechanism for ignition and extinction of homogenous mixtures, as well as selected 1-D flames over a wide range of parameters with NO concentrations between 0% and 3%. The observed worst-case relative error of the reduction is approximately 20%. The reduced mechanism was further validated with experiments involving excessive NO_x enrichment.     

Experimental and modeling study of the thermal decomposition of methyl decanoate

The experimental study of the thermal decomposition of methyl decanoate was performed in a jet-stirred reactor at temperatures ranging from 773 to 1123 K, at residence times between 1 and 4 s, at a pressure of 800 Torr (106.6 kPa) and at high dilution in helium (fuel inlet mole fraction of 0.0218). Species leaving the reactor were analyzed by gas chromatography. Main reaction products were hydrogen, carbon oxides, small hydrocarbons from C₁ to C₃, large 1-olefins from 1-butene to 1-nonene, and unsaturated esters with one double bond at the end of the alkyl chain from methyl-2-propenoate to methyl-8-nonenoate. At the highest temperatures, the formation of polyunsaturated species was observed: 1,3-butadiene, 1,3-cyclopentadiene, benzene, toluene, indene, and naphthalene. These results were compared with previous ones about the pyrolysis of n-dodecane, an n-alkane of similar size. The reactivity of both molecules was found to be very close. The alkane produces more olefins while the ester yields unsaturated oxygenated compounds.A detailed kinetic model for the thermal decomposition of methyl decanoate has been generated using the version of software EXGAS which was updated to take into account the specific chemistry involved in the oxidation of methyl esters. This model contains 324 species and 3231 reactions. It provided a very good prediction of the experimental data obtained in jet-stirred reactor. The formation of the major products was analyzed. The kinetic analysis showed that the retro-ene reactions of intermediate unsaturated methyl esters are of importance in low reactivity systems.     

GC-MS determination of low hydrocarbon species (C1–C6) from a diesel partial oxidation reformer

The partial oxidation (POx) reforming of Ultra Low Sulphur-Diesel (ULSD), rapeseed methyl ester (RME) - biodiesel and Fischer–Tropsch synthetic diesel fuels (SD) were studied by using a fixed-bed reactor. The ease of reforming the three fuels was first examined at different O/C feed ratios at constant gas hourly space velocity (GHSV) of 35 k h⁻¹ over a prototype monolith catalyst (1%Rh/CeO₂–ZrO₂). The hydrocarbon species (C₁–C₆) produced in the reformer were analyzed using direct gas injection gas chromatography mass spectrometry (GC-MS). Under the same O/C ratios for 35 k h⁻¹ the fuels conversion and process efficiency was dependent on the fuel type, and followed the general trend: SD > biodiesel > ULSD. The GC-MS analysis shows that both, biodiesel and ULSD diesel produced significantly higher amounts of alkenes compared to SD fuel. Fuel with relatively high aromatics content such diesel can be efficiently reformed to syngas over the catalyst used in this study but the reformer operating range (e.g. O/C ratio and space velocity) is limited compared to paraffinic fuels such as FT-SD. At increased GHSV of 45 k h⁻¹ and O/C = 1.75, the diesel fuel conversion efficiency to syngas (H₂ and CO) was improved significantly and the formation of intermediate species such as methane, ethylene, and propylene was reduced considerably as a result of the increased peak reaction temperatures. The reduced HC species and increased H₂ concentration in the reactor product gas from the reforming of FT-SD fuel can provide significant advantages to the IC engine applications.