Konnov Mechanism Research Articles

Update of the present decomposition mechanisms of ammonia: A combined ReaxFF, DFT and chemkin study

Ammonia, a promising carbon-free fuel, offers significant potential for reducing carbon emissions. The decomposition of NH3 is crucial for improving NH3 combustion, yet its mechanisms remain unclear. This study uses ReaxFF simulations, density functional theory (DFT), and Chemkin calculations to explore NH3 decomposition and refine the Konnov mechanism. Our results show that the NH3 to N2 conversion is highly temperature-dependent. At high temperatures, the pathway is NH3→NH2→N2H4→N2H2→N2. As temperature decreases, the N2H pathway becomes more significant, shifting the reaction to NH3→NH4→NH2→N2H→N2 pathway. Additionally, temperature affects H dissociation: at lower temperatures, NH3 to NH2 follows NH3→NH4→NH2, while the NH4 pathway's influence diminishes with rising temperatures. The updated Konnov mechanism improves NH3 decomposition prediction accuracy by about 72.79% and enhances predictions of decomposition rates and intermediate formations (NH, NH2, NH4). This research advances the understanding of NH3 decomposition mechanisms and is vital for optimizing burners for efficient and stable NH3 combustion.

Reduced Mechanism for Combustion of Ammonia and Natural Gas Mixtures

A fuel mixture of ammonia and natural gas as a low-carbon alternative for future power generation and transportation is an attractive option. In this work, a 50-species reduced mechanism, NH3NG, suitable for computational fluid dynamics simulations (CFD), is developed for ammonia–natural gas cofiring while addressing important emission issues, such as the formation of nitrogen oxides (NOx), soot, carbon monoxide, and unburnt methane/ammonia. The adoption of reduced mechanisms is imperative not only for saving computer storage and running time but also for numerical convergence for practical applications. The NH3NG reduced mechanism can predict soot emission because it includes soot precursor species. Further, it can handle heavier components in natural gas, such as ethane and propane. The absolute error is 5% for predicting NOx and CO emissions compared to the full Modified Konnov mechanism. Validation with key performance parameters (ignition delay, laminar flame speed, adiabatic temperature, and NOx and CO emissions) indicates that the predictions of the reduced mechanism NH3NG are in good agreement with published experimental data. The average prediction error of 13% for ignition delay is within typical experimental data uncertainties of 10–20%. The predicted adiabatic temperatures are within 1 °C. For laminar flame speed, the R2 between prediction and data is 0.985. NH3NG over-predicts NOx and CO emissions, similar to all other literature methods, but the NOx predictions are closer to the experimental data.

Research on NO generation characteristics of ammonia-premixed flame

Ammonia is a promising fuel with high energy density, accessible storage, and no CO2 production by combustion, but its combustion produces the pollutant NO. In this study, a Bunsen burner experimental bench was selected to investigate the concentration of NO generated by ammonia combustion at different initial oxygen concentrations. Further, the reaction pathways of NO were analyzed in depth, and sensitivity analysis was performed. The results show that the Konnov mechanism has an excellent predictive effect on NO generated by ammonia combustion. In the ammonia-premixed laminar flame at atmospheric pressure, the NO concentration peaked at an equivalence ratio of 0.9. The high initial oxygen concentration enhanced the combustion of ammonia-premixed flame and increased the conversion of NH3 to NO. NO was not only a product but a contribution to the combustion of NH3. As the equivalence ratio increases, NH2 consumes a large amount of NO and reduces NO production. The high initial oxygen concentration enhanced NO production, and the effect was more pronounced at low equivalents. The study results provide theoretical guidance for the utilization of ammonia combustion and pollutant reduction and help to drive the process of ammonia combustion toward practicality.

Study on Laminar Combustion Characteristics of Ammonia/ Hydrogen Premixed Based on Chemical Reaction Kinetics

The combustion characteristics of ammonia/hydrogen premixed laminar flow and the effect of hydrogen on the combustion performance of ammonia fuel were studied. First, the corresponding model of ammonia/ hydrogen premixed laminar combustion is established by using GRI3.0 mechanism, Konnov mechanism, Mei mechanism, Okafor mechanism, and Otomo mechanism respectively. Second, the simulation results are compared with the experimental results. It is found that the Mei mechanism and Okafor mechanism are more suitable for ammonia/ hydrogen premixed laminar combustion. On this basis, the effects of equivalent ratio, hydrogen ratio, and initial temperature on laminar flame velocity, maximum combustion temperature, and NO mole fraction were studied. The results show that the laminar flame velocity, the maximum combustion temperature, and the mole fraction of NO first increase and then decrease with the increase of the equivalent ratio, and the laminar flame velocity reaches the maximum when the equivalent ratio is 1.1. At the same time, with the increase of hydrogen ratio and initial temperature, the maximum combustion temperature increases first and then decreases. The mole fraction of NO increased with the increase of hydrogen ratio and initial temperature. The results show that mixing hydrogen in ammonia can improve the combustion characteristics of ammonia.

Comparison and reduction of the chemical kinetic mechanisms proposed for thermal partial oxidation of methane (TPOX) in porous media

The effectiveness and reducibility of the methane combustion kinetic mechanisms were examined for the TPOX process in a porous medium. To this end, TPOX was successfully simulated using ANSYS CHEMKIN-Pro through a reactor network model composed of perfectly stirred and honeycomb-monolith reactors. The efficacy of six chemical kinetic mechanisms was compared for the equivalence ratios (ERs) ranging from 2.4 to 2.6 with a constant thermal load of 1540 kW/m2. This comparison revealed that Konnov was the most successful mechanism in the prediction of the H2 and CO mole fractions. This mechanism along with the GRI-3.0 and USC-Mech 2.0 mechanisms were then reduced by the direct relation graph with error propagation (DRGEPSA) followed by the full species sensitivity analysis (FSSA). This approach reduced the number of species from 119 to 29 for the Konnov mechanism, from 53 to 23 for the GRI 3.0 mechanism, and from 111 to 34 for the USC-Mech 2.0 mechanism.

Experimental and kinetic modeling study of NO formation in premixed CH4+O2+N2 flames

The nitric oxide (NO) formation in methane (CH 4 ) flames has been widely investigated, with quite a few kinetic mechanisms available in the literature. However, studies have shown that there are often discrepancies between the simulations using various mechanisms and the experimental results. To elucidate reactions leading to these discrepancies, experiments were designed to measure the NO formation in the post flame zone of CH 4 +O 2 +N 2 flames with the oxygen ratio, x O 2 = O 2 /(O 2 +N 2 ), varying from 0.2 to 0.27. The experiments were carried out on a heat flux burner at atmospheric pressure and 298 K using saturated Laser-induced fluorescence. The equivalence ratio, ϕ , was changed from 0.7 to 1.6. The corresponding laminar burning velocity, S L , for each condition was also measured using the heat flux method. A comparison was made for the present experimental data and simulation results using the Konnov, Glarborg, NOMecha 2.0, and San Diego mechanisms, and none of them well reproduced the new NO experimental data for all investigated conditions. Numerical analyses show that the increment of NO mole fraction in stoichiometric and fuel-lean flames when the x O 2 increases is mostly defined by the thermal-NO production, which is found to be over-predicted, especially by the Konnov and San Diego mechanisms. The rate constant of reaction NO+N = N 2 +O was derived as k = 1.529 × 10 13 T − 0.0027 exp ( 185.41 [ c a l / m o l e ] R T ) cm 3 / mol s over 225–2400 K temperature range. The rate constants of four reactions controlling CH mole fraction profiles and prompt-NO formation were updated based on the analysis of the literature data that yields an improved performance of the Konnov mechanism.

A projection procedure to obtain adiabatic flames from non-adiabatic flames using heat flux method

Laminar burning velocity SL at elevated temperature Tu and its temperature dependence coefficient α in SLSL0=(TuTu0)α are important parameters for industrial applications. However, experimental systems with high unburned gas temperatures may encounter pre-dissociation, leading to significant data scattering in the measurements. To negate this, the present work proposes a projection procedure to obtain adiabatic flame parameters at various unburned gas temperatures using non-adiabatic flames on a heat flux burner, by which the preheating can be achieved within much shorter time scale than, e.g., in conventional spherical flame methods, and the advantage of good data consistency in the heat flux method is kept. Burning velocity experiments were carried out with CH4+air atmospheric flames covering Tu = 298–473 K, and the results show good agreement with the proposed projection equations. OH* spontaneous emission profiles were measured, indicating that the projection may extend to other flame characteristics. Uncertainty of the projection process was evaluated and comparisons were made with six popular kinetic mechanisms: GRI-Mech, FFCM-1 mech, Konnov mechanism, Glarborg mechanism, San Diego mechanism and Aramco mechanism. It is found that the simulated coefficients α are higher than experimental data especially at rich conditions; this is also found for literature values of high unburned gas temperature experiments. Possible reasons for this divergence are discussed.

Comparative analysis of detailed and reduced kinetic models for CH4 + H2 combustion

Directed relation graph with error propagation (DRGEP) method combined with extensive validation for 0D, 1D and 2D CFD modeling supported by sensitivity and Rate-Of-Production (ROP) analyses are implemented for comparative study of detailed and reduced kinetic mechanisms for CH4 + H2 combustion. To this end, two detailed kinetic mechanisms, namely AramcoMech 2.0 and recently updated Konnov mechanism, were validated using available measurements of ignition delay times and laminar burning velocities for hydrogen, methane and hydrogen + methane fuel mixtures. For all experimental conditions visited, both detailed mechanisms demonstrated good and close to each other performance. Two-stage DRGEP method and reaction reduction based on computational singular perturbation (CSP) were then implemented to achieve two skeletal models: 25 species and 105 reactions for AramcoMech 2.0 and 27 species and 107 reactions for the Konnov model. The conditions for skeletal models generation cover ɸ = 0.5–2.0, temperature 900–2000 K, and pressure 1–50 bar. Turbulent non-premixed flames of CH4 + H2 in the Jet in Hot Co-flow (JHC) burner for two different oxygen concentrations in a co-flow were modeled using both skeletal models. 2-D RANS simulations with OpenFOAM code of the flame structure using the two skeletal kinetic mechanisms are similar except for the mass fraction of OH and CO. To elucidate the differences between two skeletal mechanisms generated using the same reduction method, extensive validation for 0D, 1D and 2D CFD modeling were supported by sensitivity analysis for detailed and skeletal reaction models. Good agreement between the skeletal and detailed mechanisms was found in top reactions as well as their sensitivity coefficients, which affect auto-ignition process and laminar flame propagation. Further chemical and sensitivity analysis of the structure of laminar flames demonstrate that three important reactions, i.e. CO + OH = CO2 + H, H2 + OH = H + H2O, and CH4 + OH = CH3 + H2O have different rate constants in the Aramco and Konnov models that may contribute to the differences in the prediction of CO concentration profiles. The simulation predictions for CO concentrations are improved for laminar flames and JHC flame by using a 25-species modified version in which these rate constants were taken from the Konnov mechanism. It was noted that DRGEP method applied to different detailed kinetic schemes generate skeletal models with different, non-overlapping lists of retained species. The presence of CH2CHO in the Aramco 25-species skeletal mechanism and its absence in the Konnov 27-species mechanism, and the presence of CH, CH2, CH2CO in the latter and their absence in the former mechanism were analysed and explained using Rate-Of-Production analysis for conditions found in the CFD simulations.

Analysis of air-staged combustion of NH3/CH4 mixture with low NOx emission at gas turbine conditions in model combustors

The NOx formation characteristics have been numerically investigated in the NH3/CH4 fired model combustor consisting of perfectly stirred reactors (PSR) and plug flow reactors (PFR) with 84-species Tian mechanism at gas-turbine relevant conditions of 23 atm and 1873 K at the combustor outlet. Specifically, the impacts of NH3 dilution on NOx formation were first studied in a non-staged model combustor consisting of one PSR representing main combustion zone and one PFR representing post-combustion zone over a wide range of the volumetric ratio of NH3 in the NH3/CH4 fuel. It was found that the NOx emission increases sharply with the NH3 addition mainly through the enhanced HNO pathway in the main reaction zone. Recognizing that the NOx formation via the HNO pathway is limited by the availability of oxygen, the effects of fuel/air equivalence ratio on NOx formation were then quantified under different levels of NH3 dilution. It was revealed that at fuel-rich conditions, the NOx formation via HNO pathway can be offset by the reduction via other pathways, such as pathways related to NHi, leading to the overall low NOx emission. Finally, an air-staged model combustion system was proposed, in which the equivalence ratio in the primary stage was chosen to be 1.5 to take advantage of the low NOx emission under fuel-rich conditions. The air split among the two stages was determined by the fixed outlet temperature of 1873 K. A parametric study demonstrated that less than 30 ppm NOx emission can be achieved with simple two-stage combustion even when the volumetric ratio of the NH3 reaches 40%, which is dramatically lower than the one of thousands ppm in the non-staged combustion. The results were further verified with the 47-species Konnov mechanism, demonstrating the great potential of air-staged combustion for low NOx emission in the NH3/CH4 fired combustion.

Reduced Chemical Mechanisms for Ammonia/Methane Co-firing for Gas Turbine Applications

Energy storage is one of the major challenges facing the world towards its challenging 2050 climate-change targets. A potential enabler of a low-carbon economy is the energy vector hydrogen. However, issues associated with hydrogen have led to consider other molecules such as ammonia as a potential candidate for chemical storage. Apart from its relatively high stability under atmospheric temperature, ammonia has the added attraction that it can also be sold on international markets or be used for power generation, making it a very versatile and hence attractive commodity. To explore the feasibility of co-firing ammonia with other fuels, i.e. methane, detailed numerical analyses and flame chemistry are required, usually at very high computational cost. Therefore, this study intends to determine a reduced mechanism for ammonia/methane combustion for practical gas turbine combustor conditions. Five reduced mechanisms of the well-known Konnov's mechanism were compared. Ignition delay time validations (0D) under industrially relevant conditions were used for correlation purposes. Combustion products of ammonia/methane premixed laminar flames (1D) were also validated with results from the full Konnov's mechanism. Finally, CFD simulations of a turbulent flame (2D) including reduced mechanisms were performed at representative industrial conditions. Results showed a reduced reaction mechanism of 48 Species and 500 elementary reactions can provide good results for further analyses.

Laminar burning velocity of diacetyl + air flames. Further assessment of combustion chemistry of ketene

Ketene is important intermediate in high-temperature chemistry of several oxygenates, such as acetone, acetic acid, and diacetyl. Ketene reactions appear in the sensitivity spectra of calculated burning velocities of the first two species. To provide independent experimental data for validation of the ketene sub-mechanism, the laminar burning velocities of diacetyl + air flames at 1 atm and initial gas temperatures of 298 K, 318 K, and 338 K were determined for the first time. Measurements were performed using the heat flux method in non-stretched flames, stabilised on a perforated plate burner at adiabatic conditions. Recently developed detailed kinetic mechanism for acetic acid flames with updated ketene sub-mechanism has been extended by reactions of diacetyl and includes revised rate constants for reactions of acetaldehyde and acetyl radical. The model was first compared with available experimental data on ketene pyrolysis and oxidation. Its performance in prediction of C2 species formation was improved by significant reduction of the previously estimated rate constants of ketene reactions with CH3 and CH2 radicals. The updated mechanism was then compared with the new measurements for diacetyl and earlier data for acetaldehyde, acetone and acetic acid flames. The model closely reproduces burning velocity of diacetyl + air in lean and rich mixtures while underpredicts in stoichiometric and slightly rich flames. Performance of the model for acetaldehyde + air flames was much improved as compared to the Konnov mechanism version 0.6. Good agreement of the modelling with experimental data for acetone + air flames was also demonstrated. The disparity between predicted burning velocities of acetic acid and recent measurements did not change. The model was further examined using sensitivity analysis for these flames to elucidate common reactions affecting its performance. It was concluded that the mechanism performance in prediction of the burning velocities of acetic acid flames could be improved by revision of reactions between CH2CO and OH radicals, while keeping its agreement with other flames studied. Remaining uncertainties in the ketene sub-mechanism are outlined.

A Systematically Updated Detailed Kinetic Model for CH2O and CH3OH Combustion

A new detailed kinetic mechanism for formaldehyde and methanol combustion has been developed using the Konnov mechanism, version 0.6, as a starting point. In total, 82 reactions of the formaldehyde and methanol subsets were reviewed and updated using a systematic approach. Rate constants were selected based on thorough evaluation of available experimental and theoretical data. No modifications of the rate constants were performed. The mechanism was validated over a wide range of experimental conditions and devices, including shock tube and flow reactors and burner stabilized and freely propagating flames. An overall good agreement with experimental data was seen. Simulated shock tube pyrolysis and flow reactor oxidation was generally in satisfying agreement to the experimental results, and the new mechanism showed a significant improvement in predicting the laminar burning velocity of methanol.

Experimental and numerical study of CO2 content effects on the laminar burning velocity of biogas

In the present work, the adiabatic laminar burning velocity of methane diluted with carbon dioxide, the two main components of biogas, is investigated both experimentally and numerically. The burning velocity of five mixtures with different CO2 content (up to 50% in fuel volume) were measured using the heat flux method at atmospheric conditions (298K and 1atm). The experimental results were compared to data and correlations form literature and also to predictions from four detailed kinetic mechanisms. Results showed that CO2 dilution reduces the burning velocity and shifts the maximum burning velocity towards leaner mixtures. The Gri-Mech 3.0 mechanism showed the best agreement with measurements for 10% and 20% CO2 fuel content mixtures. For 30%, 40% and 50% CO2 fuel content mixtures, the Konnov mechanism presented the highest determination coefficients. The numerical model is further explored to show that the shifting of the maximum burning velocity towards leaner mixtures is determined by a combination of CO2 dilution and reaction effects.

Research on Combustion and Emission Characteristics of Ammonia under Preheating Conditions

The present study focuses on determining the potential applicability of ammonia (NH3) as a carbon-free alternative fuel through numerical simulations. The combustion and emissions characteristics of premixed NH3-air flame at preheating conditions were numerically investigated. The burning velocity and all species mole fraction were determined using the Miller and Bowman and the Reductive Konnov mechanism. The results show that both the equivalence ratio and preheating temperature have important effects on the laminar burning velocity and adiabatic flame temperature of NH3. The reaction rate of production and mole fraction of H, O, and OH radicals, together with nitrogen radicals such as NH2, NH, and N in the reaction zone, increases as the preheating temperature is increased from 298 to 573 K. Although NO concentrations in the exhaust gas increased with increasing preheating temperature under fuel-lean conditions, they could be maintained at a reasonably low level at all preheating temperatures, mainly due to the increased reduction reaction of NO with NH2, NH, N radicals. The main N radical formation in the NH3-air combustion originates from NH3 rather than N-2, which indicates that the N-2 dilution effect of NH3-air flame is negligible for NO formation. Furthermore, the equivalence ratio has a dominant effect on NO yields, because of the higher reduction reaction and lower flame temperature at higher equivalence ratio conditions, leading to a lower formation of HNO, which indicates a potential in reducing NO emissions in NH3 combustion. Therefore, the preheating temperature and equivalence ratio should be given sufficient consideration in practical operations to minimize NO formation in combustion of NH3 as a clean fuel.

Kinetic mechanism of molecular energy transfer and chemical reactions in low-temperature air-fuel plasmas.

This work describes the kinetic mechanism of coupled molecular energy transfer and chemical reactions in low-temperature air, H2-air and hydrocarbon-air plasmas sustained by nanosecond pulse discharges (single-pulse or repetitive pulse burst). The model incorporates electron impact processes, state-specific N(2) vibrational energy transfer, reactions of excited electronic species of N(2), O(2), N and O, and 'conventional' chemical reactions (Konnov mechanism). Effects of diffusion and conduction heat transfer, energy coupled to the cathode layer and gasdynamic compression/expansion are incorporated as quasi-zero-dimensional corrections. The model is exercised using a combination of freeware (Bolsig+) and commercial software (ChemKin-Pro). The model predictions are validated using time-resolved measurements of temperature and N(2) vibrational level populations in nanosecond pulse discharges in air in plane-to-plane and sphere-to-sphere geometry; temperature and OH number density after nanosecond pulse burst discharges in lean H(2)-air, CH(4)-air and C(2)H(4)-air mixtures; and temperature after the nanosecond pulse discharge burst during plasma-assisted ignition of lean H2-mixtures, showing good agreement with the data. The model predictions for OH number density in lean C(3)H(8)-air mixtures differ from the experimental results, over-predicting its absolute value and failing to predict transient OH rise and decay after the discharge burst. The agreement with the data for C(3)H(8)-air is improved considerably if a different conventional hydrocarbon chemistry reaction set (LLNL methane-n-butane flame mechanism) is used. The results of mechanism validation demonstrate its applicability for analysis of plasma chemical oxidation and ignition of low-temperature H(2)-air, CH(4)-air and C(2)H(4)-air mixtures using nanosecond pulse discharges. Kinetic modelling of low-temperature plasma excited propane-air mixtures demonstrates the need for development of a more accurate 'conventional' chemistry mechanism.

Numerical study of combustion characteristics of ammonia as a renewable fuel and establishment of reduced reaction mechanisms

With its high hydrogen density and already existing infrastructure, ammonia (NH3) is believed to be an excellent green fuel that can be used in energy generation and transportation systems. Combustion of ammonia has certain challenges (associated with its low flame speed and fuel bond NOx emissions) that need to be addressed before its widespread use in practical systems. The primary objective of this study is to develop a reduced reaction mechanism for the combustion of ammonia which can be used to expedite the design of effective ammonia combustors through numerical simulations of realistic combustor geometries with accurate kinetics models. First we have investigated the combustion characteristics of NH3/H2/air mixtures at elevated pressure and lean conditions which are encountered in practical systems such as gas turbine combustors. Laminar premixed freely propagating flame model is used to calculate the combustion properties. The results of sensitivity study of total NOx formation with respect to the equivalence ratio indicates the possibility of localized rich combustion as an effective way to reduce the NOx concentration down to levels that are the same order as the modern gas turbine engines. In the second part of the study, by considering a wide range of conditions in terms of pressure, fuel mixture, and equivalence ratio we have developed two reduced mechanisms based on the Konnov mechanism. The reduced mechanisms are capable of predicting the total NOx emission level and the laminar flame speed at an acceptable accuracy over a wide range of conditions. Evaluating the performance of the reduced mechanisms with respect to the full mechanism and experimental data shows that the mechanisms are able to predict the combustion properties almost at the same accuracy level as the Konnov mechanism, but at a nearly five times less CPU time expense.

Laminar Burning Velocity of Methane–Air Mixtures at Elevated Temperatures

The measured and computed laminar burning velocities of methane–air mixtures at higher mixture temperatures are reported in this paper. The experiments and computations were performed for a wide range of mixture temperatures and equivalence ratios. The unburned mixture temperature ranges from 370 to 650 K. Computational predictions of burning velocities were carried out with GRI-Mech 3.0, San Diego mechanism, and Konnov mechanism for methane–air mixtures. The measured burning velocities match very well with the numerical predictions for all mixture temperatures and existing experimental results for mixtures at ambient temperature. Another contribution of the present work is the variation of the measured power-law temperature exponent with mixture equivalence ratios. The maximum burning velocity (even at high mixture temperatures) and minimum temperature exponent magnitudes were observed to exist for slightly richer mixtures.

The critical tube diameter and critical energy for direct initiation of detonation in C2H2/N2O/Ar mixtures

In this study, the dynamic detonation parameters, namely, the critical tube diameter and critical energy for direct initiation, of C2H2/N2O/Ar mixtures were measured at various initial conditions. Using chemical kinetics with the Konnov mechanism and experimental measured data, a simple correlation to evaluate the critical tube diameter of C2H2/N2O/Ar detonation is developed, given by dc=594.8φ0.623(1-XAr)0.2176(p/po)-0.0246ΔI which is applicable for stoichiometric mixtures with initial pressures ranging from 50 to 130kPa, equivalence ratios from 0.625 to 2.5 and stoichiometric mixtures with maximum percentage of argon dilution up to 50% at the atmospheric pressure. By combining this correlation function with a theoretical model based on a simple work done concept, the critical energy for direct blast initiation can be predicted; and the theoretical predictions are found to be in good agreement with the critical energy measured experimentally. To assess the detonation sensitivity, the critical energy results for direct initiation of detonation in C2H2/N2O/Ar mixtures are compared with those in H2/N2O/Ar mixtures [Zhang et al., Int. J. Hydrogen Energy 39 (2011) 5707–5716]. The results indicate that C2H2/N2O/Ar is more sensitive than H2/N2O/Ar at the same initial conditions. This is also supported qualitatively by the chemical kinetic calculation which shows that the ZND induction length scale for C2H2/N2O/Ar is relatively smaller than that of H2/N2O/Ar mixture. Lastly, the effect of the oxidizer on the detonation sensitivity is studied by comparing the critical energy between C2H2/N2O/Ar and C2H2/O2/Ar mixture. It is found that a higher energy is required to successfully initiate a spherical detonation in C2H2/N2O/Ar than in C2H2/O2/Ar mixtures and equivalently, the results agree qualitatively with the ZND induction length analysis.

Validation and analysis of detailed kinetic models for ethylene combustion

Present work aims at evaluation of contemporary comprehensive detailed kinetic mechanisms for ethylene combustion, including the Konnov mechanism, LLNL nButane mechanism, San Diego (UCSD) mechanism and USC mechanism. These models have been validated by extensive comparison with available experimental data on ethylene ignition and flame propagation. The experimental data from the literature have been carefully examined to accurately assess the models’ predicting performance. Noticeable differences in the predictions of ethylene ignition and flame propagation under a variety of conditions have been observed. Moreover, sensitivity analysis has been conducted to identify important reactions for the prediction of ethylene ignition and flames. For ethylene ignition, it was found that C2H4 consumption reactions with radicals OH, O and subsequent reactions of vinyl with oxygen have dominant effect on predicted ignition delays. The pathway analysis has also been performed for each mechanism to identify different reaction pathways in ethylene ignition process. For ethylene flames, sensitivity analysis shows that H–O and C1 chemistry reactions significantly influence the laminar burning velocity in lean ethylene/air flames, while C2 chemistry reactions become of increasing importance in fuel-rich flames. Furthermore, to better understand the models’ predicting behavior, the differences in the reaction rate constants and routes of C2H4 and vinyl chemistry have been analyzed and discussed.

Photofragmentation laser-induced fluorescence imaging in premixed flames

Two-dimensional measurements of primarily hydroperoxyl radicals (HO 2) are, for the first time, demonstrated in flames. The measurements are performed in different Bunsen-type premixed flames (H 2/O 2, CH 4/O 2, and CH 4/air) using photofragmentation laser-induced fluorescence (PF-LIF). Photofragmentation is done by laser radiation at 266 nm, and the generated OH photofragments are probed through fluorescence induced by a laser tuned to the Q 1(5) transition at 282.75 nm. The signal due to naturally occurring OH radicals, recorded by having the photolysis laser blocked, is subtracted, providing an image that reflects the concentration of OH fragments generated by photolysis, and hence the presence of primarily HO 2, but also smaller contributions from H 2O 2 and, for the methane flames, CH 3O 2. For the methane flames the measured radial profiles of OH photofragments and natural OH agree well with corresponding profiles calculated for laminar, one-dimensional, premixed flames using CHEMKIN-II with the Konnov detailed C/H/N/O reaction mechanism. An interfering signal contribution is observed in the product zone of the methane flames. It is concluded that the major source for the interference is most likely hot CO 2, from which O atoms are produced by photolysis, and OH is rapidly formed as the O atoms react with H 2O and H 2. This conclusion is supported by the fact that the interference is absent for the hydrogen flame, but appears when CO 2 is seeded into the flame. Another strong indication is that the Konnov mechanism predicts a similar buildup of OH after photolysis.