Laminar Flame Speed Research Articles

Application of static integrated skeletal reduction and tabulation of dynamic adaptive chemistry in the combustion simulation of ethylene-fueled scramjet combustor.

The high-fidelity reduced mechanism is one of the key elements in the combustion simulation of scramjet combustors to reveal their combustion and flow phenomena. In the present work, the hierarchically constructed NUIGMech1.2 (2857 species and 11 814 reactions) is applied to the combustion simulation of an ethylene-fueled scramjet combustor using the method of static integrated skeletal reduction and tabulation of dynamic adaptive chemistry (TDAC). The integrated skeletal reduction strategy successively consists of species elimination using the revised directed relation graph with error propagation method of fixed species scheme and improved sensitivity analysis method, and reactions elimination based on computational singular perturbation importance index. A preferred ethylene skeletal mechanism (26 species and 117 reactions) is obtained through the integrated skeletal reduction strategy under target working conditions of temperature range of 900-1800 K, pressure range of 1-4 atm, and equivalence ratio range of 0.25-5.0. The compact skeletal mechanism is comprehensively validated against the experimental results of ignition delay times, laminar flame speeds, and key species concentration profiles. Meanwhile, it shows consistent results with the detailed mechanism on the adiabatic flame temperature profiles and "S"-curves. When applying this skeletal mechanism to combustion simulations of ethylene-fueled scramjet combustor with double parallel cavities, the path flux analysis method and in situ adaptive tabulation algorithm of TDAC is further utilized to speed up the chemical reaction solution process at run-time. Under the scramjet and ramjet modes, the corresponding simulation results in terms of flame luminosity images, schlieren images, and static pressure distributions, coincide well with those of experimental measurements. The combustion and flow characteristics of the two modes are investigated and analyzed comparatively based on above results and combustion performance parameters. Present work contributes to the application of fuel kinetic mechanisms in scramjet combustor combustion simulation.

Local statistics of turbulent spherical expanding flames for NH3/CH4/H2/air measured by 10 kHz PIV

In this paper, we present high-speed particle image velocimetry measurements to quantify combustion-induced turbulence and turbulent stretch rate effects on flame propagation in a fan-stirred constant-volume combustion vessel. Three stoichiometric fuel/air mixtures including methane/air, methane/hydrogen/air and ammonia/methane/air were tested. The expansion rate of flame radius is found to scale with a half-power law of the mean-radius based Reynolds number. Measurements of velocity in the reactant zone show that the gas mean and rms velocity increase within the flame brush then gradually decrease when approaching the detection edge. Combustion enhances the measured non-reacting flow turbulence by ∼64 % within the reactant side of the flame brush of this expanding spherical flame. The evolution of rms velocity is found to be insensitive to pressure but sensitive to initial turbulent intensity and laminar flame speed, scaling with Reynolds number in a similar manner as the rate of expansion of the flame radius. Hydrogen blending shows the largest absolute value of rms velocity increase, but ammonia blending leads to the largest increase in rms velocity relative to laminar flame speed. The pdfs of 2D curvature are broadened as u′ and p increase but gradually saturate at high u′ for all mixtures, those of stretch rate are widened by flame speed. The temporal evolution of statistical flame stretch rates suggests the tangential stretch rate is larger than and anti-correlated with the normal stretch, and that there is a strong positive correlation between instantaneous flame propagation rate and the rms strain rate. Finally, the mean flame curvature and mean stretch rate are found to be near zero as flame propagates outwardly.

Experimental investigation and modeling of boundary layer flashback for non-swirling premixed prevaporized n-propanol/air and /isopropanol/air flames

The idea of utilizing lean, premixed and prevaporized (LPP) combustion could be a step towards sustainable aviation in the future, with the absolute absence of soot particles and very low nitrogen oxide emissions. However, premixing with compressed hot air presents a challenge for both conventional jet fuel and synthetically produced sustainable aviation fuel (SAF) due to self-ignition and flashbacks. Our group is thus seeking LPP-compatible advanced sustainable aviation fuels, which are evaluated based on their flashback propensity. Preliminary studies indicate that short-chain alcohols, with their higher ignition delay times and lower flame velocities, may be appropriate for LPP combustion concepts. In this study, the isomers of propanol, n-propanol, and isopropanol, are the first representatives of the short-chain alcohols to be investigated experimentally and numerically with respect to their flame stability limits. The experimental determination of stability limits for n- and isopropanol/air premixed flames was conducted on a generic, non-swirled atmospheric burner for an equivalence ratio range of 0.82 to 1.11. A fully automated measurement procedure was used to generate 119 measurement points with Reynolds numbers in the range from 1430 to 2750. The influence of the positional isomerism and of the preheat temperature (422 K, 446 K and 466 K) of the unburned mixture are investigated. A clear influence of the isomeric fuel structure is found. A theoretical analysis shows that it can be related to the different laminar flame speed and laminar flame thickness of the isomers. Moreover, a detailed analysis shows that the critical velocity gradient concept to describe boundary layer flashback is well usable with a Péclet number model.

Control of Intrinsic ThermoAcoustic instabilities for methane and hydrogen air flames: DNS and network analysis

Replacing methane by hydrogen in premixed burners is a typical decarbonation scenario. This is usually done by ensuring a constant flame speed, i.e. by using ultra-lean H2 mixtures. This change can modify the thermoacoustic properties of burners. This study focuses on one class of thermoacoustic instabilities, Intrinsic ThermoAcoustic (ITA) modes, for flames stabilized on a diaphragm. An acoustic network approach is built to construct an ITA stability criterion (called ITAS criteria) for a three-duct configuration and a fully analytical expression is obtained in the limit of thin diaphragms. It shows that instability will grow when the Flame Transfer Function (FTF) gain n−π, located at the frequency at which u′-q′ phase ϕ=−π, is greater than a critical gain nc expressed by the ITAS criteria. Two-dimensional DNS, with anechoic terminations, of methane–air and hydrogen–air flames stabilized on a diaphragm are simulated with similar laminar flame speed sL0 and theoretical flame length, with an equivalence ratio of 0.73 and 0.4 respectively. The theory developed allows to analyze the differences with these two strong unstable ITA modes. For ultra-lean hydrogen flame, preferential diffusion near the diaphragm lips and tip opening reduces the flame time delay and shifts the ITA frequency towards higher values compared to methane. The assessment of two new control strategies is possible with the help of the analytical network model by increasing nc. A first strategy involves the use of preheated leaner mixtures: increasing Tu and reducing ϕ while keeping the flame speed sL0 constant. A second strategy consists of modifying the combustor geometry by changing the section ratio S2/S0, between plenum and combustion chamber. Both strategies successfully mitigate the hydrogen–air ITA modes and seems promising strategies for methane cases despite the fact that instabilities are not fully mitigated for this fuel at the conditions of interest in this study.

Stabilization regimes and flame structure at the flame base of a swirled lean premixed hydrogen–air injector with a pure hydrogen pilot injection

Hydrogen (H2) has become a key in the decarbonation process of the aeronautical field and new gaseous injection systems must be designed to suit hydrogen’s specific properties (high laminar flame speed, high adiabatic temperature, large flammability limits and fast diffusivity (Le<1)). In this study, Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS) are performed for a co-axial injector including an annular premixed hydrogen–air, swirled injection surrounding a central axial pure hydrogen lance. LES of the injector installed in a full chamber are performed at 12bar and the LES resolution issues associated with these high pressures are presented. LES reveals that the flame is stabilized on the rim separating the premixed and the pure hydrogen streams. The stabilization is produced by a structure called RSE for Rim Stabilized Edge Flame. To avoid resolution issues of the LES, the RSE flame structure is analyzed using CANTERA 1D flames and a 2D DNS of the attachment region. Extinction limits are described and implications for flame stability are discussed.

Experimental and numerical simulation study of the influence of CF3CHFCF3 on characteristic of hydrogen/methane/air explosion

The effect of CF3CHFCF3 on the explosion of hydrogen/methane/air mixtures at low hydrogen doping ratios (XH2=10 %) has been investigated on a closed visualisation experimental platform. Different equivalence ratios (? = 0.8, 1 and 1.2) and CF3CHFCF3 concentrations (XCF3CHFCF3= 0% -5%) were considered. The results showed that the suppression effect of CF3CHFCF3 on the mixture was good under the conditions of different equivalence ratios. After the addition of CF3CHFCF3, both of the maximum explosion pressure and the maximum pressure rise rate decrease, which shows CF3CHFCF3 has an inhibitory effect on the explosion. However, pressure peak occurs earlier indicates that CF3CHFCF3 accelerated the progression of the explosion and facilitates its occurrence. The simulation results indicate that CF3CHFCF3 changes the mole fractions of the major species and increases the consumption of the O, H, and OH radicals. Besides, the results of sensitivity analyses show that CF3CHFCF3 not only play an inhibitory role but also enhances the laminar flame speed of the explosion flame. Meanwhile, the effect of CF3CHFCF3 on the reaction H + CH3 (+M) &lt;=&gt; CH4 (+M) is different under different equivalence ratios. This study can provide theoretical support for the safe use of hydrogen/methane/air mixtures.

Experimental study of the influence of Lewis number, laminar flame thickness, temperature, and pressure on turbulent flame speed using hydrogen and methane fuels

To experimentally explore the influence of Lewis number Le, laminar flame thickness δL, pressure P, and unburned gas temperature Tu on turbulent flame speed ST, a set of conditions is designed by adjusting nitrogen mole fraction in lean H2/O2/N2 (Le=0.35) and stoichiometric CH4/O2/N2 (Le≈1) mixtures. The adjustment is performed by simulating complex-chemistry laminar flames to obtain the same laminar flame speeds SL not only for different fuels, but also for different pressures (1, 2, and 5 atm). The mixtures are characterized by significantly different SL at Tu=300 K and 400 K, whereas variations in δL with the temperature are sufficiently weak. Moreover, laminar flame thicknesses are approximately equal for H2-based and CH4-based mixtures at the same P, but are significantly decreased with increasing pressure. For this set of conditions, ST is measured by applying schlieren imaging techniques to film expansion of centrally ignited, statistically spherical flames in homogeneous isotropic turbulence generated by a dual-chamber, constant-pressure, fan-stirred explosion facility. Analyses of the measured data show the following trends. First, turbulent flame speed is increased by both P and Tu, whereas ST/SL is decreased with increasing Tu. Second, turbulent flame speed measured at different P and Tu can be predicted by allowing for SL(P,Tu) and δL(P,Tu). Thus, the present data do not call for explicitly substituting normalized pressure or temperature into a turbulent flame speed approximation. Third, ST is increased with decreasing laminar flame thickness. Fourth, speeds of the lean H2/O2/N2 flames are higher when compared to the stoichiometric CH4/O2/N2 flames, with this difference is increased (reduced) by P (Tu, respectively). Fifth, all measured data on ST can quantitatively be described by substituting SL and δL with the counterpart characteristics of highly strained twin laminar flames. The latter finding supports leading point concept of premixed turbulent combustion.

Combustion mechanism study of ammonia/n-dodecane/n-heptane/EHN blended fuel

Due to the challenges of ammonia, such as high ignition energy and slow flame propagation speed, the utilization of ammonia in engines might necessitate the implementation of dual-fuel combustion modes along with the use of cetane improvers. To optimize the performance of ammonia-fueled engines, and achieve efficient and clean combustion, three-dimensional computational fluid dynamics (CFD) simulations are imperative. However, prior to conducting a three-dimensional CFD study, an accurate reaction mechanism for each component is essential. In this study, the blending mechanisms of ammonia, n-dodecane, and 2-ethylhexyl nitrate (EHN) were constructed using CHEMKIN software, including 243 species and 1293 reactions. The calculated results of ignition delay, laminar flame velocity, and the concentration of significant species agreed well with experiment results. The ignition delay, laminar flame velocity, adiabatic flame temperature, and the concentration of major products were analyzed in different blending ratios. The use of high cetane fuel shortens ignition delay and increases laminar flame speed. As the equivalence ratio of ammonia increases, the concentration of NO decreases while the concentration of H2 increases. The combustion process is also analyzed based on optical diagnostic results, and the impact of the ammonia/n-dodecane mixtures blended with EHN on the combustion process is investigated. The addition of EHN proved to shorten the ignition delay of the mixture. Furthermore, the introduction of EHN exhibits marginal influence on NOx generation, with the predominant augmentation of NOx emissions stemming from fuel-derived NOx resulting from the fuel's inherent nitrogen content. Due to the small amount of EHN added, the overall impact is relatively small.

DNS of Turbulent Premixed Ammonia/Hydrogen Flames: The Impact of Thermo-Diffusive Effects

Direct Numerical Simulations (DNS) of three-dimensional premixed turbulent hydrogen-air flames enriched with 19%, 36%, 44% and 57% of NH3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_3$$\\end{document} (in volume) are performed. Starting from an equivalence ratio of 0.44 for the case with 19% of NH3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_3$$\\end{document}, richer mixtures of ϕ=\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\phi =$$\\end{document} 0.54, 0.69 and 0.95 are considered when increasing NH3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_3$$\\end{document} concentration to obtain comparable laminar flame speeds, i.e., 0.17 m/s for 19% and 36 % NH3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_3$$\\end{document} enriched case, and 0.30 m/s when NH3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_3$$\\end{document} concentration is increased to 44 and 57%. The composition and characteristics of the studied mixtures enable to investigate the effects of thermo-diffusivity in a turbulent flow and the role of chemistry and stretch effects in the development of the flames. Given a composition of ammonia and hydrogen and an equivalence ratio, a predictive method is described to identify compositions where thermo-diffusive effects impact the flame and predict the stretch factors. Two maps are proposed to achieve this: the first one is based on the Markstein number and the second one is based on the ratio of consumption speed of strained flames over the laminar unstretched flame speed. This prediction can guide model selection and help manufacturers and experimentalists identify relevant operating points based on desired energy output.

The Ammonia Combustion Engine for Future Power Generation Applications

Energy storage is one of the big challenges for the energy transition and ammonia as a hydrogen carrier is considered a promising candidate for efficient and medium‐ to long‐term chemical storage. Gas engines are well suited for power generation using ammonia as a carbon‐free fuel, making it unnecessary to convert ammonia back to hydrogen for the utilization in the power plant. This study investigates operating strategies for a prechamber combustion concept for a large‐bore gas engine suitable for power generation applications. To overcome the high ignition energy and low laminar flame speed of ammonia, different quantities of hydrogen as a promoting agent to improve the combustion properties of ammonia have been employed in the experimental investigations on a single‐cylinder research engine. The combustion concept shows a wide operating range with stable and robust combustion from 0.3 to 2.5 MPa indicated mean effective pressure and will be transferred to a multicylinder engine for the first combined heat and power demonstration.

The combustion chemistry of ammonia and ammonia/hydrogen mixtures: A comprehensive chemical kinetic modeling study

Ammonia (NH3) is relatively less reactive compared to hydrocarbon fuels. Therefore, ammonia mixtures blended with hydrogen (H2) have been shown to be a promising fuel for internal combustion engines. In this study, a detailed NH3/H2 chemical kinetic model is developed over a wide range of engine-relevant conditions and comprehensively validated to describe the combustion of NH3/H2 mixtures using available experimental literature data, including ignition delay times, laminar flame speeds and species concentration profiles. The new model captures very well the combustion properties of pure NH3 and NH3/H2 mixtures at most conditions. By performing sensitivity and reaction path flux analyses the key reactions controlling fuel reactivity at high-temperature (≥ 1500 K) and low-to-intermediate temperature (1000 ≤ T ≤ 1500 K) regimes are identified. Moreover, the formation and consumption pathways of nitrogen oxides (NOx) in NH3/H2 combustion at different conditions have also been investigated, which are found to be highly coupled to the underlying chemical reactions that dictate fuel reactivity. The kinetic data for the important reactions and species thermochemistry data used in our model are rigorously evaluated and are discussed in detail.

Shock-tube laminar flame speed measurements of ammonia/airgon mixtures at temperatures up to 771K

Laminar flame speed measurements of lean (φ = 0.75), stoichiometric (φ = 1.0), and rich (φ = 1.25) mixtures of ammonia in “airgon” (79% Argon, 21% O2) were conducted at room temperature (295 K) and various pressures in a static setup, and for the first time at temperatures above 500 K behind reflected shock waves. Constant-pressure static experiments were performed in the cylindrical, Constrained Reaction Volume (CRV) section of the Imaging Shock-tube (IST) to study flame propagation and estimate minimum ignition energies. Ammonia/air experiments were also conducted at room temperature to assess the effect of the diluent gas. The shock-tube flame speed method allowed access to the temperature range of 477 K–771 K at atmospheric pressure. Simultaneous side-wall schlieren and end-wall OH* chemiluminescence diagnostics permitted time-resolved tracking of the propagation of outwardly expanding flames spark-ignited by an Nd:YAG laser system. An area-averaged linear-curvature model was employed to compute unstretched laminar flame speeds. Measurements were compared with 1-D, planar flame speed simulations based on different chemical kinetic models for ammonia oxidation, including those by Glarborg et al., Okafor et al., Zhang et al., Chen et al., Otomo et al., and Shrestha et al. These models agree well with experimental results and each other at room temperature, but clearly diverge at higher temperatures. Experimental results agree well with the models of Okafor et al. and Zhang et al. at high temperatures. Flame speed sensitivity analyses were performed and key reactions, as predicted by the models, were identified.

Experimental and modeling study of the combustion of ethyl methyl carbonate, a battery electrolyte

Ethyl methyl carbonate (EMC) is one of the main constituents of the electrolyte in lithium-ion batteries (LIBs). To understand better fire events involving LIBs and try to prevent them, the combustion of EMC was studied experimentally, and the results were analyzed using modern detailed kinetics models. Shock tubes were used to collect ignition delay times (IDTs) and CO time histories, while laminar flame speeds were measured in a closed vessel. All experiments were carried out near atmospheric pressure. The shock tube experiments were performed for three equivalence ratios (0.5, 1.0, and 2.0) with the IDTs measured with “fuel-air” mixtures and the CO spectroscopic laser measurements in mixtures highly diluted in 99.25% He/Ar. Comparisons with results obtained for other linear carbonates typically found in LIBs, such as dimethyl carbonate and diethyl carbonate, are also presented. Numerical predictions from recent detailed chemical kinetics mechanisms (Takahashi et al., 2022, Grégoire et al., 2023) did not capture well the combustion behavior of EMC in some of our conditions. Sensitivity analyses suggest that EMC decomposition reactions and subsequent combustion chemistry of the formed intermediates, i.e. methanol and ethylene, still need improvement.

A CFD ignition model to predict average-cycle combustion in SI engines with extreme EGR levels

Control of the combustion process in Spark-Ignition (SI) engines operated with extreme dilution from exhaust gas re-circulation (EGR) represents one of the major limitations in the industrial design of such technology. Numerical approaches able to describe in detail the formation of the early flame kernel become essential to face such an ambitious task. This work presents a RANS-based multi-dimensional model of the combustion process, including an advanced description of the ignition stage to consider its stochastic re-ignitions within the average cycle prediction. The spark-channel is described as a column of Lagrangian parcels that represent early flame kernels, whose growth is controlled by the laminar flame speed and energy input from the electrical circuit. The spatial evolution of each parcel is computed according to a scaled value of the average-flow speed, to mimic the smooth but short elongation of the mean-cycle channel produced by stochastic restrikes affecting the single-cycle arcs. To clarify this phenomenon and assess the proposed CFD method, a series of experiments are performed in a single cylinder SI engine with optical access, running at a low-load cruise-speed operating condition. Increasing EGR levels are tested up to the onset of misfire, with measurements of the secondary-circuit features and of the flame evolution through high-speed imaging. Satisfactory results are achieved in terms of numerical-experimental comparison of the cycle-averaged in-cylinder pressure, discharge parameters, and spatial flame distribution, demonstrating the reliability of the proposed numerical approach.

Systematic construction of combustion reaction model for large hydrocarbon fuels based on a two-step reaction scheme

Understanding the combustion chemistry of large hydrocarbons in practical gasoline, diesel and jet fuels is critical for the reduction of pollutant emissions and increase of fuel and engine efficiencies. The primary objective of the present study is to illustrate the methodology and application of a simplified modeling approach based on a two-step reaction scheme (TSRS) for describing the combustion chemistry of large hydrocarbon fuels under engine-relevant high-temperature conditions. Under such conditions, the combustion process of large hydrocarbons consists of two distinct subsequent reaction steps: the first step is the prompt fuel decomposition, converting the parent fuel to critical intermediate products; the second step is the oxidation reactions of the intermediates. Particularly, the second step is slower, thus rate-limiting, controlling the heat release and formation of final combustion products of the entire combustion reaction process. A systematic analysis by mathematical methods was performed to obtain a unified treatment for hydrocarbons with different molecular sizes. Based on the analysis of characteristic time scale of elementary reaction and steady-state assumption, TSRS used a fuel decomposition submodel to describe the fuel molecule decompose into critical intermediate species, including ethene, propene, butenes and methane for n-alkanes, and used a detailed foundational fuel chemistry mechanism to describe the oxidation reaction of the intermediates. The entire TSRS reaction model for large hydrocarbons is compact, consisting ∼110 species and ∼800 reactions, which can be further reduced. It was shown in the present study that, by revealing the relationship between the functional group composition of linear alkanes and the distribution of intermediates, and in combination with the rate criterion, the fuel decomposition submodel can be systematically generated to facilitate the automatic construction of compact kinetic models. One unified TSRS combustion reaction model was created and demonstrated for several large hydrocarbons ranging from C8 to C16, which was validated by comprehensive datasets, including speciation, ignition delay times and laminar flame speeds.

A wide ranging experimental and kinetic modeling study of TMEDA pyrolysis and oxidation

Traditional liquid propellants like unsymmetrical dimethylhydrazine include several issues, such as toxicity, short storage time and preservation difficulty at low temperatures. For this reason, exploring green and stable propellant fuels is imperative in the development of propellants. Tetramethyl ethylenediamine (TMEDA) is considered to be a promising green propellant fuel with high specific impulse, non-toxicity, and long-term storage stability. Thus, investigating the combustion characteristics of TMEDA and constructing a detailed chemical kinetic mechanism are of fundamental importance in describing its combustion in computational fluid simulation. For this work, ignition delay times of TMEDA were measured in a shock tube at equivalent ratios of 0.5, 1.0 and 2.0, at pressures of 2, 10, and 20 bar, and in the temperature range of 870–1500 K with fixed fuel concentration at 2 %. After the reflected shock at 990–1500 K, the pyrolysis products of 2000 and 5000 ppm TMEDA in Argon were also measured at 5 and 10 bar conditions in a single pulse shock tube. In addition, laminar flame speeds of TMEDA were measured at initial pressures of 1 bar, initial temperatures of 333 and 353 K, and equivalence ratios ranging from 0.8 to 1.5 in a constant volume reactor. The experimental results were simulated using a detailed TMEDA kinetic mechanism. Reaction path analysis and sensitivity analysis were provided to clarify the chemical kinetics of TMEDA oxidation and pyrolysis. The current work provides new experimental data and fundamental analyses for understanding the oxidation and pyrolysis characteristics of TMEDA, and sheds light on optimal directions for future models.

A Numerical Investigation on Laminar Flame Speed of Syngas in Air with Ozone Addition

In the context of energy and propulsion systems, in recent years engineering research has increased its efforts to develop more efficient technologies with a lower environmental impact. Specifically, new combustion systems have been investigated, such as ultra-lean combustion and Homogeneous Charge Compression Ignition (HCCI) combustion, and sustainable fuels have been employed. With respect to the latter option, syngas and hydrogen have emerged as attractive choices. However, the low specific energy of syngas and the presence of diluents can cause flame failure and instability. On the other hand, it is well known that ozone-assisted combustion enhances the Laminar Flame Speed (LFS), reduces the ignition delay time, and improves the flame stabilization of fuels such as methane, n-heptane and iso-octane, since it affects the chemistry in the Low Temperature Combustion (LTC) regime. In this context, the aim of the present work is to evaluate the effect of ozone on the LFS of syngas/ozone/air mixtures. Specifically, 1-D simulations have been carried out to compute the LFS of CO/H2 and CO/H2/N2 syngas mixtures. The model has been validated against experimental data available in the scientific literature by considering four different reaction mechanisms. Then, the model has been used to perform a parametric analysis by considering different conditions in terms of syngas composition (three different CO/H2 ratios) and equivalence ratio (from 0.5 to 5). The results show that the addition of 2500 ppm of ozone in air results in an increase in LFS for all equivalence ratios examined. The four different mechanisms give comparable results in terms of LFS and LFS increase for CO/H2 syngas, and are also able to predict ozone effect on LFS enhancement for CO/H2/N2 mixtures. Finally, the results show that the influence of ozone, in terms of LFS percentage increase, is greater for CO/H2 ultra lean mixtures with a lower CO/H2 ratio.

Hydrogen, methane and one of their fuel blends combustion: CFD analysis and numerical-experimental comparisons of fixed and mobile applications

The capabilities of Computational Fluid Dynamics (CFD) coupled with detailed chemistry simulations are examined in both steady jet diffusion flames and in an internal combustion engine case fuelled with hydrogen. Different approaches to turbulence-chemistry interaction such as the “Laminar Flame Concept” the “Eddy Dissipation Concept” and the “Turbulent Flame Speed Closure” are considered and tested. The results are compared with the experimental data available. Concerning the jet diffusion flames, the combustion processes of hydrogen, methane and one of their fuel blends are investigated on two burner geometries. Different sensitivities (i.e. mesh, turbulence model, turbulent Schmidt number, chemical mechanism) are performed. The study demonstrates that despite the burner geometry considered and the chemical composition of the fuel, the Complex Chemistry with “Eddy Dissipation Concept” is the model that better describes the behaviour of the turbulent flames. On the other hand, the “Laminar Flame Concept” sub-model is characterized by an higher fuel consumption rate, which causes an overestimation of the temperature peak. As for the in-cylinder unsteady simulations, the hydrogen combustion process is better described by the “Turbulent Flame Speed Closure” sub-model, which, unlike the other two, requires the specification of both laminar and turbulent flame speed. Despite different variations being considered, the “Laminar Flame Concept” adoption leads to an unphysically high burning rate, while the Eddy Dissipation Concept sub-model is characterized by an underestimation of the apparent heat release rate, and thus of the pressure peak inside the combustion chamber.

A phenomenological model for predicting the early development of the flame kernel in spark-ignition engines

This work presents a simple and effective phenomenological model for the prediction of the early growth of the flame kernel in SI engines, including its initiation as a result of the electrical breakdown of the fuel/air mixture between the spark plug electrodes. The present model aims to provide an improved description of the ignition-affected early phases of flame kernel development compared to the majority of models currently available in literature. In particular, these models focus on electrical energy supply and turbulence, whereas the stretch-induced kernel growth slowdown is quantified with linear models that are inconsistent with the small kernel radius. For the flame kernel initiation, this model replaces the current methods that rely on 1D heat diffusion within a plasma column with a more consistent analysis of post-breakdown conditions. Concerning the kernel growth, the present model couples the mass and energy conservation equations of a spherical kernel with the species and temperature profiles outside of it. This combination leads to a non-linear description of the flame stretch, according to which the kernel development is controlled by the Lewis-number-dependent balance between the heat gained via combustion and the heat lost via thermal diffusion. As a result, the kernel temperature differs from the adiabatic flame temperature, causing the laminar flame speed to change from its adiabatic value and ultimately affecting the overall kernel development. Kernel growth predictions are conducted for laminar flames and compared to literature data, showing a satisfactory agreement and highlighting the ability to describe the stretch-induced kernel slowdown, up to its possible extinction. A good agreement with literature data is also obtained for kernel expansions under moderately turbulent conditions, typical of internal combustion engines. The simple formulation of the present model enables swift integration into phenomenological combustion models for sparkignition engines, while simultaneously offering useful insight into the early kernel development even for CFD-based approaches.

Influence of E85 on performance and efficiency of a motorcycle engine

E85 (85 vol% ethanol and 15 vol% gasoline blend) is one of the most promising sustainable fuels for SI engines, thanks to the optimum trade-off between pollutant emissions and cost of implementation, starting from a pure gasoline baseline.From the point of view of engine performance, the most relevant differences from such a baseline are related to the heat of vaporization and to the laminar flame speed. The higher heat of vaporization helps to reduce combustion temperature, thus the risk of knocking, but it also slows down the air-mixing process; the small amount of Oxygen in the fuel molecule leads to a slightly different combustion behavior.The goal of this study is to compare commercial gasoline (E5, 5 vol% ethanol and 95 vol% gasoline blend) and E85, by means of CFD 1D (GT-Power) and 3D (AVL-FIRE) simulations, using experimentally calibrated models. The reference engine is a single-cylinder, four-stroke, PFI motorcycle unit, with a displacement of 463 cc and a maximum power > 30 kW at 9500 rpm.After the calibration, carried out on the E5 version, the fuel type is changed to E85 in the 1D model, in order to provide accurate Initial Conditions (ICs) and Boundary Conditions (BNDs) to the CFD-3D analysis. Then, a series of combustion simulations are carried out at maximum power operative point (9500 rpm – WOT), varying spark advance and equivalence ratios.Results reveal that an increase of fuel flow rate and a new calibration of spark timing are needed when the engine runs on E85 to reach performances comparable to the ones obtained with E5. Simulations also show that, moving from E5 to E85, combustion efficiency can be significantly increased, with a small reduction in engine performance.An estimation of specific emissions, provided by ECFM-3Z combustion model, show that, using E85, CO and HC emissions can be significantly reduced with a small increase of NO emissions, compared to gasoline case.