Laminar Flame Speed Measurements Research Articles

Laminar flame speed measurement and combustion mechanism optimization for ethylene–air mixtures

AbstractEthylene plays a crucial role as an intermediate component in the cracking and combustion processes of large molecular alkane and olefins. In this article, the laminar flame speed of ethylene–air mixtures was measured using the heat flux method. The mechanism of ethylene was simplified by utilizing the error propagation directed relationship graph (DRGEP) and sensitivity analysis (SA), and the Arrhenius pre‐exponential factors for 20 selected reactions in the skeletal mechanism were optimized using the particle swarm optimization (PSO) algorithm. Finally, an ethylene optimization mechanism including 39 species and 85 reactions was obtained. The prediction results for flame speed, ignition delay time, and species concentration were compared with experimental data and other mechanisms, covering a wide range of temperatures (298–1725 K), pressures (1–22.8 atm), and equivalence ratios (0.5–2.0). The findings demonstrate that the optimization mechanism not only improves the prediction results of laminar flame speed in the rich combustion zone and low oxygen environment but also enhances the prediction accuracy of the ignition delay time at high pressure and in the lean combustion zone, as well as the prediction accuracy of C2H4 and H2O radicals. In conclusion, the optimized mechanism exhibits higher accuracy and broader applicability.

Ammonia/hydrogen laminar flame speed measurements at elevated temperatures

Laminar flame speeds of ammonia/hydrogen blends diluted in a mixture of 79% Ar and 21% O2 (referred to as “airgon”) were measured for the first time at high temperatures exceeding 500 K and atmospheric pressure using the shock tube flame speed method. Lean (ϕ = 0.8), stoichiometric (ϕ = 1) and rich (ϕ = 1.2) mixtures, with hydrogen fuel fractions xH2 = 0.2, 0.4, and 0.6 were studied in the temperature range of 500–820 K behind reflected shock waves. A significant enhancement in flame speeds and improved flame morphology was observed with increasing concentrations of hydrogen. Equivalent fuel/air data were computed from fuel/airgon measurements using a mixture-scaling approach and compared against available literature data. Although current data do not overlap with the temperatures of data in the literature, the full 300–800 temperature range of experimental data remain consistent with a non-Arrhenius correlation, while a power-law fit does not capture the full data trend. 1-D flame speed simulations were conducted using five recent chemical kinetic models of ammonia combustion available in the literature. Large disagreement was found among the models, with the model by Zhou et al. agreeing well with experiments at all conditions studied in this work. Sensitivity analyses were performed, and the most relevant reactions involved in the computation of flame speed, according to the models, were identified. Overall, the experimental data and modeling demonstrate the value of using blends of ammonia and hydrogen to improve the combustion characteristics of ammonia as a carbon-free fuel.

Laminar flame speed measurement and combustion mechanism optimization of methanol–air mixtures

AbstractLaminar flame speeds of methanol/air mixtures at 338–398 K are measured by the heat flux method, extending the range of equivalence ratio up to 2.1. And a new optimized methanol mechanism with 94 reactions is proposed by using the particle swarm algorithm, adjusting 20 Arrhenius pre‐exponential factors in their uncertainty domains. The optimized model is compared with eight methanol combustion mechanisms and experimental data published in recent years, covering a wide range of initial temperatures (298–1537 K), pressures (0.04–50 atm) and equivalence ratios (0.5–2.1). The results show that the optimized mechanism not only improves the accuracy of ignition delay time with rapid compression machine at low temperature but also moderately improve the description of laminar flame speed in lean and stoichiometric conditions. Meanwhile, the optimized model significantly enhances the prediction accuracy of CH3 and CH2O radical, and perfectly captures the evolution trend of HCO radical in laminar flat flame. Overall, the optimized mechanism provides the best overall description of the currently available measurements, leading to more accurate and comprehensive prediction of ignition delay time, laminar flame speed and species concentration.

Chemical Kinetic Analysis of High-Pressure Hydrogen Ignition and Combustion toward Green Aviation

In the framework of the “Multidisciplinary Optimization and Regulations for Low-boom and Environmentally Sustainable Supersonic aviation” project, pursued by a consortium of European government and academic institutions, coordinated by Politecnico di Torino under the European Commission Horizon 2020 financial support, the Italian Aerospace Research Centre is computationally investigating the high-pressure hydrogen/air kinetic combustion in the operative conditions typically encountered in supersonic aeronautic ramjet engines. This task is being carried out starting from the zero-dimensional and one-dimensional chemical kinetic assessment of the complex and strongly pressure-sensitive ignition behavior and flame propagation characteristics of hydrogen combustion through the validation against experimental shock tube and laminar flame speed measurements. The 0D results indicate that the kinetic mechanism by Politecnico di Milano and the scheme formulated by Kéromnès et al. provide the best matching with the experimental ignition delay time measurements carried out in high-pressure shock tube strongly argon-diluted reaction conditions. Otherwise, the best behavior in terms of laminar flame propagation is achieved by the Mueller scheme, while the other investigated kinetic mechanisms fail to predict the flame speeds at elevated pressures. This confirms the non-linear and intensive pressure-sensitive behavior of hydrogen combustion especially in the critical high-pressure and low-temperature region which is hard to be described by a single all-encompassing chemical model.

Measurements of methane laminar flame speeds at temperatures up to 1320 K

Atmospheric-pressure laminar flame speed measurements of methane diluted in airgon (79 % Ar/21 % O2) and air were performed at temperatures up to 1320 K using the shock-tube flame speed method. A mixture-scaling approach was used to convert methane/airgon measurements to methane/air equivalents for additional high-temperature data relevant to practical applications. Measurements and scaled data were compared with literature values, showing good agreement at overlapping conditions. Five kinetic models optimized for methane oxidation were used to simulate flame speeds. The reduced model by Stagni et al. (2015) and GRI 3.0 were the best in reproducing the present experimental data. Additionally, empirical correlations based on the power-law and non-Arrhenius models are reported, with the latter more accurately representing the experimental results.

Enhancing lithium-ion battery safety: Investigating the flame-retardant efficacy of bis(2,2,2-trifluoroethyl) carbonate during ethyl methyl carbonate combustion

Bis(2,2,2-trifluoroethyl) carbonate (BtFEC) is a candidate fire suppressant for lithium-ion batteries (LIBs). The high flammability of the electrolyte is the reason behind the frequent fire incidents reported with LIBs. These incidents are due to various factors, such as a default in the conception of the battery or operational abuse. The flame-retardant effectiveness of BtFEC was investigated by measuring its effect on the oxidation of ethyl methyl carbonate (EMC), a common LIB electrolyte component. Laminar flame speed (LFS) measurements for EMC-BtFEC in air mixtures were carried out at 403 K, 1 atm, for equivalence ratios (ϕ) between 0.8 and 1.4, and with 0.5 % vol. BtFEC. Additional measurements were made at ϕ = 1.1 with BtFEC addition ranging from 0.1 up to 1.4 % vol. To further understand the combustion chemistry of the EMC-BtFEC system, ignition delay times (time at the maximum peak production of the excited radical OH*) and CO time histories were measured in a shock tube with temperatures ranging from 1217 to 1732 K, at near-atmospheric pressure (1.24–1.42 atm), and for three equivalence ratios (ϕ = 0.5, 1.0, and 2.0). These new results constitute an experimental database that was used to evaluate the performance of a model assembled for both BtFEC and EMC using previous work from our group. The updated, detailed chemical kinetics mechanism presented in this study consists of 237 species and 1630 reactions. Sensitivity, rate-of-production, and reaction pathway analyses permitted the oxidation of the BtFEC-EMC system to be described. A flame sensitivity analysis exhibited the significant effect from the reactions H + CF2O ⇆ HF + CFO and CF3 + H ⇆ CF2 + HF, which consume H radicals that are involved in the branching reaction H + O2⇆ OH + O, ultimately reducing the LFS. This study shows that improvements in the base fluorine chemistry are still necessary to obtain better agreement with the experiments, especially at the speciation level.

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.

Laminar flame speed measurements of ethanol, iso-octane, and their binary blends at temperatures up to 1020 K behind reflected shock waves

A shock tube augmented with a laser-spark ignition system was used to measure the atmospheric-pressure (1 atm ±2%) laminar flame speeds of neat ethanol and iso-octane as well as ethanol/iso-octane blends in a 21% O2-79% Ar (so-called “airgon”) oxidizer. Low-temperature (453 K–524 K) validation experiments of atmospheric-pressure, stoichiometric ethanol/air were first performed for comparison with literature. Two fuel blends with 50% and 85% ethanol (by volume) in iso-octane (E50 and E85, respectively) were then studied in airgon at high unburned-gas temperatures (637 K–1020 K) and at various equivalence ratios (0.75, 1, and 1.2). The low-temperature ethanol/air measurements showed good agreement with literature data, confirming the accuracy of flame speeds measured using a shock tube. High-temperature fuel/airgon flame speeds were simulated with various kinetic models, and significant model disagreement was found, highlighting the value of the new flame speed data in the high-temperature regime. To make the present fuel/airgon measurements compatible with practical engineering applications using air as the oxidizer, the ratio of fuel/airgon and fuel/air flame speeds was calculated using 1D simulations for each experiment, then subsequently used to perform mixture-scaling for the fuel/airgon data to obtain equivalent fuel/air flame speeds. Fuel/air laminar flame speed correlations as a function of the unburned-gas temperature were determined for the studied fuel blends and equivalence ratios. Finally, flame speed sensitivity analyses conducted for high-temperature ethanol/airgon and iso-octane/airgon flames revealed three key governing reactions. Assigning common rate expressions to these reactions across different kinetic models improved the agreement between model-predicted and experimentally measured ethanol flame speeds, but did not have a similar effect for iso-octane, suggesting the need for further exploration of iso-octane-specific reaction rates to facilitate better model agreement.

Development of a reduced combustion model for ammonia/hydrogen combustion

Ammonia/hydrogen receives increasing interest as a carbon-free fuel in novel combustion techniques. Simulation aided design often requires coupling of flow field and chemical kinetics, whose feasibility relies essentially on an affordable (reduced) reaction model. However, existing ammonia/hydrogen reduced models are scarce and cannot reasonably reproduce available experimental data over a comprehensive set of conditions. Hence, a reduced ammonia/hydrogen model was developed in the present work using a combined method: the directed relation graph with error propagation method was applied first followed by a contribution to species production/consumption screening method. The model reduction method substantially reduces the size of the model, from 37 species and 237 reactions to 25 species and 108 reactions. The reduced model was compared to available reduced models over a comprehensive set of experimental data, including ignition delay time measurements in shock-tube and rapid compression machine, laminar flame speed measurements and species profile data measured in jet stirred reactor and burner stabilized flame. The obtained model maintains comparable prediction accuracy compared to the detailed model and is superior to the other reduced models.

On the ignition kernel formation and propagation: an experimental and modeling approach

The next generation of advanced combustion devices is being developed to operate under ultra-high-pressure conditions. However, under such extreme conditions, flame tends to become unstable and measurement of fundamental properties such as the laminar flame speed becomes challenging. One potential method to resolve this issue is measuring the ignition-affected region during spherically expanding flame experiments. The flame in this region is more resistant to perturbations and remains smooth due to the high stretch rates (i.e. small radii). Stable flame propagation allows for improved flame measurement, however, the experimentally observed kernel propagation is a function of both inflammation and ignition plasma. Therefore, the goal of the present study is to better understand the plasma formation and propagation during the ignition process, which would allow for reliable laminar flame speed measurements. To accomplish this goal, thermal plasma operating at high pressures is studied with emphasis on the spark energy effects on the formation of the ignition kernel. The thermal effect of the plasma is experimentally observed using a high-speed Schlieren imaging system. The energy dissipated within the plasma is measured with the use of voltage and current probes with a measurement of plasma sheath voltage drop as an input to numerical modeling. The measured kernel propagation rate is used to assess the accuracy of the model. The experiments and modeling are conducted in dry air at 1, 3, and 5 atm as well as in CH4-N2 mixtures at 1 atm, and kernel radius, temperature, and mass are reported. The voltage-drop (as a non-thermal loss) is measured to be approximately 330 5 V (dry air at 1 atm) for glow plasma with a large dependency on pressure, gas composition, electrode surface quality, electrode geometry, electrode shape, and current density. The same loss within the arc plasma is measured to be 15 5 V, however the arc phase loss which agrees with arc propagation is significantly higher (∼45 V) which suggest additional unaccounted for phenomena occurring during the arc phase. With these losses, the modeling results are shown to predict the final kernel radius within 10%–20% of the observed kernel size. The difference found between the modeling and experimental results is determined to be a result of assuming that the primary loss mechanism (voltage drop across sheath formation) remains constant for the duration of glow discharge. The discrepancy for arc discharge is discussed with several potential sources, however, additional studies are required to better understand how the arc formation affects the kernel propagation.

Atmospheric-pressure shock-tube measurements of high-temperature propane laminar flame speed across multiple equivalence ratios

Laminar flame speeds of premixed propane in 21%O2-79% Ar (so-called ”airgon”) were experimentally measured in a shock tube at atmospheric pressure (1 atm±2%) for a range of equivalence ratios (0.5, 0.75, 1, and 1.2) and unburned-gas temperatures (644–1,044 K). Simultaneous side-wall schlieren and end-wall CH* emission imaging was utilized to track the propagation of outwardly expanding laminar flames initiated by laser-induced spark ignition behind reflected shock waves. The linear curvature model with an area-averaged formulation was used to extrapolate laminar flame speeds to zero-stretch. The high-temperature laminar flame speed results were compared with 1D simulations performed using two detailed kinetic models, NUIG 1.3 and Aramco 3.0, and two skeletal models, USC Mech II and San Diego 2016. The experimental results agree with the NUIG 1.3 model within 3.5%, while the predictions of the models disagree by up to 16%. Sensitivity analyses conducted using the models revealed six chemical reactions that strongly impact the prediction of laminar flame speed. In order for the present data to be compatible with practical combustion systems using air as the oxidizer, the propane/airgon measurements were scaled to obtain laminar flame speeds of propane/air using ratios calculated from 1D simulations. The scaled experimental measurements showed good agreement with literature propane/air results at 650 K. Empirical correlations of laminar flame speed as a function of unburned-gas temperature are then reported for the studied equivalence ratios.

Laminar flame speed measurements of a gasoline surrogate and its mixtures with ethanol at elevated pressure and temperature

Laminar Flame speed measurements of a gasoline surrogate and mixtures of it with ethanol were conducted using a heated, constant-volume vessel. A spherical propagating flame was observed using a high-speed camera, and laminar flame speed was determined therefrom. The gasoline surrogate, which serves as the baseline for the current study, consisted of four components, namely, iso-octane, n-heptane, toluene, and 1-hexene. Different mixtures of the gasoline surrogate and ethanol were studied, governed by the ethanol percentage in the mixture. That is, E0, E30, E50, and E85 mixtures represent 0%, 30%, 50%, and 85% ethanol in the gasoline surrogate mixture by liquid volume, respectively. Initial temperatures of 335, 359, and 408 K and initial pressures of 1 and 3 bar were investigated. The findings of this study are compared to results in the literature, which show good agreement for E0 but some deviation for the E30 blend. In general, the study showed an increase in laminar flame speed as the ethanol percentage increases in the mixture. Similarly, increasing the initial temperature with fixed ethanol percentage resulted in an increase in laminar flame speed, as expected. In contrast, increasing the initial pressure with fixed Ethanol percentage showed a decrease in laminar flame speed. Finally, the results are compared to an existing chemical kinetics model designed for ethanol and gasoline. Although agreement between the model and data is reasonable and mostly within about 10%, some improvement to the kinetics model is needed to uniformly lower the calculated flame speeds.

Study of water and hydrogen addition on the pyrolysis and combustion characteristics of isooctane

Water injection and hydrogen combustion technologies have gained widespread attention in recent years. Combining H2O and H2 injection could bring more desirable combustion based on the needs of different engine conditions. To fully explore what effects the simultaneous injection of H2O and H2 would have on the combustion process of isooctane, three different subtests were performed in this study: investigation on chemical reaction mechanism, the laminar flame speed measurements and real engine tests. Firstly, H2O and H2 were added to the JSR (Jet Stirred Reactor) reactor to analyze the synergistic effect of H2O and H2 on the pyrolysis process of isooctane. The experimental results showed that the addition of H2O and H2 leads to an increase of OH and H radicals, which increases the concentrations of C1-C3 species and decreases the concentrations of C4 species. To further investigate the effect of H2O and H2 on the radical reaction of isooctane combustion process, this study then measured the laminar flame speed of IC8H18/H2/O2/H2O gas mixture by using constant volume combustion bomb, and found that water injection would reduce the laminar flame speed, and H2 would increase the laminar flame speed accordingly. Sensitivity analysis showed that it was more sensitive to reactions R1, R97 and R12, and was more sensitive to H2O than to H2. Finally, real engine tests were performed on a modified 1.5L gasoline equipped with port H2O and H2 injection systems at 1300 rpm with 30% throttle opening. The results showed that the use of H2O and H2 permitted the manipulation of engine combustion speed. Specifically, H2O injection led to a decrease in the peak cylinder pressure; and the addition of H2 increased the cylinder pressure again·H2O injection resulted in increased ignition delay and slower flame spread, slowing the engine's burn rate when needed. Using H2 addition led to a short ignition delay and accelerated combustion velocity, which counteracted the negative impacts of H2O injection and elevated the engine combustion speed to optimize efficiency. In summary, this paper revealed the complementary coordination between H2O and H2 through experimental and simulation studies. It will provide theoretical guidance for the development of H2O injection with H2 technology for gasoline engines.

Experimental measurements of laminar flame speeds for highly N[formula omitted]-diluted ethanol flames under microgravity conditions

Nitrogen has been used as an alternative for CO2 in the fire suppression industry. The high nitrogen concentrations added to a reactive mixture slows down its flame propagation in case of a fire outbreak. To describe quantitatively the flame spread as a function of nitrogen dilution, it is mandatory to evaluate the mixture’s laminar flame speed. Performing experiments under earth gravity using the RWTH high-pressure, high-temperature spherical combustion chamber showed that the laminar flame speed extraction is impossible for such highly dilute conditions due to severe flame-front distortions. To overcome the influence of buoyancy, experiments were carried out under microgravity conditions in a parabolic flight campaign onboard the Airbus A310 Zero-G. The setup consists of a pressure-release-type dual chamber adequate for aircraft-related safety requirements, and a conventional shadowgraphy system to visualize the flame morphology under quasi-isobaric pressure. Only a limited number of liquid energy carriers are allowed onboard, and ethanol was chosen because of its short molecular structure and low boiling point compared to longer chain liquid alkanes such as n-heptane. Results showed a flame speed reduction induced by radiation in the order of 10% to 45% for near-stoichiometric to very rich conditions, respectively. To correct laminar flame speeds for radiation effects, an empirical correlation presented by Yu et al. (Combust. Flame 161 (2014) 2815–2824) is applied. Excellent agreement between radiation-corrected experimental data and simulations using the chemical mechanism of Cai et al. (Combust. Flame 37 (2019) 639–647) is found. This indicates that high dilution levels with nitrogen can be captured well by kinetic schemes derived for fast-burning flames. The unique experimental data obtained here, which were extremely difficult to measure and post-processed very carefully, can be used as a reference to validate/improve existing and newly developed kinetic mechanisms.

A comprehensive kinetic modeling of oxymethylene ethers (OMEn, n=1–3) oxidation - laminar flame speed and ignition delay time measurements

This work reports on the development and experimental validation of a detailed reaction mechanism for the oxidation of polyoxymethylene dimethyl ethers (OMEn, n = 1–3). The validation is done by constant-volume chamber laminar flame speeds (393 K and 443 K, 1 to 5 bar, and equivalence ratio 0.8 to 1.6) and Rapid Compression Machine ignition delay times (550–680 K, 10 and 15 bar, equivalence ratios of 0.5–2.0) in OMEn/air mixtures. Using our new experimental and published data, the validation basis for the new kinetic model comprises the pyrolysis and oxidation of OMEn (n = 1–3) in freely propagating flames, auto-ignition in rapid compression machines and shock tubes, and speciation in jet-stirred and flow reactors as well as burner-stabilized premixed flames. The model provides a reasonable agreement with the experimental data for a broad range of conditions investigated. The performance of the developed model is compared against the recent literature models. OMEn (n = 1–3) all have the same laminar flame speed. The model suggests that the chemistry of OME0 (DME) and CH3OCHO (methyl formate) is the one that dictates the flame chemistry. Under the same pressure and equivalence ratio conditions, ignition delay times of OME2 and OME3 are similar for the investigated temperature range. This work helps to improve the understanding of OMEs chemistry. The model developed in this work will serve as the base mechanism for higher chain length OMEs (n>3).

Measurements of propane–O2–Ar laminar flame speeds at temperatures exceeding 1000 K in a shock tube

Laminar flame speed (SL) measurements of stoichiometric propane in an oxygen-argon oxidizer were performed in a shock tube at unburned-gas temperatures of 296–1234 K and near-atmospheric pressures. Non-intrusive laser-induced breakdown is used to ignite expanding flames following the reflected-shock passage. Flame propagation is recorded using schlieren imaging in a recently implemented side-wall imaging flame test section (SWIFT). In a refined approach to account for flame distortion and the slight residual motion of the post-reflected-shock gas, an area-averaged formulation of the linear-curvature model (the AA-LC model) is derived for use extrapolating flame data to zero stretch. Measured SL values extracted using the AA-LC model closely agree with previous experimental measurements performed in a conventional kinetics shock tube (CKST) using much smaller flame kernels, providing evidence of the earlier data having been ignition affected. Below the chemistry-affected limit of 1050 K, experimental SL values fall in the range of values simulated with the detailed AramcoMech 3.0 kinetic mechanism and propane-specific mechanisms from NUIG and San Diego but exhibit a stronger temperature dependence than predicted by the mechanisms. Over the wide temperature range of the present data, the ubiquitous power-law form of empirical fit is shown to be inadequate for capturing the SL temperature dependence; a non-Arrhenius form is shown to perform favorably. The uncertainties of flame speed measurements performed in the SWIFT average 3.0% and 4.4% for experiments performed under static and post-reflected-shock conditions, respectively, a reduction from the 5.8% average uncertainty of CKST experiments. This work represents a significant step forward in the development of experimental capabilities for high-temperature flame speed measurements. The present results illustrate the potential value of the shock-tube flame speed method to provide measurements useful for informing kinetic model tuning and validation at conditions for which experimental data were not previously obtainable.

How fast can we burn, 2.0

We review the state of the art in measurements and simulations of the behavior of premixed laminar and turbulent flames, subject to differential diffusion, stretch and curvature. The first part of the paper reviews the behavior of premixed laminar flames subject to flow stretch, and how it affects the accuracy of measurements of unstrained laminar flame speeds in stretched and spherically propagating flames. We then examine how flow field stretch and differential diffusion interact with flame propagation, promoting or suppressing the onset of thermodiffusive instabilities. Secondly, we survey the methodology for and results of measurements of turbulent flame speeds in the light of theory, and identify issues of consistency in the definition of mean flame speeds, and their corresponding mean areas. Data for methane at a single operating condition are compared for a range of turbulent conditions, showing that fundamental issues that have yet to be resolved for Bunsen and spherically propagating flames. Finally, we consider how the laminar flame scale response of flames to flow perturbations interacting with differential diffusion leads to very different outcomes to the overall sensitivity of the burning rate to turbulence, according to numerical simulations (DNS). The paper concludes with opportunities for future measurements and model development, including the perennial recommendation for robust archival databases of experimental and DNS results for future testing of models.

An updated empirical correlation formalism for laminar flame speeds: Application to a TRFE gasoline surrogate in highly diluted conditions

In order to comply with present and future stringent environmental policies, engine manufacturers have to improve engine design and control to achieve combustion with high dilution ratios. The use of CFD simulations with complex combustion chemistry remains prohibitive, and alternatives to assess quickly the laminar flame speeds at local grid cell conditions (temperature, pressure, equivalence ratio, and dilution ratio) are desired, such as empirical correlations. Whereas these correlations should ideally be obtained from experimental measurements, comprehensively validated kinetic models can help extend databases to conditions that cannot be achieved practically. Hence, the present study proposed a reduced kinetic mechanism, containing 593 species and 3698 reactions, for one gasoline surrogate, namely Toluene Reference Fuel with Ethanol addition (TRFE). It is obtained by first compiling sub-mechanisms from the literature for the four components (isooctane, n-heptane, toluene, and ethanol) with updates of some key rate constants, and then is reduced for 1-D flame speed computations. The model was first validated against recent experimental laminar flame speed measurements of TRFE/air/diluent mixtures for various temperatures, pressures, equivalence ratios, and dilution ratios and then employed to extend the experimental database for the TRFE surrogate to higher dilution ratios and temperatures. A new formalism, including new mathematical expressions for the reference and dilution terms, and the temperature exponent are proposed. This new formalism exhibits improved abilities in fitting the laminar flame speeds, especially at high dilution ratios and in very fuel-lean and fuel-rich conditions. A new mathematical correlation based on these formulas was developed, whose correlation parameters were obtained by fitting both experimental data and mechanism predictions. Regardless of the conditions, the present correlation is observed to show overall good agreements with available experimental data in the literature for laminar flame speeds and their dependence on equivalence ratio, temperature, pressure, and dilution. Results show that the effect of dilution in reducing flame speeds is not linear and depends on the equivalence ratio. A parameter μ is defined in the correlation formulas to evaluate the dilution effectiveness of the diluent, which is found to be composition-specific and possibly follows a linear mixing rule for diluent mixtures.

Laminar flame speeds and ignition delay times for isopropyl nitrate and propane blends

Laminar flame speed and ignition delay time measurements are presented for blends of isopropyl-nitrate and propane. The laminar flame speeds were measured in a spherical bomb, with the majority of the experiments performed at 373 K and 1 bar, with equivalence ratios between 0.6 <ϕ< 2.1; the fuel composition was 0%, 10%, and 100% isopropyl-nitrate. Additional experiments were performed at 300 K for pure propane as a fuel, and at 0.5 bar for pure isopropyl-nitrate as fuel. The ignition delay measurements were performed in a shock tube, with reflected shock temperatures between 1050 K and 1530 K and a nominal reflected shock pressure of 20 bar. Equivalence ratios of 0.5, 1.0, and 1.5 were considered for fuel blends with 0% and 1% isopropyl-nitrate in propane; additional experiments were performed with 10% isopropyl-nitrate at ϕ = 1. In comparison with the pure propane flames, the pure isopropyl-nitrate have a similar peak flame speed (SL0=60.3 cm/s at ϕ=1.09 for isopropyl-nitrate versus SL0=57.9 cm/s at ϕ=1.10 for propane), but the nitrate is considerably faster under fuel lean conditions, and it exhibits a much broader domain. The ignition delay times for 1% isopropyl-nitrate in propane are indistinguishable from the pure propane experiments for all three equivalence ratios. The 10% isopropyl-nitrate in propane mixture, in contrast, is significantly more reactive. Transition state theory calculations were performed for select RH + NO2 reactions. These calculations were added to a recently developed mechanism for isopropyl-nitrate and propane. Detailed simulations of the experiments were performed in Cantera. The agreement between the models and experiments is excellent. The largest discrepancies occur for the ignition delay times for 10% isopropyl-nitrate at T5<1150 K and for the flame speeds for ϕ>1.8, which suggests that further work needs to be done under these conditions. To the best of our knowledge, these data represent the first published results on the laminar flame speeds of any alkyl nitrate.

Experimental and Numerical Studies of Ignition Delay Time and Laminar Flame Speed of JP-10 at Elevated Temperature Conditions

In this work, the laminar flame speeds of JP-10/air mixtures and ignition delay time of JP-10/O2/Ar mixtures have been studied over a wide range of experimental conditions using a premixed Bunsen flame and a shock tube. Laminar flame speed measurements of the JP-10/air mixture were measured at T = 360–453 K, φ = 0.7–1.3 and p = 1 atm. The ignition delay times of JP-10 with fuel mole fraction of 0.2% JP-10/2.8% O2/97% Ar were measured with a heated shock tube behind the reflected shock wave at 1183–1478 K and pressure of 3.4 atm. Numerical calculation and sensitivity analysis of the laminar flame speed and ignition delay time of JP-10 were performed by using published detailed and skeletal kinetic mechanisms. The comparison of experimental and numerical results showed that all models tend to overestimate the laminar flame speed under the studied conditions, especially under lower temperature conditions (360–423 K). A temperature dependency empirical correlation of laminar flame speed was then proposed by power law theory. The sensitivity analysis identified three important reactions [H+O2 = O+OH, C5H5+H (+M) = C5H6(+M), C3H3+C2H2 = C5H5] that determined the laminar flame speed and ignition delay time.