Heat Flux Burner Research Articles

An experimental and modeling study on extinction strain rate in C2HX flames with varied oxygen content

The extinction strain rates (ESR) is an important physical-chemical properties of fuels. Therefore, an experiment study on ESR is crucial for studying emission reduction techniques. In this work, the ESR and laminar burning velocity (LBV) of C2HX fuels under a wide range of conditions are determined experimentally. Moreover, a series of the reduced mechanisms for predicting ESR and LBV of C2 hydrocarbons (C2H2, C2H4, C2H6) is provided, because the research of mechanism reduction seldom pay attention to ESR and LBV. The contribution of this work is therefore the improvement of understanding of the oxidation characteristics of light C2 hydrocarbons and to provide reduced skeletal mechanisms for those fuels with different levels of accuracy for their interaction with reduced oxygen content. For these gas mixtures, the extinction strain rates (ESR) and the laminar burning velocity (LBV), two important physical-chemical properties, were investigated in a counterflow and a heat flux burner setup under atmospheric conditions (1 bar and 298 K). The LBV of the premixed flames, as well as the ESR of the non-premixed C2Hx/O2/N2 flames, are determined experimentally and compared to the numerical predictions of a detailed mechanism (USC II) and the seven corresponding reduced skeletal mechanisms. The present study highlights the non-linear influence on the ESR of C2 hydrocarbons that were found with decreasing fuel content and decreasing oxygen content in either case. Since C2H2 has a significantly higher reactivity than C2H4 and C2H6, this is manifested in lower ESR and higher LBV for the same conditions. The recently developed species-targeted global sensitivity analysis (STGSA) is used to develop reduced mechanisms, showing that they can reproduce the ESR trends and are within the determined measurement uncertainty in a wide range of operating conditions. In addition, these developed significantly reduced mechanisms remain good at predicting the laminar burning velocity. The research generated a unique experimental dataset of the ESR and LBV of C2Hx which was also used to develop reduced mechanisms. Sensitivity analysis for the ESR and LBV gives insight into the difference between ESR and LBV calculations.

Experimental and numerical investigations of the laminar burning velocities of premixed fuel-rich methane oxy-fuel and oxygen-enhanced flames

In the context of Oxy-fuel combustion, there is a strong industrial interest in highly fuel-rich flames, characterized by an equivalence ratio (Φ) exceeding 2.5, for the production of H2-CO-rich synthesis gas. The validation of existing chemical reaction mechanisms for this specific regime pose a significant challenge due to the scarcity of experimental data for this particular range of equivalence ratio.The scope of this study was therefore the determination of the laminar burning velocity sL of ultra-rich CH4-O2 flames under varying conditions, such as high equivalence ratios and preheating temperatures using the heat flux burner method. The laminar burning velocity was determined for equivalence ratio within the range of 2.3 < Φ < 3.6 and preheating temperature ranging from 300 K to 455 K. Additionally, the impact of the of reducing the oxygen content in the oxidizer on sL was investigated by argon addition in molar percentages ranging from 40 % to 100 %.The results of our study reveal laminar burning velocities ranging from 5 cm/s to 45 cm/s for pure oxy-fuel CH4 flames. Preheating is shown to increase sL and this effect can be represented by a power law correlation sL = sL,0(T/T0)α with α = 1.53. Furthermore, the dilution of the oxidizer with argon leads to a significant decrease in sL of approximately 50 % compared to pure oxy-fuel flames. It is worth noting that detailed reaction mechanisms in combination with molecular transport data are typically not validated within the selected equivalence ratio range.To assess the performance of such reaction schemes in predicting laminar burning velocity under the specified selected conditions, we evaluated 13 different reaction mechanisms sourced from the literature. We compared the laminar burning velocities obtained through calculations of laminar premixed 1D flames and, additionally compared these with experimental data. A sensitivity analysis of the laminar burning velocity were carried out for selected mechanisms. Among the mechanisms, the CalTech2.3 mechanism exhibited the most consistent and accurate performance, particularly at high equivalence ratios.

Insight into premixed diethoxymethane flames: Laminar burning velocities, temperatures, and emissions behaviour

Diethoxymethane ((CH3CH2O)2CH2, DEM) is a promising carbon-neutral fuel. DEM is a diether or acetal with a molecular structure similar to oxymethylene ethers (CH3O–(CH2O)n–CH3, OMEn). Thus, DEM can be expected to have a similar combustion behavior to OMEs, reducing harmful emissions such as NOx and particulate matter (PM) in internal combustion engines. From both experimental and kinetic modeling, fundamental studies on DEM are scarce in the literature. More studies are required to gain a detailed insight into the oxidation kinetics of DEM. Laminar burning velocity (LBV) is a critical property that allows a detailed assessment of the potential application of DEM in combustion devices. Unfortunately, the literature on the LBV of DEM is limited. Therefore, in this study we have investigated the LBV of DEM using two reactors for the first time, namely a heat flux burner and a combustion chamber. The experimental data is reported for equivalence ratio between 0.7 and 1.7, initial temperatures of 368–423 K, and initial pressure of 1–5 bar. In addition, we developed a detailed kinetic model extending our recent work of Shrestha et al. (Combust. Flame. 246 (2022) 112,426) to characterize the combustion behavior of DEM utilizing the new experimental data from this work and the literature data. Our model performs remarkably well in capturing the newly measured LBV experimental data over various experimental conditions. We found that DEM and dimethoxy methane (DMM) have similar values of LBVs (within ±1.5 cm/s) for a given condition, which indicates that intermediate chemistry governs the flame chemistry. Despite DEM being a larger molecule that is expected to have slightly lower LBVs than DMM, its effect on the measured values of LBVs is negligible. Finally, we experimentally measured NOx formation in DEM flame for the first time. The stochiometric flame has the highest NOx formation. The proposed model predicted the equivalence ratio dependence of NOx nicely. However, it overestimates the NOx formation for stoichiometric DEM/air mixtures by ∼30 %. The model suggests that the thermal NO formation route is favored at lean and stochiometric conditions. In contrast, the prompt NO formation route is enhanced for rich mixtures.

Structure and dynamics of hexagonal cells in H2/CO2 flames

Transition from fossil fuels to sustainably produced hydrogen remains an important goal to achieve current climate targets. Hydrogen from gasification and steam reforming is produced alongside CO2. Instead of separating the CO2, it can be kept in the product gas, leading to possible reductions in NOx and improved safety aspects. However, the addition of CO2 makes hydrogen flames more prone to cellular instabilities, which can lead to problems when designing burners for hydrogen-based flames. To understand the formation and dynamics of these instabilities, fully resolved 3D simulations of H2/CO2 cellular flames on a heat flux burner are performed. The numerical configuration follows an experimental setup that observed cellular and band-like structures stabilizing on the burner plate. With the simulations employing finite rate chemistry and detailed diffusion models, hexagonal flame structures are identified and characterized. The simulations show good qualitative agreement of the flame structure with the measurements. The influence of mass flow rate, burner plate temperature and equivalence ratio on the flame structure is investigated. A transition from band-like to hexagonal structures is described in terms of cusp formation, which become the corner points of the hexagonal cells, and the stabilization mechanism is explained via the high local flame stretch above holes in the burner plate. A Markstein number-based correlation for the cell size with respect to the burner plate temperature is proposed and the simulation data is provided as a public database for further analysis.

Chemical insights into ethyl acetate flames from experiment and kinetic modeling: Laminar burning velocity, speciation and NOx emission

Oxygenated fuels, such as alcohols, ethers, and esters, are promising alternatives to conventional fuels. These fuels can help reduce detrimental emissions like carbon monoxide and unburned hydrocarbons and enhance octane ratings. Among these oxygenates, ethyl acetate (EA), a small alkyl ester sourced from biomass, emerges as a clean, promising energy carrier. It serves as a surrogate fuel to facilitate investigations into the combustion behaviours of biodiesel. Despite its importance, the literature knowledge of EA combustion characteristics is limited. Therefore, this study aims to broaden the knowledge of the combustion behaviour of this type of oxygenated fuel compound. In this study, we measured the laminar burning velocities of EA by employing a heat flux burner and a closed combustion vessel over the equivalence ratios of 0.7 – 1.7, pressures of 1 – 10 bar and temperatures ranging from 353 – 423 K. Further, we also measured the NOx emissions in exhaust gas of the premixed flames fueled by EA/air for the first time over the equivalence ratio of 0.8 – 1.2. Additionally, we employed a non-premixed counterflow flame setup for extensive characterisation of species and their concentration under diverse conditions encompassing various strain rates and oxygen concentrations. Finally, we utilized these newly measured data to construct and validate a detailed kinetic model developed as part of this work. The newly developed model will help characterize the combustion properties of EA.

Measurements and simulations on effects of elevated pressure and strain rate on NOx emissions in laminar premixed NH3/CH4/air and NH3/H2/air flames

Efforts to reduce NOx emissions by utilizing ammonia-fired combustors are made through various methods; this work investigates the possibility of combining the effects of pressure and stretch rate to reduce NOxemissions in laminar premixed flames. NOx emissions, including NO, NO2, N2O, NH3,and HCN, from NH3/CH4/air and NH3/H2/air unstrained and strained laminar premixed flames were separately measured and simulated with a high-pressure heat flux burner and CHEMKIN software. The key points are that increasing the pressure can decrease most NOx emissions, including NO, N2O, and HCN. However, it slightly increases the unburnt ammonia emissions and largely increases the NO2 formation. Medium XNH3 conditions are the most favorable for reducing NO emissions by increasing pressure. High strain rates can reduce NO emissions, but only in stoichiometric and rich conditions; they reversely increase NO emissions for lean equivalence ratios before the NO peak equivalence ratio (ϕ). The reason is explained by kinetic and reaction pathway analyses, which showed that in high-strain flames, more fuel-N species participated in the reaction to form N2 instead of NO for higher ϕ, while O and OH increased for lower ϕ, promotingNO formation. The combination of increasing pressure and strain rate can efficiently reduce NO emissions but ensure full ammonia oxidation for the medium ammonia content at ϕ = 0.9 ∼ 1.1. This sheds light on the utilization of highly strained ammonia-blend flames in more practical high-pressure burners to reduce NOx emissions.

Extremely Low Temperatures for the Synthesis of Ethylene Oxide

The partial oxidation of ethylene in a methane atmosphere by pure oxygen is the most important industrial process for the synthesis of ethylene oxide. However, due to the high reactivity and exothermicity of the reaction system, the overall production is limited by kinetic and safety issues. The shift toward mild operative conditions can support the management of undesirable side reactions, enhancing the performance of the whole process. In the future, the direct use of liquefied ethylene and oxygen-enriched air, a low-cost waste in membrane-based nitrogen production, can provide convenient sources for heat removal as well as promote the use of innovative and more sustainable solutions. This work is focused on the experimental and numerical characterization of the oxidation of ethylene/methane/nitrogen/oxygen mixtures at different operative conditions, including extremely low temperatures, and oxidant compositions. To this aim, the laminar burning velocity and flammability limits were first measured utilizing the heat flux burner and compared with detailed kinetic mechanisms and experiments retrieved from the current literature. The reported data were adopted to identify the operational limits for innovative processes, paving the way to unlocking the potential of innovative chemistry at extreme conditions. Eventually, key performance indicators accounting for kinetic and safety aspects were defined to identify the most sustainable and convenient operative conditions.

Impact of carbon dioxide and nitrogen addition on the global structure of hydrogen flames

Investigations into the combustion characteristics and flame structure of hydrogen/carbon dioxide/air flames have been carried out experimentally and numerically. The aim was to determine the variations of the combustion properties and flame shape for different carbon dioxide dilutions in the H2-air mixtures. For this purpose, premixed flames at various compositions of hydrogen-carbon dioxide mixtures are investigated experimentally in a heat flux burner configuration at varying equivalence ratios from 0.5 to 1.1, fresh gas temperatures and under atmospheric conditions. Optical investigations are performed to detect the number of cellular structures on the flame front. Furthermore, the laminar burning velocity and the influence of carbon dioxide addition were examined numerically. A detailed kinetic mechanism was implemented for the identification of the most representative intermediates via a reaction path analysis, and the most influential species and reactions are identified through sensitivity analyses at conditions relevant to the studied application. Considering the nature of the adopted mechanism, the presence of CO2 has the potential to shift the production/consumption rate of some hydrogen-containing radicals. Hence, numerical investigations employing an inert species having the same thermodynamic and transport properties as CO2 (referred to as fictitious CO2, FCO2) were compared and discussed in this work. To investigate the effect of CO2 on the dynamics of the hydrogen flames, one-dimensional and two-dimensional detailed simulations of the flame structure have been carried out. The addition of CO2 makes the flame more prone to thermo-diffusive instabilities through a decrease in the mixture’s thermal diffusivity. This results in a decrease of the Markstein number and an enhanced formation of characteristic cellular structures on spherically expanding flames. Overall, the comparison between the experimental and numerical investigations reveals similar conclusions.

On the laminar flame propagation of C5H10O2 esters up to 10 atm: A comparative experimental and kinetic modeling study

Biodiesel is a family of renewable engine fuels with carbon-neutral nature. In this work, three C5H10O2 esters (methyl butanoate, methyl isobutanoate and ethyl propanoate), which can serve as model compounds of biodiesel and represent linear and branched methyl esters and linear ethyl esters, were investigated to characterize their laminar flame propagation characteristics up to 10 atm and unravel the effects of isomeric fuel structures. A high-pressure constant-volume cylindrical combustion vessel was used to achieve laminar burning velocity measurements at 1–10 atm, 423 K and equivalence ratios of 0.7–1.5, while comparative experimental work was performed on a heat flux burner at 1 atm, 393 K and equivalence ratios of 0.7–1.6 for methyl butanoate and ethyl propanoate. The laminar burning velocity generally decreases with increasing pressure and increases in the order of methyl isobutanoate, methyl butanoate and ethyl propanoate, which shows distinct fuel isomeric effects. A kinetic model of C5H10O2 esters was developed and validated against the new data in this work and previous data in literature. Modeling analyses were performed to provide insight into the fuel-specific flame chemistry of the three esters isomers. Remarkable differences in radical pools of three ester isomers are concluded to be responsible for the observed fuel isomeric effects on laminar flame propagation. The feature of two ethyl groups connected to the ester group in ethyl propanoate facilitates the ethyl production and inhibits the methyl and allyl production, making it propagate fastest among the three isomers. The branched structure feature of methyl isobutanoate with methyl and i-propyl groups connected to the ester group prevents the ethyl formation and results in considerable CH3 and allyl production, which decelerates its laminar flame propagation.

Laminar burning velocities and nitric oxide formation in premixed dimethyl ether/air flames: Experiments and kinetic modeling

Adiabatic laminar burning velocities and post-flame NO mole fractions for flat, non-stretched, premixed dimethyl ether/air flames were experimentally determined with a heat flux burner combined with laser-induced fluorescence diagnostics, over equivalence ratios ranging from 0.7 to 1.6, at atmospheric pressure and initial temperatures from 298 to 338 K. The present burning velocity measurements were then compared with selected data available in the literature obtained with different techniques. The comparison showed reasonably good agreement with recent datasets obtained at different temperatures, as well as possible outliers not suitable for the validation of kinetic models. The detailed kinetic mechanism of the authors was extended by the reactions of dimethyl ether. A comparison of experimental and computational results using two contemporary detailed chemical kinetic mechanisms, along with the one from the authors presented in this work, was also conducted and discussed. Discrepancies between experiments and model predictions and among models themselves were observed, especially under rich conditions. Further numerical analyzes were performed to identify the main causes of the observed differences. Notwithstanding, the present model showed overall good performances in reproducing both laminar burning velocities and nitric oxide mole fractions. Kinetic modeling was also performed to enhance the understanding of the intrinsic NOx emission characteristics of dimethyl ether by comparing the present measurements with those obtained for ethanol/air flames under identical conditions.

Antioxidant effect of 2–4 xylenol on fuel oxidation in liquid and gas phase over a wide temperature range

The inhibiting effect on fuel ageing of 2,4-xylenol, a phenolic antioxidant that can be produced from biomass, was investigated experimentally in the liquid phase, while its impact on combustion was studied in the gas phase. The antioxidant effect of 2,4-xylenol in liquid phase has been examined in n-decane and in a surrogate, containing 65%vol. toluene and 35%vol. n-heptane. Experiments were performed in a PetroOxy at initial oxygen pressure 7 bar and temperature held at 180 °C. The impact of 2,4-xylenol on the gas-phase reactivity of the surrogate fuel was investigated on ignition delay times and laminar flame velocities. Ignition delay times were measured in the rapid compression machine at 10 bar, from 675 to 1000 K at two equivalence ratios, 0.5 and 1. Laminar flame velocity measurements were conducted in a heat flux burner at atmospheric pressure and an initial temperature of 398 K. For all experiments considered in this work, the doping level of 2,4-xylenol was kept below 1%mol. Results are compared to effect of other usual additives, EHN and ferrocene. Results show that 2,4-xylenol exhibits a remarkable antioxidant effect in liquid phase, without affecting the main combustion characteristics of the fuel.

Laminar Burning Velocity of Lean Methane/Air Flames under Pulsed Microwave Irradiation

Laminar burning velocity of lean methane/air flames exposed to pulsed microwave irradiation is determined experimentally as part of an effort to accurately quantify the enhancement resulting from exposure of the flame to pulsed microwaves. The experimental setup consists of a heat flux burner mounted in a microwave cavity, where the microwave has an average power of up to 250 W at an E-field in the range of 350–380 kV/m. Laminar burning velocities for the investigated methane/air flames increase from 1.8 to 12.7% when exposed to microwaves. The magnitude of the enhancement is dependent on pulse sequence (duration and frequency) and the strength of the electric field. From the investigated pulse sequences, and at a constant E-field and average power, the largest effect on the flame is obtained for the longest pulse, namely 50 μs. The results presented in this work are, to the knowledge of the authors, the first direct determination of laminar burning velocity on a laminar stretch-free flame exposed to pulsed microwaves.

Oxidation kinetics of methyl crotonate: A comprehensive modeling and experimental study

The current study explores the combustion behavior of methyl crotonate (CH3CH=CHC(=O)OCH3), which is a short ester representative of large unsaturated methyl esters. Starting with a detailed kinetic model for methyl butanoate (CH3CH2CH2C(=O)OCH3) oxidation, revisions are introduced to the C0-C4 chemistry based on the recent Aramco mechanism 3.0. The resulting mechanism is combined with a short model for methyl crotonate, derived from a suitable reference mechanism. Several new classes of reactions are included and the rate constants of the existing reactions are revised based on various theoretical studies and analogies to reactions of similar species. Furthermore, the low-temperature chemistry of methyl crotonate has been implemented in the current study to extend the validity of the mechanism to lower temperatures. The resulting methyl crotonate combustion mechanism has been comprehensively validated using various experiments in the literature. In addition, experiments are performed using a heat flux burner at atmospheric conditions to measure the laminar burning velocities of methyl crotonate at different unburnt mixture temperatures (318, 338, and 358 K). The mechanism is found to reproduce the experimental data for high-temperature combustion of methyl crotonate satisfactorily. The mechanism is also found to predict the low-temperature ignition delays accurately. Sensitivity and path flux analysis are performed to delineate the importance of the different reaction classes in methyl crotonate chemistry. The current study presents a comprehensive mechanism for methyl crotonate combustion, along with a new set of experimental results complementing the existing experimental database in the literature.

Laminar burning velocities, CO, and NOx emissions of premixed polyoxymethylene dimethyl ether flames

Polyoxymethylene dimethyl ethers (OMEn) with the chemical structure of CH3O(CH2O)nCH3 are a promising synthetic e-fuel class that can be produced from renewable electricity and improves the emission characteristics of internal combustion engines. However, knowledge of their fundamental combustion properties, such as the laminar burning velocity, is limited in the literature, preventing the full assessment of their application potential in combustion devices. Thus, a heat flux burner is applied in this study to measure the laminar burning velocities of OME1 and OME2 over a wide range of equivalence ratios of 0.6–1.9 and at initial temperatures of 383–401 K under atmospheric pressure for the first time. The obtained data are compared with computed results using a recently developed reaction mechanism for OME1–4. Good agreement is observed. The experimental results are also discussed in the context of a comparison to literature data of dimethyl ether and OME3, demonstrating the minor impact of chain length of (OMEn) on their burning velocities, which is further analyzed in terms of sensitivity analysis on reaction kinetics. Furthermore, the exhaust gas composition of NOx and CO is determined experimentally for flames of OME1 and OME2 using a probe method and compared to the numerical results, observing mostly good agreement, where the remaining differences could be explained on the basis of combined experimental and model uncertainties. This provides unique knowledge on the emission performance of the combustion of (OMEn).

Measurements of the laminar burning velocities and NO concentrations in neat and blended ethanol and n-heptane flames

Adiabatic laminar burning velocities and post-flame NO mole fractions for neat and blended ethanol and n-heptane premixed flames were experimentally determined using a heat flux burner and laser-induced fluorescence. The flames were stabilized at atmospheric pressure and at an initial temperature of 338 K, over equivalence ratios ranging from 0.6 to 1.5. These experiments are essential for the development, validation and optimization of chemical kinetic models, e.g. for the combustion of gasoline-ethanol fuel mixtures. It was observed that the addition of ethanol to n-heptane leads to an increase in laminar burning velocity that is not proportional to the ethanol content and to a decrease of NO formation. Such a NO reduction is due to the slightly lower flame temperatures of ethanol, which decrease the production of thermal-NO at 0.6 < Φ < 1.2, while under fuel-rich conditions this behavior is due to the lower concentrations of CH radicals, which decrease the production of prompt-NO. At Φ > 1.3, the lower NO formation through the prompt mechanism in the ethanol flames is partially offset by a lower rate of NO consumption through the reburning mechanism. New experimental results were compared with predictions of the POLIMI detailed chemical kinetic mechanism. An excellent agreement between measurements and simulated results was observed for the laminar burning velocities over the equivalence ratio range investigated; however, discrepancies were found for the NO mole fractions, especially under rich conditions. Further numerical analyses were performed to identify the main causes of the observed differences. Differences found at close-to stoichiometric conditions were attributed to an uncertainty in the thermal-NO mechanism. In addition, disagreement under rich conditions could be explained by the relative importance of reactions in hydrogen cyanide consumption pathways.

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.

Methyl-3-hexenoate combustion chemistry: Experimental study and numerical kinetic simulation

This work represents a detailed investigation of combustion and oxidation of methyl-3-hexenoate (CAS Number 2396-78-3), including experimental studies of combustion and oxidation characteristics, quantum chemistry calculations and kinetic model refinement. Following experiments have been carried out: Speciation measurements during oxidation in a jet-stirred reactor at 1 atm; chemical speciation measurements in a stoichiometric premixed flame at 1 atm using molecular-beam mass-spectrometry; ignition delay times measurements in a shock tube at 20 and 40 bar; and laminar burning velocity measurements at 1 atm using a heat-flux burner over a range of equivalence ratios. An updated detailed chemical kinetic mechanism for methyl-3-hexenoate combustion based on previous studies was proposed and validated against the novel experimental data and the relevant data available in literature with satisfactory agreement. Sensitivity and reaction pathway analyses were performed to show main decomposition pathways of methyl-3-hexenoate and underline possible sources of disagreements between experiments and simulations.

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.

Insight into fuel isomeric effects on laminar flame propagation of pentanones

Laminar flame propagation was investigated for pentanone isomers/air mixtures (3-pentanone, 2-pentanone and 3-methyl-2-butanone) in a high-pressure constant-volume cylindrical combustion vessel at 393–423 K, 1–10 atm and equivalence ratios of 0.6–1.5, and in a heat flux burner at 393 K, 1 atm and equivalence ratios of 0.6–1.5. Two kinds of methods generally show good agreement, both of which indicate that the laminar burning velocity increases in the order of 3-methyl-2-butanone, 2-pentanone and 3-pentanone. A kinetic model of pentanone isomers was developed and validated against experimental data in this work and in literature. Modeling analysis was performed to provide insight into the flame chemistry of the three pentanone isomers. H-abstraction reactions are concluded to dominate fuel consumption, and further decomposition of fuel radicals eventually produces fuel-specific small radicals. The differences in radical pools are concluded to be responsible for the observed fuel isomeric effects on laminar burning velocity. Among the three pentanone isomers, 3-pentanone tends to produce ethyl and does not prefer to produce methyl and allyl in flames, thus it has the highest reactivity and fastest laminar flame propagation. On the contrary, 3-methyl-2-butanone tends to produce allyl and methyl instead of ethyl, and consequently has the lowest reactivity and slowest laminar flame propagation.

Experimental and modeling study of laminar burning velocities and nitric oxide formation in premixed ethylene/air flames

Adiabatic laminar burning velocities and post-flame NO concentrations for flat, non-stretched, premixed C2H4/air flames were experimentally determined with a heat flux burner of improved design, over equivalence ratios ranging from 0.7 to 2, at atmospheric pressure and initial temperature of 298 K. Recognizing that C2H4 is a main intermediate in high-temperature oxidation pathways of heavy hydrocarbons, these data are essential for the development, validation and optimization of kinetic models for any fuel.The present measurements were then compared with the data available in the literature obtained with different techniques under the same experimental conditions. Regarding burning velocity measurements, the comparison showed considerable scatter among existing stretch-corrected data, which corroborate the necessity for the present adiabatic, non-stretched results. Regarding NO concentrations, an excellent agreement was observed between the present in situ, non-intrusive laser-induced fluorescence measurements and the only dataset available in the literature, determined by the phenol disulfonic acid method.A comparison of experimental and computational results using two contemporary comprehensive, detailed chemical kinetic mechanisms, along with one from the authors presented in this work, was also conducted and discussed. Discrepancies between experiments and model predictions and among models themselves were observed under rich conditions. Notwithstanding, the present updated model showed overall good performances in reproducing both laminar burning velocities and nitric oxide concentrations. Further numerical analyses were performed to identify the main causes of the observed differences. The results showed that the description of the relative importance of reactions involving vinyl and hydrogen cyanide consumption pathways, due to remaining uncertainties, lead to the different model behaviors.