Flame Oxidation Research Articles

Cellular instabilities of expanding hydrogen-propane-nitrous oxide spherical flames: Experiment and theory

The cellular instabilities of expanding hydrogen-propane-nitrous oxide spherical flames were experimentally and theoretically investigated at various equivalence ratios (ϕ=0.4–1.6), hydrogen fractions (XH=0–0.8), and initial pressures (Pu=1.1–1.5 bar). The experiments were conducted in a constant volume combustion chamber (CVCC) using high-speed schlieren technology to record the flame morphology, while the theoretical analysis was based on linear stability theory. The controlling parameters, critical conditions for the onset of instability, flame front oscillation intensity, logarithmic growth rate of perturbation, etc. were studied. The results indicate that the instability of premixed hydrogen-propane-nitrous oxide flames is dominated by hydrodynamic instability at XH≤0.6, while the competitive mechanism between thermal-diffusion and hydrodynamic instabilities is considered at XH=0.8. The oscillation intensity in the horizontal direction of the flame profile is more prominent than those in other directions, and with the increasing hydrogen fraction, the equivalence ratio at which the highest oscillation strength occurs shifts from ϕ=1.0 to ϕ=0.6. The trends of the experimental critical radius and Peclet number with each initial condition are completely identical to that of the theoretical value, but the experimental value is somewhat underpredicted. The three main reasons are proposed in the paper.Novelty and Significance Statement: This work is the first study on the cellular instabilities of hydrogen-propane-nitrous oxide flames by experiment and theory. It is under a small scale and reduced initial pressure condition that the cellular destabilization of hydrogen-propane flames is realized in the nitrous oxide atmosphere, while in the air atmosphere, the initial condition at which flame cellular destabilization occurs is harsher. Flame front oscillation strength, growth rate of disturbance, etc. were used to characterize quantitatively the degree of wrinkles and instabilities. Some novel findings were presented in the study. The present work is of great significance for the combustion community to understand deeply the cellular instabilities characteristic of expanding spherical flames supported by nitrous oxide, as well as for the engineering community to optimize the engine, propulsion, and energy supply systems, etc.

Advancing building fire safety through heat resistant and flame retardant hybrid silicone sealant

Rapid urbanization and population growth have led to a significant transformation in the urban landscape, characterized by the proliferation of high-rise buildings. However, this urban development has brought about new challenges, particularly in terms of fire safety. The escalating risk of disasters, especially fires, underscores the critical importance of implementing effective fire safety measures across all building types, necessitating legislative reforms. Despite their crucial role in ensuring structural integrity, sealants have often been overlooked in building safety standards, resulting in a lack of comprehensive regulations. The research addresses these challenges by focusing on the development of fire-resistant silicone sealants designed to enhance the fire resistance of building materials. These sealants were formulated independently, incorporating three fire retardants and four heat-resistant agents. While metal oxide flame retardants generally exhibited excellent performance, excessive mixing led to surface deformation and reduced workability. Optimal combinations of flame retardants and heat-resistant agents were identified, with the FR2 specimen demonstrating promising fire resistance performance. Full-scale fire performance tests conducted on FR2 revealed a maximum temperature difference of 40.6 °C between heated and unheated surfaces over a 2-h period, indicating high thermal insulation performance. These findings validate the efficacy of the proposed method and offer significant enhancements to building safety, thereby mitigating fire damage in urban environments.

Real-time monitoring of charcoal layer resistance for investigating the synergistic flame retardant mechanism of zinc oxide and intumescent flame retardants in SBS

Metal oxides as synergists of intumescent flame retardant (IFR) lead to an extremely complex formation process of intumescent char layers. The lack of means to detect the evolution of the char layer in real time makes it challenging to investigate the synergistic flame retardant mechanism between metal oxides and IFR. Herein, an innovative method of detecting real-time changes in the resistance of the char layer under flame ablation is used to investigate its changes in morphology, thus assisting in revealing the synergistic flame retardant mechanism of zinc oxide (ZnO) in IFR/SBS. ZnO showed a significant synergistic flame retardant effect with IFR without MEL and no synergistic effect with MEL-containing IFR. The analysis of the char layer structure revealed that ZnO played a role in promoting gas release and char layer formation and increasing the viscosity of the molten char layer regardless of the presence or absence of MEL.The difference in the morphology of the char layer could not reveal the experimental phenomenon that there is no synergistic flame retardant effect between ZnO and MEL-containing IFR. The analysis of the real-time resistance of the charcoal layer reveals that ZnO achieves its synergistic flame retardation with the MEL-free IFR by increasing the expansion of the charcoal layer at the initial stage of its formation. For MEL-containing IFR, 0.5% ZnO inhibits the rupture of the char layer by promoting its formation in the early stages of ablation, and 1% ZnO leads to the rupture of the char layer instead, due to the promotion of gas release. Excess ZnO (2%) caused pyrolysis gases to escape from the voids of the char layer rather than allowing it to expand. The analysis of the real-time resistance of the charcoal layer can help to accurately reveal the flame retardant mechanism.

Recyclable porous core-shell Mn-Co-O flame retardant for improving flame retardancy and smoke suppression of epoxy resin

The development of metal oxide flame retardants that are easy to synthesize, efficient, and recyclable would be more beneficial to circular economy. In this work, a porous core-shell Mn3CoOx was developed to efficiently reduce heat, smoke as well as hazardous gas during the combustion of epoxy resin (EP). Compared with pure EP, the peak heat release rate (pHRR), total heat release (THR), peak smoke production rate (pSPR), total smoke production (TSP), peak CO production rate (pCOPR), and peak CO2 production rate (pCO2PR) of EP/Mn3CoOx are reduced by 47.5 %, 21.5 %, 34.6 %, 17.6 %, 58.6 %, and 55.4 %, respectively. It has been shown that Mn3CoOx not only catalyzes the formation of high-quality char layer but also effectively catalyzes the conversion of hazardous gas products into CO2. Recovery experiments of Mn3CoOx filler in EP composites show that the recovery of Mn3CoOx can reach 90.0 %. This work provides insights into the design of efficient metal oxide flame retardants and ideas for the development of recycled flame retardants.

NO measurements in high temperature hydrogen flames: The crucial role of the hydrogen oxidation chemistry for accurate NO predictions

The current work investigates the formation of Nitric Oxide (NO) in hydrogen–air flames, over a wide range of flame temperatures. The use of hydrogen allows improved focus on the thermal-NO pathway by removing the complexity introduced by the prompt-NO pathway, which has been shown to be an important contributor to inaccurate predictions of absolute post-flame NO concentrations in hydrocarbon flames. This experimental study is conducted at atmospheric pressure using stoichiometric, premixed, laminar stagnation flames. Adiabatic flame temperatures ranging from 1600K to 2300K are achieved by varying the argon concentration in air. One-dimensional velocity, temperature, and NO concentration profiles are measured using non-intrusive laser diagnostics: Particle Tracking Velocimetry (PTV), NO multiline thermometry, and NO Laser Induced Fluorescence (NO-LIF), respectively. Results show that the experimental velocity profiles are incorrectly captured by the studied mechanisms, especially at low and high temperatures. This suggests that major inaccuracies are present in the hydrogen oxidation chemistry of the thermochemical models, regardless of their optimisation methodology. Furthermore, NO-LIF profiles show major discrepancies between all the studied mechanisms and the experiments, especially at elevated temperatures. The disagreement stems from an inaccurate description of the base chemistry of the models. These inaccuracies arise specifically from the description of the radical pool driving the flame behaviour and NO formation. This study demonstrates the need for model optimisation on experimental measurements using pure hydrogen 1D flames to obtain an accurate description of the hydrogen oxidation chemistry at play. This would lead to an improved description of the NOx sub-chemistry of any hydrogen, or hydrocarbon, combustion system.

High-performance intercalated composite solid electrolytes for lithium metal battery

Composite solid electrolytes (CSEs) combining the advantages of both inorganic and organic solid-state electrolytes, are expected to become the most promising solid electrolyte owning to their favorable interfaces with electrodes. However, low room-temperature ionic conductivity restricts the application of CSEs in lithium metal batteries. Herein, we design an intercalated CSE based on poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). Kaolin (2SiO2-Al2O3-2H2O), employed as an oxidative flame retardant, imparts nonflammability to the material. The polar molecule dimethyl sulfoxide (DMSO) is inserted between the kaolin layers as the pre-intercalation treatment, serving as an organic additive within the PVDF-HFP based SPE. The intercalated structure of CSE provides rapid Li+ transport channels, resulting in a high ionic conductivity (8.58 × 10−4 S cm−1) and large Li+ transference number at room temperature. The Li||Li symmetrical cell with prepared CSE exhibits outstanding cyclic stability of over 1400 h at a current density of 0.2 mA cm−2 for the capacity of 0.2 mAh cm−2. Moreover, the assembled Li||LiFePO4 cell delivers a high initial capacity of 140.5 mAh g−1 with a capacity retention of 81.2 % after 800 cycles at 0.5 C. In this paper, we present a novel approach for constructing high-performance solid-state lithium metal batteries.

Structure and NOx Emissions of Stratified Hydrogen-Air Flames Stabilized on a Coaxial Injector

Abstract In recent years, the need for low-carbon power has seen hydrogen emerge as a potential fuel to replace conventional hydrocarbons in combustion to limit CO2 emissions in several sectors, including aeronautics. The challenges posed by hydrogen combustion are similar to the issues of kerosene flames but more challenging, like nitrogen oxide (NOx) emissions and flame flashback. One potential solution to address these problems is to burn a rich mixture of hydrogen and air in globally lean conditions on a coaxial injector to obtain a stable and staged combustion and attempt to reduce emissions. In this article, the evolution of NOx production as more air is mixed into the fuel is studied, as well as the changes in flame size and structure. In particular, the appearance of a secondary flame front is observed and increasing the proportion of air in the fuel mixture both shortens the flame and reduces the NOx emission index. Additionally, the effect of the global equivalence ratio and flame thermal power is studied. Finally, existing models for NOx emission of hydrogen flames on a coaxial injector based on average flame residence time and strain rate are tested and shown to have promising results.

Effects of ferrocene addition on soot formation characteristics in laminar premixed burner-stabilized stagnation ethylene flames

The influence of ferrocene addition (∼60 ppm on a molar basis) on soot formation in laminar premixed burner-stabilized stagnation ethylene flames (with an equivalence ratio of 2.0) is investigated both experimentally and computationally. The results show that ferrocene addition has a very small effect on both flame temperature and concentrations of small soot precursors from kinetic modeling, but apparently promotes soot inception and particle size growth from particle size distribution measurements, and decreases soot maturity from elemental analysis. Transmission electron microscope (TEM) images and energy dispersive spectrometry (EDS) results show that distinct iron cores are present in large soot aggregates, consistent with the kinetic modeling results that iron predominates over iron oxide in ferrocene-doped flames. Iron in the incipient soot may be too dispersed or form too small cores to be observed by TEM; and as the soot particles coagulate, iron in the soot coalesces into large cores. Since iron is not exposed on the soot surface, and the concentrations of soot precursors are also not significantly increased by ferrocene, ferrocene promotes soot size growth mainly through enhanced coagulation rather than surface growth.

Study on the Combustion Mechanism of Diesel/Hydrogen Dual Fuel and the Influence of Pilot Injection and Main Injection

Hydrogen is a clean and renewable alternative fuel. In this paper, the combustion mechanism of diesel/hydrogen dual fuel is constructed and verified. The mechanism is combined with three-dimensional numerical simulation to study the effects of pilot injection and main injection on the combustion and emissions of a diesel/hydrogen dual fuel engine. The mechanism uses a 70% mole fraction of n-decane and 30% mole fraction of α-methylnaphthalene as diesel substitutes, and it combines n-decane, α-methylnaphthalene, NOX, PAH, soot and H2/C1-C3 sub-mechanisms to form a diesel/hydrogen dual fuel combustion mechanism. The mechanism was verified by chemical kinetics, including the ignition delay time, JSR (Jet Stirred Reactor) oxidation and laminar flame speed, and then, it was verified by computational fluid dynamics. The results show that the simulated values are in good agreement with the experimental values of cylinder pressure, heat release rate and emissions data. The mechanism can well predict the combustion and emissions of a diesel/hydrogen dual fuel engine. Compared with single injection, the peak heat release rate, peak cylinder pressure and MPIR (Maximum Pressure Rise Rate) increase with the increase in pilot mass percent from 5% to 20%, which makes the phase of CA10 and CA50 advance and reduces CO emissions, but NOX emissions increase. With the advance of pilot injection timing from 10° CA BTDC to 30° CA BTDC, the peak cylinder pressure increases, the peak heat release rate decreases, CA10 and CA50 advance, CO emissions decrease, NOX emissions increase and NOX emissions peak at 30° CA BTDC. When the pilot injection timing is further advanced from 30° CA BTDC to 50° CA BTDC, the peak cylinder pressure decreases, the peak heat release rate increases, CA10 and CA50 are delayed, CO and NOX emissions are reduced, and NOX emissions at 50° CA BTDC are lower than those at 10° CA BTDC. With the advance of main injection timing from 0° CA BTDC to 8° CA BTDC, CO emissions decrease, NOX emissions increase, the peak cylinder pressure increases, the peak heat release rate decreases slightly first and then increases, and the peak cylinder pressure and peak heat release rate corresponding to the overall phase shift forward. When the main injection timing is advanced to 6° CA BTDC, MPIR is 1.3 MPa/° CA, exceeding the MPIR limit of diesel engine 1.2 MPa/° CA.

Ameliorating structural and electrochemical properties of traditional poly-dioxolane electrolytes via integrated design of ultra-stable network for solid-state batteries

Poly(1,3-dioxolane) (PDOL)-based solid electrolytes hold great potential for solid-state lithium metal batteries (SLMBs) due to their high ionic conductivity, good lithium metal compatibility, and facile synthesis through in-situ polymerization. However, traditional PDOL electrolyte suffers from inferior structural stability and low Li-ion transference number (tLi+), which has impeded PDOL from authentic commercialization. Here we design and attain an ultrathin crosslinked polymer electrolyte (viz. PTADOL) to significantly upgrade the functional properties of PDOL. The in-situ formed PTADOL has rational O-Li+ coordination for fast Li+ transport, which enhances both ionic conductivity and tLi+. The unique integrated network structure stabilizes the electrode/electrolyte interface, and achieves additional favorable features, including improved oxidative stability, thermal stability, and flame retardancy. Based on the ultra-stable PTADOL polymer electrolyte, the high-voltage LiNi0.8Mn0.1Co0.1O2||Li solid batteries exhibit excellent operation stability with suppressed polymer degradation. This work provides not only a practical approach to the design of highly stable solid polymer electrolytes for SLMBs, but also the deep understanding of enhancement mechanism.

High Temperature Oxidation and Oxyacetylene Ablation Properties of ZrB2-ZrC-SiC Ultra-High Temperature Composite Ceramic Coatings Deposited on C/C Composites by Laser Cladding

In order to improve the high temperature oxidation and ablation resistance of C/C composites, ZrB2-ZrC-SiC ultra-high temperature composite ceramic coatings were prepared on C/C composites by laser cladding using Zr, B4C, and Si as raw materials. The microstructure of the coating was characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Air isothermal oxidation (1600 °C, 80 min) and oxyacetylene flame ablation (2400 kW/m2, 300 s) were used to evaluate the high-temperature oxidation and ablation properties of the coating, respectively. The results show that the microstructure of laser cladding coating is a totem of black and white. The white part is mainly the first solidified high melting point ZrB2 phase, and the black part is the latter solidified eutectic structure, which is mainly composed of ZrB2(ZrB12)-ZrC or ZrB2(ZrB12)-SiC two phases. After oxidation at 1600 °C and 80 min, the coating is mainly composed of ZrO2 and ZrSiO4 phases, and ZrSiO4 is basically distributed among ZrO2 particles. The high temperature oxidation and ablation properties of the coating are better than the C/C composite matrix, and the mass ablation rate of the coating is about 1/4 of the latter.

Investigation of soot sensitivity to strain rate in ethylene counterflow soot formation oxidation flames

Soot sensitivity to strain rate is mainly responsible for soot formation intermittence in practical combustion devices. This work provides a fundamental study on soot formation in Soot Formation Oxidation (SFO) counterflow flames at varying strain rates. While the problem has been extensively studied in Soot Formation (SF) configurations, where the dominant process is nucleation, investigations remain scarce in the corresponding SFO cases. In the latter, the high temperatures and strong oxidative environments make the surface reactions prevail over nucleation. The work provides a new dataset for ethylene SFO flames in a wide range of strain rates and sheds light on the main processes concurring in determining soot strain rate sensitivity in such conditions. In particular, the peak of soot volume fraction (SVF) is primarily controlled by surface growth and oxidation. The latter becomes progressively more dominant on the side of the SVF distribution toward the oxidizer nozzle, where the presence of oxidizing agents is significant. The soot mechanism adopted predicts a SVF distribution and sensitivity to strain rate in agreement with experimental data. The latter is found similar to corresponding SF cases, although soot loads in the two configurations differ by almost an order magnitude, and the SVF sensitivity is known to be more accentuated for lower soot loads. A deeper investigation revealed that the nucleation process through dimerizations primarily controls the SVF sensitivity, providing the onset of soot necessary for further growth. Then, the latter tends to reduce SVF sensitivity depending on its impact. PAH sensitivities mostly agree with theoretical observation even though further validations on the kinetic mechanism are needed to improve its predictions in lean conditions. The simplistic yet effective model based on the hybrid method of moments and the employment of a reduced kinetic mechanism makes the approach amenable for turbulent computational fluid dynamic (CFD) simulations.

A wide-range experimental and kinetic modeling study of the pyrolysis and oxidation of 1-butyne

Allyl and propargyl radicals are involved in the production of the first aromatic ring, which is considered a crucial process in forming polycyclic-aromatic hydrocarbons and soot. 1-Butyne decomposes to propargyl radicals during its pyrolysis and oxidation. To improve our knowledge of the kinetics of 1-butyne, its pyrolysis, oxidation, and laminar flame speed properties have been measured. Pyrolysis experiments were performed in a single-pulse shock tube at 2 bar in the temperature range 1000 – 1600 K. Ignition delay times for 1-butyne/‘air’ mixtures were measured at pressures of 1, 10, 30, and 50 bar, in the temperature range 680 – 1580 K, at equivalence ratios of 0.5, 1.0, and 2.0 using rapid compression machines and shock tubes. Furthermore, laminar flame speeds were measured at ambient temperature, at p = 1, 2, 3 atm, over an equivalence ratio range of 0.6 – 1.9. A new detailed mechanism for 1-butyne has been developed and widely validated using the new experimental data and those available in the literature. Important reactions of 1-butyne pyrolysis and oxidation are determined through flux and sensitivity analyses.

Oxidation study of n-propylamine with SVUV-photoionization molecular-beam mass spectrometry

This work reports the experimental results of n-propylamine (NPA) oxidation in a jet-stirred reactor at 1 atm within 625–875 K, equivalence ratios from 0.5 to 2.0. Oxidation products and intermediates were identified and quantified with synchrotron vacuum ultraviolet photoionization mass spectrometry. Apart from various hydrocarbons, oxygenated and nitrogenous species reported in previous studies of amines, several intermediates were newly detected, including formamide (H2NCHO), nitromethane (CH3NO2), nitrous acid (HNO2), 2-propen-1-ol (C3H5OH) and 2-propenal (C2H3CHO). A detailed kinetic model consisting of 277 species and 2314 reactions was developed with reasonable predictions against the measured data. The rate-of-production and sensitivity analyses results show that NPA oxidation at low temperatures is dominated by the reaction with HO2. Particular attention was paid to the main oxidation product HCN, because its formation is affected by both fuel structure and reaction temperature. The equivalence ratio changes have an opposite effect on HCN concentration in NPA oxidation compared with the pyrrole oxidation and ethylamine flame. In the current study, the peak mole fraction of HCN decreases with increasing equivalence ratio, because the formation of CN triple bond in HCN requires successive H-abstractions, dominantly controlled by the concentrations of OH/HO2 radicals and O2. In addition, a comparison between the experimental results of NPA oxidation and pyrolysis was performed to illustrate the effect of O2 concentration on reaction routes. Current results provide a preliminary insight into the combustion kinetics of more complicated aliphatic amines.

A wide-range experimental and kinetic modeling study of the pyrolysis and oxidation of 2-butyne

To reduce particulate emissions leading to a cleaner environment, it is important to understand how polycyclic-aromatic hydrocarbons (PAHs) and their precursors are formed during combustion. 2-butyne can decompose to propargyl and allyl radicals. These radicals can produce benzene and other PAHs, leading to the formation of soot. In the present study, pyrolysis, oxidation, and laminar flame speed experiments were performed for 2-butyne. The pyrolysis experiments were conducted in a single-pulse shock tube at 2 bar in the temperature range 1000 – 1500 K. Ignition delay times for 2-butyne/‘air’ mixtures were measured in the pressures range 1 – 50 bar, over the temperature range 660 – 1630 K, at equivalence ratios of 0.5, 1.0, and 2.0 using rapid compression machines and shock tubes. Moreover, laminar flame speed (LFS) experiments were performed at ambient temperature, at p = 1 – 3 atm, over an equivalence ratio range of 0.6 – 1.8. A new, detailed chemical kinetic model for 2-butyne has been developed and widely validated against the data measured in this study and those available in the literature. The significant reactions for 2-butyne pyrolysis, ignition, and oxidation are identified and discussed using flux and sensitivity analyses.

Nitric Oxide Generation in N2-Diluted H2–N2O Flames: A Computational Study

A computational study was conducted on the addition of nitrogen (0–60 vol%) to stoichiometric hydrogen–nitrous oxide flames at various initial pressures (1–10 bar) and temperatures (300–500 K), with a special emphasis on NO, which is a major pollutant that is generated by these flames. The detailed kinetic modeling of H2–N2O–N2 flames, which was performed using the GRI 3.0 mechanism, produced the structure of the H2–N2O–N2 flames for the temperature and mass fraction profiles of 53 species, which participated in 325 elementary reactions. The NO profiles that were computed by the detailed kinetic modeling followed the trend of experimental NO profiles that have been reported in the literature for laminar premixed flames at sub-atmospheric pressures. For the examined H2–N2O–N2 flames, an increase in initial pressure resulted in a significant reduction in the NO mass fraction of the burned gas. The increase in initial temperature was also accompanied by an increase in the mass fractions of the generated NO, as well as the flame temperature.

Effects of the addition of a high energy density fuel, adamantane to diesel on its cetane number, sooting propensity, and soot nanostructural properties

Soot particles, released during diesel combustion, adversely affect human health and the environment. Some studies suggest blending diesel with oxygenated fuels to reduce soot emissions, though it also reduces fuel economy and may produce new pollutants such as aldehydes. Another method to reduce soot could be to increase its reactivity by altering its physicochemical properties (e.g., introducing curvatures in its nanostructures) so that it oxidizes inside the engine or flame. In this direction, a cyclic high-density hydrocarbon fuel, adamantane is studied as an additive for diesel to determine its effect on cetane number, threshold sooting index, soot reactivity, and soot nanostructural properties. The addition of up to 6.75 wt% of adamantane in diesel significantly increased the smoke point of diesel from 18 to 29 mm (and threshold sooting index decreased by 44%), indicating a drastic drop in fuel sooting tendency. Adamantane addition in a small amount to diesel minimally affected the cetane number. To understand the impact of adamantane addition to diesel on the characteristics of soot particles, different techniques including TGA, HRTEM, XRD, EDX, and EELS were used. It is observed that adamantane enhances curvatures in soot nanostructures and reduces the size of fringes in soot and its aromatic content, and all of these property variations improve soot reactivity to improve its oxidation in flames or engines and reduce soot emission.

Modeling soot particles as stable radicals: a chemical kinetic study on formation and oxidation. Part II. Soot oxidation in flow reactors and laminar flames

Recent experimental and theoretical studies have shed light on the persistent radical behavior of large polycyclic aromatic hydrocarbons (PAHs) and soot particles. The first detailed soot model developed based on these findings is here proposed. While its good performances in predicting soot formation and growth when soot oxidation is negligible are presented in the companion paper of this work (Part I), the model is here validated against 45 cases of particle size distribution functions (PSDF) and soot volume fraction under oxidizing conditions. Notably, we show that by considering stable radical particles and aggregates it is possible to reproduce soot oxidation by O2 in plug flow reactors under very diluted conditions, whereas existing models could not predict particle consumption in the absence of particle surface activating species. Therefore, this work paves the way for a different approach to soot modeling that could help to better interpret soot reactivity. Oxidation of soot produced from different fuels (ethylene, n-heptane, and toluene, i.e. leading to different particle graphitization) at different temperatures (950–1073 K) and O2 concentrations (1000–10000 ppm) is successfully modeled adopting reference oxidation rates of gas-phase resonantly stabilized PAH radicals, featuring analogous reactivity compared to soot species, and by also accounting for different particle hydrogenation levels. Good model performances are also obtained in both rich and lean premixed ethylene flames (Tmax∼1480–1510 K) and in two sets of counterflow ethylene soot formation/oxidation flames (SFO, Tmax∼2560–2825 K), where OH radical becomes the dominant oxidizer. Finally, model limitations emerging in some of the cases considered are discussed through a sensitivity analysis and with references to other oxidation mechanisms proposed in the literature in order to highlight possible future improvement of the model predictive capability

A Theoretical Study on Cool Flame Oxidation as an Effective Way for Fuel Reforming: Emphasis on Ignition Characteristics and Chemical Analysis

ABSTRACT Cool flame oxidation of hydrocarbon fuels produces substantial intermediates with various levels of chemical reactivity, which can be used for combustion control of advanced engines with low-temperature combustion. Previous investigation has evaluated the performance of low-temperature reforming under high fuel-rich conditions, whereas the potential of a broader scope of reforming conditions (especially fuel-lean scenarios) is not clear. In this work, cool flame oxidation as a reforming way was proposed to investigate the effect of reforming products from various reforming conditions on ignition characteristics of typical operating conditions, and the role of the reforming products from typical reforming conditions on ignition characteristics of different operating conditions was clarified under engine-relevant conditions. The results show that the reforming products via cool flame oxidation are more sensitive to reforming temperature than pressure and equivalence ratio. Residence time also plays an important role in reforming products, and depending on the oxidation degree, two kinds of reforming products can be identified, i.e. mild reforming and severe reforming, manifesting distinct chemical reactivity. Meanwhile, it is found that different from the previous findings of very fuel-rich reforming conditions, the ignition delay time of different blends is significantly shortened under most operating conditions, except for some scenarios (e.g. the negative temperature coefficient region) manifesting a “cross-point” behavior. Moreover, the promotion on ignition processes becomes enhanced under higher temperature, higher pressure, leaner mixture, and severe reforming conditions. Globally, cool flame oxidation as a reforming way can reduce the ignition delay time by 30 ~ 50% under engine-relevant conditions. The current work can provide new perspectives of cool flame oxidation as an effective way to control advanced engine combustion processes.

Reduced Chemical Kinetic Mechanism for the Oxidation of Methyl Methacrylate in Flames at Atmospheric Pressure

Methyl methacrylate (MMA) is the main pyrolysis product of the widely used polymer polymethyl methacrylate; therefore, a compact mechanism of MMA oxidation is of interest for CFD modeling of flame propagation over this polymer. A reduced mechanism of MMA combustion consisting of 263 elementary reactions involving 66 species was developed based on a detailed chemical-kinetic mechanism of MMA oxidation in flames using Chemical Workbench software. The developed mechanism was validated against experimental data on the speed of premixed MMA/air flame at an equivalence ratio $$0.9 < \phi < 1.3$$ and against literature data on the structure of a MMA/O2/Ar ( $$\phi$$ = 1) flame stabilized on a flat burner at a pressure of 1 atm. The proposed reduced kinetic model for MMA describes the experimental data with satisfactory accuracy, and the modeling results for the full and reduced mechanisms of MMA oxidation are in good agreement with each other in terms of the concentrations of the major flame species and the concentrations of most combustion intermediates, including hydrogen, methane, ethylene, acetylene, propane, acetaldehyde, methyl acrylate, etc.