Universal calibration of quantitative OH-LIPF measurements in hydrocarbon flames at elevated pressures

Quantitative measurements of radical concentrations are important in studies of the structure of flames. In this research, a quantitative analysis was performed using laser-induced predissociative fluorescence of OH radicals (OH-LIPF) in high-pressure (1–5 atm) premixed methane-air and propane-air flat flames (ϕ = 0.7–1.3). OH was excited (A ²Σ⁺, ν′ = 3 − X ²Π, ν″=0, and P₂8) using a KrF excimer laser and (3,2) band fluorescence was observed. OH fluorescence intensities were calibrated against the OH concentrations calculated from flame simulation results in the postflame zone using the CHEMKIN premix flame code in conjunction with the GRI-Mech 3.0 reaction mechanism. The accuracy and spatial resolution of temperature measurements are important factors for the correctness of the corresponding flame simulations, especially in the reaction zone near the burner surface. In this work, a carefully constructed thermocouple (R-type, 50 μm) positioning system was used to identify the temperatures above the burner surface. With careful evaluations of quenching rates, Voigt profiles, and normalization against room-air N₂ Raman scattering intensity, a universal calibration constant [C _T = (1.076 ± 0.174) · 10¹⁶ molecules/cm³] was determined. The OH concentrations obtained by flame simulations showed good agreement with the quantitative OH-LIPF measurements in all methane-air flames and fuel lean (ϕ = 0.7−0.8) propane-air flames. However, a 2-fold to 5-fold discrepancy was obtained in propane flames at ϕ > 0.9. This may be caused by the lack of C₃ reaction paths in the GRI mechanism and/or the inaccuracy of the thermochemical data for large molecules. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 4, pp. 67–76, July–August, 2009.     

Influence of organophosphorus inhibitors on the structure of atmospheric lean and rich methane-oxygen flames

The inhibition of atmospheric laminar methane-oxygen flames of various compositions by trimethyl phosphate was studied experimentally and by numerical modeling using mechanisms based on detailed kinetics. The H and OH concentration profiles in flames with and without the addition of trimethyl phosphate were measured and calculated. It was shown that the addition of the inhibitor reduced the maximum (in the reaction zone) concentrations of H and OH in lean and rich flames. The concentration reduction was higher in rich flames than in lean flames. The concentration profiles of the phosphorus-containing products PO, PO₂, HOPO, HOPO₂, and (HO)₃PO in lean and rich flames stabilized on a flat burner were measured and calculated. Tests of the previously developed model of flame inhibition by phosphorus compounds showed that the model provides adequate predictions of many experimental results. __________ Translated from Fizika Goreniya i Vzryva, Vol. 43, No. 2, pp. 23–31, March–April, 2007.     

高温烟气伴流的温度和氧浓度对甲烷/氢气扩散火焰的影响(英文)

This paper investigates the effects of coflow O2 level and temperature on diffusion flame of a CH4/H2 jet in hot coflow(JHC) from a burner system similar to that of Dally et al.The coflow O2 mass fraction(y*O2) is varied from 3% to 80% and the temperature(T*cof) from 1200 K to 1700 K.The Eddy Dissipation Concept(EDC) model with detailed reaction mechanisms GRI-Mech 3.0 is used for all simulations.To validate the modeling,several JHC flames are predicted under the experimental conditions of Dally et al.[Proc.Combust.Inst.,29(1),1147-1154(2002)] and the results obtained match well with the measurements.Results demonstrate that,when y*O2 decreased,the diffusion combustion is likely to transform from traditional combustion to MILD(Moderate or Intense Low-oxygen Dilution) combustion mode.When cof T* is higher,the temperature distribution over the whole domain trends to be more uniform.Reducing y*O2 or cof T* leads to less production of intermediate species OH and CO.It is worth noting that if y*O2 is high enough(y*O2>80%),increasing y*O2 does not cause obvious temperature increase.     

Experimental study on CH* chemiluminescence characteristics of impinging flames in an opposed multi‐burner gasifier

The bandpass filtered images of impinging flames in an opposed multi‐burner (OMB) gasifier was visualized by a CCD camera combined with a high temperature endoscope. A filtering and image processing method by use of three bandpass filters was applied to subtract soot and CO₂* contributions in the CH* band and obtain the CH* chemiluminescence of impinging flames. The results show that a clear reaction core is generated in the impinging zone of four‐burner impinging flames. The size of the reaction core is affected by the O/C equivalence ratio ([O/C]_e) and the impingement effect is relatively stronger at lower [O/C]_e. The flame lift‐off length in the gasifier is jointly controlled by the syngas concentration and the diesel atomization effect. The impingement effect shortens the flame lift‐off length. The relationship between the syngas concentration and the maximum CH* intensity makes it possible to evaluate the syngas concentration from CH* intensity. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2007–2018, 2017     

Using terahertz radiation to detect OH radicals and NO molecules in flames

The possibility of detecting OH radicals and stable paramagnetic NO species in flames using the effect of rotation of the polarization plane of terahertz radiation (1–10 THz) in a magnetic field (Faraday effect) was experimentally demonstrated. Experimental data on detection of NO in a cell and OH radicals in an atmospheric-pressure H₂/O₂/N₂ flame using this method are given. The proposed method can be used to study highly scattering media which are opaque in the visible region, for example, dusty or sooty flames.     

Problem of investigating the hydrogen flame radiation using the radiation of the intermediate products of reaction

Results are presented of the measured excited OH radical total radiation from turbulent flames of homogeneous hydrogen-air mixtures within the excess air coefficient range between 0.15 and 3.0 and of the diffusional submerged hydrogen flames at various outflow velocities. Information is obtained about the influence of the radiation from heated quartz glasses and of the natural flame absorption on the measurement accuracy. A possibility is demonstrated of using the data on the radiation of laminar flames for a quantitative analysis of turbulent flames while simultaneously determining the shapes and size of the radiating volume and in complicated cases also the pressure distribution. Institute of Theoretical and Applied Mechanics of the Siberian Division of the Russian Academy of Sciences, 630090 Novosibirsk. Translated from Fizika Goreniya i Vzryva, Vol. 31, No. 6, pp. 64–73, November–December, 1995.     

Diffusion Flame of a CH4/H2 Jet in a Hot Coflow: Effects of Coflow Oxygen and Temperature

This paper investigates the effects of coflow O₂ level and temperature on diffusion flame of a CH₄/H₂ jet in hot coflow (JHC) from a burner system similar to that of Dally et al. The coflow O₂ mass fraction (y_O2^*) is varied from 3% to 80% and the temperature (T_cof^*) from 1200 K to 1700 K. The Eddy Dissipation Concept (EDC) model with detailed reaction mechanisms GRI-Mech 3.0 is used for all simulations. To validate the modeling, several JHC flames are predicted under the experimental conditions of Dally et al. [Proc. Combust. Inst., 29 (1), 1147-1154 (2002)] and the results obtained match well with the measurements. Results demonstrate that, when y_O2^* decreased, the diffusion combustion is likely to transform from traditional combustion to MILD (Moderate or Intense Low-oxygen Dilution) combustion mode. When T_cof^* is higher, the temperature distribution over the whole domain trends to be more uniform. Reducing y_O2^* or T_cof^* leads to less production of intermediate species OH and CO. It is worth noting that if y_O2^* is high enough (y_O2^*>80%), increasing y_O2^* does not cause obvious temperature increase.     

Laser Studies of Small Radicals in Rich Methane Flames: OH,HCO, and 1CH2

Chemical mechanisms for combustion processes are often developed for nearly stoichiometric flames, a perfect balance of fuel and oxidizer. As the fuel content is increased and flames become richer, these models will eventually break down because they lack reactions leading to the formation of larger hydrocarbons and ultimately soot. In this contribution, we investigate the behavior of two methane combustion models, the GRI 2.11 and Prada–Miller mechanisms, in a series of rich flames of stoichiometry 1.0, 1.2, 1.4, and 1.6. Using non-intrusive laser diagnostics, laser-induced fluorescence, and cavity ring-down spectroscopy, the concentration profiles as a function of height above burner for OH, HCO, and ¹CH₂ have been measured in these rich low-pressure methane flames. The experimental results are compared to the calculated profiles. In general, good agreement is found for all species over the range of stoichiometries investigated. The agreement of the OH and HCO profiles with the models is particularly striking. For ¹CH₂, the width, position, and relative height of the profiles match the model predictions for flames of stoichiometry 1.0 and 1.2. Discrepancies are noted for the richer flames investigated, those with stoichiometrics of 1.4 and 1.6. An analysis of the models suggests that additional pathways for ¹CH₂ formation and removal should be considered. Even under fairly rich conditions, stoichiometry of 1.6, both models perform well for the radicals investigated here. There is no indication of the requirement for >C₂ chemistry to reproduce the present data.     

Experimental and numerical study of CH4/CH3Cl/O2/N2 premixed flames under oxygen enrichment

A comprehensive experimental and numerical study has been conducted to understand the influence of CH₃Cl addition on CH₄/O₂/N₂ premixed flames under oxygen enrichment. The laminar flame speeds of CH₄/CH₃Cl/O₂/N₂ premixed flames at room temperature and atmospheric pressure are experimentally measured using the Bunsen nozzle flame technique with a variation in the amount of CH₃Cl in the fuel, equivalence ratio of the unburned mixture, and level of oxygen enrichment. The concentrations of major species and NO in the final combustion products are also measured. In order to analyze the flame structure, a detailed chemical kinetic mechanism is employed, the adopted scheme involving 89 gas-phase species and 1017 elementary forward reaction steps. The flame speeds predicted by this mechanism are found to be in good agreement with those deduced from experiments. Chlorine atoms available from methyl chloride inhibit the oxygen-enhanced flames, resulting in lower flame speeds. This effect is more pronounced in rich flames than in lean flames. Although the molar amount of CH₃Cl in the methane flame is increased, the temperature at the post flame is not significantly affected, based on the numerical analysis. However, the measured concentration of NO is reduced by about 35% for the flame burning the same amount of methyl chloride and methane at the oxygen enrichment of 0.3. This effect is due to the reduction of the concentration of free radicals related to NO production within the flame. In the numerical simulation, as CH₃Cl addition is increased, the heat flux is largely decreased for the oxygen-enhanced flame. It appears that the rate of the OH + H₂ → H + H₂O reaction is reduced because of the reduction of OH concentration. However, the function of CH₃Cl as an inhibitor on hydrocarbon flames is weakened as the level of oxygen enrichment is increased from 0.21 to 0.5. __________ Translated from Fizika Goreniya i Vzryva, Vol. 42, No. 6, pp. 103–111, November–December, 2006.     

OH concentration fluctuations in turbulent natural gas jet flames

OH radical concentrations in a turbulent non-premixed natural gas flame were measured using laser-induced fluorescence. Instantaneous concentration profiles along a line were obtained using a diode array camera. Investigation of the molecular transitions during laser excitation shows that concentrations, where calibrated with a one-dimensional laminar premixed flame, are biased with a factor of about two. This bias is similar for different flames, so that results of different flames can be compared with an accuracy of about 20%. Three different flames were studied, with fuel jet Reynolds numbers of 9.7 × 10³, 6.8 × 10³ and 4.9 × 10³. Average concentrations and probability density functions show that concentrations close to the nozzle in the flame with highest turbulence are low, which may indicate local extinction. Integral length scales and Taylor micro scales, derived from spatial correlation, exhibit minima at radical locations where OH fluctuations exhibit maxima.     

乙醇和二甲醚微射流火焰燃烧特性研究

采用实验及数值计算研究了乙醇和二甲醚微圆管射流火焰燃烧特性。通过实验观察到不同燃料流速下乙醇和二甲醚火焰都具有四种典型的火焰形态；使用平面激光诱导荧光测试系统获得了微射流火焰的OH基元分布，实验结果表明在较高流速下稳定燃烧的乙醇火焰比二甲醚火焰直径小，且略高于二甲醚火焰；采用考虑详细化学反应机理的数值计算对乙醇和二甲醚火焰进行了数值模拟，计算结果与实验现象吻合较好；利用一维非预混对冲火焰计算进一步研究了这两种燃料的化学反应路径，分析结果表明乙醇和二甲醚火焰的中间产物有显著差异，两种燃料化学反应特性的差异导致了不同的微火焰结构。     

Fire behaviour and external flames in corridor and tunnel‐like enclosures

This work investigates how the inflow, the burning and the outflow develop in a corridor open to one end having a fire at either the closed or open end. The situation of a corridor fire having a fire source at the close end is a situation similar to a tunnel having a fire source at the centre of the tunnel without ventilation. A gaseous propane burner is used to produce the fire at a prescribed fuel flow rate in a long corridor of aspect ratio up to 6:1 having a rectangular cross section and varying door‐like openings. Gas temperatures using thermocouple trees, heat fluxes in the corridor and on its façade, flame heights of emerging flames and total heat release rates (HRRs) are measured as the fuel flow rate of propane increases gradually and linearly with time to a preset maximum value. For over‐ventilated conditions, the flames remain near the fire source at the closed end of the corridor. Unexpectedly, it is established for under‐ventilated conditions that the inflow of air is not affected by the aspect ratio of the corridor or the location of the burner in the corridor and that the vertical distribution of gas temperatures inside the enclosure is nearly uniform with height everywhere. In addition, the flame heights and heat fluxes on the façade are the same as those for aspect ratios of the corridor from 1:1 to 3:1 examined in previous work. Moreover, as the conditions changed from over‐ventilated to under‐ventilated conditions, the flames migrated in a ghostly manner from the closed end to the open end of the corridor as soon as under‐ventilated conditions were established. The speed of migration of the flames from the back to the front has also been inferred from the thermocouple tree measurements, which also indicate that the flow conditions ahead and after the passing of the front are changed. These results can be applied to interpret some of the observed behaviours of fires in long corridors or tunnels without ventilation. Copyright © 2012 John Wiley & Sons, Ltd.     

A First-Principles Study of the Role of Quaternary-N Doping on the Oxygen Reduction Reaction Activity and Selectivity of Graphene Edge Sites

Density functional theory (DFT) was used to investigate O₂ chemisorption on the edge sites of graphene doped with quaternary nitrogen (N-graphene). The location of the doped quaternary N within the graphene cluster was systematically varied to determine the effect of interior versus edge doping on the reactivity of the edge graphene sites. Model 1b, where a quaternary-N atom is at the zigzag edge of the graphene cluster, is found to be the most favored structure and strongly adsorbs O₂ molecule via a “two feet” geometry. For this most stable O₂ binding configuration, the potential-dependent free energy of reaction for the subsequent oxygen reduction reaction (ORR) steps was evaluated. The favored four electron-proton transfer mechanism passes through a dissociative O*+OH* state instead of an OOH* intermediate, followed by a series of reduction steps to produce water. At the equilibrium potential for ORR of 1.23 V-NHE, the protonation of O* and OH* both show uphill steps, but the production of O* is facile with a small overpotential. An applied potential of ?0.15 V-NHE is required to facilitate the protonation of OH* to water, a larger overpotential than observed experimentally. While solvent effects may reduce this overpotential, our results suggest that the edge of the N-graphene is very active towards activation of O₂ and production of O* and OH* but because of strong binding of the oxygen atom, the subsequent steps of the ORR reaction will be hindered. Mechanisms that have OH* formed at the edge site and then move to adjacent sites for more facile protonation will have to be explored in the future.     

Numerical simulation of the effect of the addition of NO and NO<Subscript>2</Subscript> on a rich hydrogen flame using the tracer method

The effect of the addition of nitric oxides (NO and NO₂) on rich hydrogen-air flames was studied using the tracer method in numerical simulation. It is shown that the effects of these additives are not similar. Both oxides promote the formation of OH and H₂O in the low-temperature zone of the front. The addition of NO reduces the first maximum of the OH profile and the burning velocity. The addition of NO₂ increases the first maximum of the OH profile and does not change the burning velocity. __________ Translated from Fizika Goreniya i Vzryva, Vol. 45, No. 3, pp. 19–25, May–June, 2009.     

Time-of-flight SIMS analysis of polypropylene films modified by flame treatments using isotopically labeled methane fuel

The surface of polypropylene (PP) film was oxidized by exposure to a flame fueled by isotopically labeled methane (CD₄). The isotopic sensitivity of static secondary ion mass spectrometry (SIMS) was then used to gain new insights into the mechanism of flame treatment. SIMS analysis indicated that much of the oxidation of PP occurring in fuel-lean flames is not deuterated, while for PP treated in fuel-rich flames, some of the affixed oxygen is deuterated. These observations imply that O₂ is the primary source of affixed surface oxygen in fuel-lean flame treatments, but that OH may be a significant source of affixed oxygen in fuel-rich flame treatments. Hydroxyl radicals are primarily responsible for hydrogen abstraction in fuel-lean flames, while H is the primary active gasphase species in fuel-rich flames. SIMS also detected trace quantities of oxidized nitrogen groups affixed to the flame-treated PP.     

Ozonation of Exhausted Reactive Dye-Bath Analogues: the Use of Atrazine as an OH* Probe Compound

Ozonation and O₃/H₂O₂ treatment of a spent reactive dye–bath analogue containing aminofluorotriazine type reactive dyestuffs were examined in semi–batch experiments at different pH (2, 7 and 12), bicarbonate (0.18 M at pH7) and carbonate (0.19 M at pHl2) alkalinity. Atrazine (23.2 µM) was added as an OH* – sensitive probe compound to the reactive dye–bath analogue to evaluate the individual contributions of both direct molecular (O₃) and indirect radical (OH*) type reaction pathways for the treatment of spent dye–bath ingredients. From the obtained results it was evident that particularly de–aromatization (expressed in terms of UV₂₅₄ and UV₂₇₀ removal) kinetics were significantly retarded in the presence of HCO₃, CO₃ ^2‐ and particularly H₂O₂. O₃ decomposition and consumption increased appreciably when H₂O₂ (0.125 mM at pH7) was added as compared in the presence of carbonate/bicarbonate alkalinity when H₂O₂ was added indicating that H₂O₂ and alkalinity could act as radical chain reaction promoters (carriers) and inhibitors, respectively. Atrazine degradation and UV₂₇₀ abatement rates paralleled each other revealing that particularly de–aromatization of the spent dye–bath constituents mainly involves the action of OH*. Nitrate formation was distinctly higher at pH7 than at pH 12 as a consequence of the stronger OH* scavenging effect of carbonate alkalinity at pH12.     

Surface modification of polypropylene film using N2O-containing flames

Contact-angle measurements and X-ray photoelectron spectroscopy (XPS or ESCA) were used to characterize polypropylene (PP) films that were exposed to laminar premixed air: natural gas flames containing small quantities of nitrous oxide. During combustion, the nitrous oxide generates gas-phase nitrogen oxides that lead to the affixation of nitrogen-containing functional groups to the PP surfaces. Treatment of PP in nitrous oxide-containing flames also leads to an increase in surface oxidation and markedly improves wettability when compared with standard flame treatments. The chemical form of the nitrogen affixed to the PP surface is strongly dependent on the flame equivalence ratio. Fuel-lean flames tend to affix highly oxidized forms of nitrogen such as nitrate and nitro groups, while fuel-rich flames tend to affix less-oxidized nitrogen groups such as nitroso, oxime, amide, and amine. A computational model, SPIN, was used to elucidate the chemistry of the flame as it impinges upon the cooled PP surface. The SPIN modeling indicates that the principal reactive gas-phase species at or near the PP surface are O₂, OH, H, NO, NO₂, HNO, and N₂O. A number of possible reactions between these species and the PP can account for the formation of the various nitrogen functional groups observed.     

CO2稀释对甲烷反扩散火焰结构的影响研究

基于火焰光谱诊断平台,利用由高分辨率CCD相机成像系统和光纤光谱仪组成的光谱成像系统对甲烷反扩散火焰的光谱辐射特性进行研究。获得了不同氧燃当量比和CO₂稀释水平下CH₄/O₂同轴射流反扩散火焰的OH*、CH*二维辐射分布并进行了Abel反卷积处理。结果表明：随氧燃当量比的增加,OH*火焰逐渐中空,火焰锋面被拉伸,轴向高度和火焰面积均先增大后减小。CH*火焰核心反应区位置和形状随当量比增加变化不明显。随CO₂稀释剂体积分数的增加,OH*火焰被拉伸,并由完全包络状过渡为对称包络状,火焰面积逐渐减小。CH*火焰被拉伸并靠近中央轴线,火焰面积逐渐增大。对比OH*火焰层,CH*火焰层较薄且峰值强度低。     

Formation of Mullite Basedon Hydromica Clay – Aluminum Oxide Compositions

The results of studying mullite formation based on hydromica clay – Al(OH)₃ compositions are described. It is shown that formation of mullite (or a solid solution of mullite with corundum) is the most intense at temperatures of 1150 – 1200°C in compositions with a content of hydromica clay equal to 50 – 60%.