气流床气化炉内撞击火焰的Na~*、K~*二维辐射特性

气流床气化炉内火焰的辐射光谱包含着大量与火焰结构、火焰温度及当量比等有关特性的信息,因此对火焰进行有效的光谱诊断,是实现燃料高效利用、监控和优化气化过程的必要手段.基于实验室规模多喷嘴对置式气化实验平台,以柴油及水煤浆为原料,重点研究了气化炉内撞击火焰Na~*、K~*二维辐射分布特征,进而表征气化炉内火焰温度分布.结果表明,相比较两喷嘴火焰,四喷嘴火焰撞击区自由基辐射强度明显增大;Na~*的二维强度分布明显强于K~*的二维强度分布,且柴油中K含量高于Na,说明在相同条件下,Na比K更加容易被激发;随O/C摩尔比增大,Na~*及K~*辐射强度均增大,撞击区温度升高;在不同O/C摩尔比条件下,撞击中心辐射强度均最大且撞击中心火焰温度最高.     

烷烃燃料微小平板催化燃烧的火焰特性

对微平板燃烧器内4种烷类燃料（C₁ ~ C₄）进行铂催化燃烧实验,获得其点火过程和静态火焰的特征,并进行对比分析。当量比相同时,点火过程火焰传播速度大小顺序为甲烷 > 乙烷 > 丁烷 > 丙烷。随着当量比增大,火焰传播速度加快,稳态火焰根部位置向气流上游移动。观察可见光、430 nm（OH*光谱）、516 nm（C₂*光谱）成像火焰发现,当量比越大,火焰亮度越大,OH*和C₂*浓度越高。当量比相同时,乙烷的OH*、CH*和C₂*浓度最高,而甲烷和丙烷的则较低。     

负压下CH_4/空气层流预混火焰的自发辐射

采用实验与数值模拟相结合的方法,对负压下甲烷空气层流预混火焰自发辐射进行了研究.由数值模拟建立起同时模拟OH(A)、CH(A)和C2(d)共3种激发态粒子自发辐射的一维模型,实验拍摄平面火焰自发辐射图像,提取激发态粒子峰值光强和峰值粒子数密度作为研究层流预混火焰自发辐射的主要参数.结果表明,当量比小于或等于1时,OH(A)最先达到峰值,C2(d)紧接着达到峰值,CH(A)最后达到峰值;当量比大于1时,OH(A)最先达到峰值,CH(A)紧接着达到峰值,C2(d)最后达到峰值;峰值比值OH(A)/CH(A)和C2(d)/CH(A)是当量比的单调函数.     

不同燃烧方式下甲烷与空气火焰光谱特性研究

为研究不同燃烧方式下甲烷燃烧火焰光谱特性,利用高速光纤光谱仪分别对预混和扩散燃烧方式下甲烷与空气燃烧火焰光谱进行采集,对比分析了不同燃烧工况中火焰辐射光谱特点,结果表明:在可见光波长范围内,扩散燃烧方式下的光谱强度明显高于预混燃烧;随着空气流量增加, C₂^*辐射强度呈先增加后下降的趋势,其余三种自由基辐射强度均呈小幅增加趋势,且扩散燃烧火焰中C₂^*辐射强度峰值明显高于预混燃烧;随着甲烷流量增加,在两种燃烧方式下各自由基辐射强度均呈减少趋势;根据C₂^*、OH^*辐射强度大小与可见光波长范围内连续光谱强度可以判断火焰燃烧方式。     

污泥热解气/氨旋流火焰燃烧特性

污泥热解气因难以被有效利用导致了大量的能源浪费,而其中所含的氢气、甲烷等可燃组分可有效提升氨燃料的燃烧性能。对污泥热解气掺氨旋流火焰的结构及燃烧特性进行分析,基于化学发光法,通过实验考察当量比、掺氨比对火焰结构的影响。结果表明,旋流燃烧火焰中的OH^*在化学当量条件(φ=1.0)下辐射强度最大,CH^*在富燃条件(φ=1.2)下辐射强度最大,OH^*可以对火焰稳定性进行更好地表征;污泥热解气/氨气混合燃料中,随着氨气比例增大,旋流火焰稳定性下降。     

水雾作用下甲烷/空气预混火焰的光谱特性

利用中阶梯光栅光谱仪,对甲烷,空气层流预混火焰以及水雾作用过程中的火焰发射光谱特性进行了实验研究,对比分析了水雾对预混火焰燃烧过程中自由基离子发射光谱的影响,探讨了水雾抑制甲烷燃烧过程中的化学作用机理.结果表明,火焰阵面OH*、CH*自由基离子发射光谱强度随着火焰阵面水雾载荷比的增大而减小;足量的水雾作用可抑制预混火焰中OH*、CH*和HCO*等链引发自由基的生成,增强甲烷燃烧链式反应中的三体反应过程,促进甲烷预混火焰的链销基反应的发生,在抑制甲烷预混火焰燃烧过程中起重要作用.     

基于集成深度玻尔兹曼机和最小二乘支持向量回归的燃烧过程NO_x预测算法

通过研究燃烧过程中的火焰自由基图像与NO_x排放之间的关系,提出了集成深度玻尔兹曼机和最小二乘支持向量回归的NO_x预测算法.首先采用深度玻尔兹曼机对4类火焰自由基图像(OH*、CN*、CH*和C*_2)进行自动图像特征学习,然后用最小二乘支持向量回归来拟合图像特征与NO_x排放量之间的关系,进而对NO_x排放量进行预测.结果表明:NO_x排放预测值与NO_x排放参考值具有一致性;与已有的基于图像的NO_x预测算法相比,所提方法在预测精度方面具有明显的优势.     

热薄材料表面火焰传播的三维效应

利用能够限制自然对流的水平窄通道,对薄材料表面逆风传播火焰的三维效应进行了实验研究,参数包括气流速度、氧气浓度、燃料宽度等.结果表明,在足够宽的通道内,火焰传播随燃料宽度的变化,表现出随氧气浓度和气流速度的不同而变化的三维特性.侧面热损失和氧气扩散对火焰传播的影响,在各种氧气浓度和气流速度下,都限于燃料宽度小于10倍扩散长度.     

预混火焰中温度对碳烟表面官能团和氧化活性的影响

基于甲烷/氧气预混火焰燃烧系统及毛细管取样技术,采用傅里叶变换红外光谱仪(FT-IR)、X射线光电子能谱仪(XPS)和热重分析技术(TGA),研究了火焰温度对碳烟颗粒表面官能团相对含量和氧化反应活性的影响规律.结果表明:随着火焰温度的升高,碳烟颗粒表面脂肪族碳氢官能小(aliphatic C—H)的当量峰高比IC—H/IC=C减小,含氧官能团(C—OH和C=O)的摩尔分数也减小,表明碳烟颗粒表面官能团的相对含量随火焰温度的升高而降低;碳烟颗粒氧化反应特征温度和表观活化能都随火焰温度的升高而增大,表明较高的火焰温度产生的碳烟颗粒的氧化反应活性较低;碳烟颗粒表面官能团的含量与其氧化反应活性有直接关系,表面官能团含量越少,其氧化反应活性越低.     

CH_4/空气和C_3H_8/空气预混湍流火焰结构及湍流火焰速度测量

利用OH-PLIF技术探测了CH4/空气和C3H8/空气混合气在不同湍流强度下的预混湍流瞬时火焰前锋面结构,通过湍流产生板产生了弱湍流区内的不同湍流强度,通过控制当量比调节火焰密度比和路易斯数,计算了实验中的层流火焰特征参数,分析了湍流强度和火焰特征参数对预混湍流火焰结构、湍流火焰速度的影响.结果表明:火焰前锋面结构的褶皱程度随湍流强度的增大而明显增强.在相同湍流强度下,CH4/空气火焰比C3H8/空气火焰更加褶皱.通过提取火焰OH-PLIF图像锋面,分别用角度法和面积法计算了湍流火焰速度,获得了弱湍流条件下湍流火焰速度与湍流强度的一般关系式.在相同密度比下,即相同流体动力学不稳定性下,热扩散不稳定性更大的CH4/空气比C3H8/空气的湍流火焰速度更大,而在路易斯数接近且热扩散不稳定性处于次要地位时,密度比大的CH4/空气流体动力学不稳定性更大,其对湍流火焰速度的影响也更大.     

Correlation analysis of chemiluminescent and pollutant emissions of a liquid-fueled turbulent swirl burner

Real-time combustion and emission control is an ongoing challenge in combustion technology and science. Hence, the scope of the present paper is the investigation of the relationship between the chemiluminescent signal and the CO and NO_X emissions. Flame emission spectrometry measurements were carried out to determine the characteristic free radicals of the spectra. For the experiments, a lean premixed liquid fuel burner equipped with an airblast atomizer was used in a test rig at 15 kW combustion power. The following measurement parameters were modified: combustion air flow rate, atomizing pressure, and the vertical alignment of the spectrometer. Furthermore, various half-cone angle quarls were mounted on the burner lip to extend the lean flame blowout stability limit. The CO and NO_X emissions and the chemiluminescence intensity ratios of the strongest peaks of OH*, CH*, C₂*, HCO*, and CH₂O* were evaluated separately at first. Then a correlation analysis of the intensity ratios and the pollutant emission components was carried out. A notable linear correlation was found between both the HCO*/C₂* and OH*/C₂* intensity ratios and the CO emission in certain parameter combinations.     

Turbulence and combustion interaction: High resolution local flame front structure visualization using simultaneous single-shot PLIF imaging of CH, OH, and CH2O in a piloted premixed jet flame

High resolution planar laser-induced fluorescence (PLIF) was applied to investigate the local flame front structures of turbulent premixed methane/air jet flames in order to reveal details about turbulence and flame interaction. The targeted turbulent flames were generated on a specially designed coaxial jet burner, in which low speed stoichiometric gas mixture was fed through the outer large tube to provide a laminar pilot flame for stabilization of the high speed jet flame issued through the small inner tube. By varying the inner tube flow speed and keeping the mixture composition as that of the outer tube, different flames were obtained covering both the laminar and turbulent flame regimes with different turbulent intensities. Simultaneous CH/CH₂O, and also OH PLIF images were recorded to characterize the influence of turbulence eddies on the reaction zone structure, with a spatial resolution of about 40 μm and temporal resolution of around 10 ns. Under all experimental conditions, the CH radicals were found to exist only in a thin layer; the CH₂O were found in the inner flame whereas the OH radicals were seen in the outer flame with the thin CH layer separating the OH and CH₂O layers. The outer OH layer is thick and it corresponds to the oxidation zone and post-flame zone; the CH₂O layer is thin in laminar flows; it becomes broad at high speed turbulent flow conditions. This phenomenon was analyzed using chemical kinetic calculations and eddy/flame interaction theory. It appears that under high turbulence intensity conditions, the small eddies in the preheat zone can transport species such as CH₂O from the reaction zones to the preheat zone. The CH₂O species are not consumed in the preheat zone due to the absence of H, O, and OH radicals by which CH₂O is to be oxidized. The CH radicals cannot exist in the preheat zone due to the rapid reactions of this species with O₂ and CO₂ in the inner-layer of the reaction zones. The local PLIF intensities were evaluated using an area integrated PLIF signal. Substantial increase of the CH₂O signal and decrease of CH signal was observed as the jet velocity increases. These observations raise new challenges to the current flamelet type models.     

Influence of fuel composition on chemiluminescence emission in premixed flames of CH4/CO2/H2/CO blends

The growing concern about pollutant emissions and depletion of fossil fuels has been a strong motivator for the development of cleaner and more efficient combustion strategies, such as the gasification of coal, biomass or waste, which have increased the interest in using a new type of fuels, mainly composed of CH₄, H₂, CO and CO₂.These new fuels, commonly called syngas, display a wide range of compositions, which affects their combustion characteristics and, in some cases, are more prone to instabilities or flashback. Since flame properties have been demonstrated to be strongly related to equivalence ratio, a precise measurement of the flame stoichiometry is a key pre-requisite for combustion optimization and prevention of unstable regimes. In particular, chemiluminescence emission from flames has been largely tested for stoichiometry monitoring for methane flames, but its use in syngas flames has been far less studied. Consequently, the main goal of this work is analyzing the effect of fuel composition on the chemiluminescence vs. equivalence ratio curves for different fuel blends, as a first approach for a wide range of syngas compositions. The experimental results revealed that the ratio OH*/CH*, which had been widely demonstrated to be the best option for methane, may not be suitable for monitoring with certain fuels, such as those with a high percent of hydrogen. Alternatively, other signals, in particular the ratio OH*/CO₂*, appear as viable stoichiometry indicators in those cases.The analysis was also completed by numerical predictions with CHEMKIN. The comparisons of calculations with different flame models and experimental data reveals differences in the chemiluminescence vs. equivalence ratio curves for the different combustion regimes, depending on the range of the equivalence ratio ranges and fuel compositions. This finding, which confirms previous observations for a much narrower range of fuels, could have important practical consequences for the application of the technique in real combustors.     

Instabilities and soot formation in high-pressure, rich, iso-octane-air explosion flames: 1. Dynamical structure

Simultaneous OH planar laser-induced fluorescence (PLIF) and Rayleigh scattering measurements have been performed on 2-bar rich iso-octane-air explosion flames obtained in the optically accessible Leeds combustion bomb. Separate shadowgraph high-speed video images have been obtained from explosion flames under similar mixture conditions. Shadowgraph images, quantitative Rayleigh images, and normalized OH concentration images have been presented for a selection of these explosion flames. Normalized experimental equilibrium OH concentrations behind the flame fronts have been compared with normalized computed equilibrium OH concentrations as a function of equivalence ratio. The ratio of superequilibrium OH concentration in the flame front to equilibrium OH concentration behind the flame front reveals the response of the flame to the thermal-diffusive instability and the resistance of the flame front to rich quenching. Burned gas temperatures have been determined from the Rayleigh scattering images in the range 1.4???1.9 and are found to be in good agreement with the corresponding predicted adiabatic flame temperatures. Soot formation was observed to occur behind deep cusps associated with large-wavelength cracks occurring in the flame front for equivalence ratio ??1.8 (C/O?0.576). The reaction time-scale for iso-octane pyrolysis to soot formation has been estimated to be approximately 7.5-10 ms.     

Investigation of local flame structures and statistics in partially premixed turbulent jet flames using simultaneous single-shot CH and OH planar laser-induced fluorescence imaging

We report on the application of simultaneous single-shot imaging of CH and OH radicals using planar laser-induced fluorescence (PLIF) to investigate partially premixed turbulent jet flames. Various flames have been stabilized on a coaxial jet flame burner consisting of an outer and an inner tube of diameter 22 and 2.2 mm, respectively. From the outer tube a rich methane/air mixture was supplied at a relatively low flow velocity, while a jet of pure air was introduced from the inner one, resulting in a turbulent jet flame on top of a laminar pilot flame. The turbulence intensity was controlled by varying the inner jet flow speed from 0 up to 120 m/s, corresponding to a maximal Reynolds number of the inner jet airflow of 13,200. The CH/OH PLIF imaging clearly revealed the local structure of the studied flames. In the proximity of the burner, a two-layer reaction zone structure was identified where an inner zone characterized by strong CH signals has a typical structure of rich premixed flames. An outer reaction zone characterized by strong OH signals has a typical structure of a diffusion flame that oxidizes the intermediate fuels formed in the inner rich premixed flame. In the moderate-turbulence flow, the CH layers were very thin closed surfaces in the entire flame, whereas the OH layers were much thicker. In the high-intensity-turbulence flame, the CH layer remained thin until it vanished in the upper part of the flame, showing local extinction and reignition behavior of the flame. The single-shot PLIF images have been utilized to determine the flame surface density (FSD). In low and moderate turbulence intensity cases the FSDs determined from CH and OH agreed with each other, while in the highly turbulent case a locally broken CH layer was observed, leading to a significant difference in the FSD results determined via the OH and CH radicals. Furthermore, the means and the standard deviations of CH and OH radicals were obtained to provide statistical information about the flames that may be used for validation of numerical calculations.     

Syngas combustion radiation profiling through spectrometry and DFCD processing techniques

Experimental investigations of H₂ and H₂-enriched syngas flame radiation properties have been conducted through spectroscopic and DFCD (Digital Flame Colour Discrimination) techniques. A spectrograph was employed to quantify the emission profile of H₂-based flames in the UV–visible spectral domain. The OH* emission was found to be the strongest in reactants with highest amount of H₂. Further addition of CO and/or CO₂ resulted in the reduction of OH* intensity with the addition of CO₂ causing greater radical intensity-loss than that of CO. The decrease in OH* intensity is accompanied by a corresponding increase in the CO–O* broadband continuum in the short-wavelength domain of the visible spectrum. Such reduction of OH* along with increase in CO–O* intensity can be related to the endothermic reaction mechanism of CO + OH => CO₂ + H, which describes the role of CO/CO₂ addition in H₂-enriched syngas flames. Comparison with direct imaging results provided additional credence to the effect of temperature reduction as flames with CO and/or CO₂ additions resembled colourations closer to typical bluish premixed hydrocarbon flame. The employment of DFCD processing effectively characterised different syngas combustion conditions by combining aspects of digital flame colour intensities with spatial combustion distributions. This colour signal quantification method was shown to yield useful characterisation of H₂-based flames, similar to the use of OH* chemiluminescence intensity variation from spectrometry. Also, DCFD analysis was able to depict the variances between the burning of different syngas gaseous constituents. Thus, useful image-based parameters related to the H₂-based combustion can be derived and potentially applied as a practical monitoring and characterisation mean for syngas combustion.     

LIMIT OF EQUIVALENCE RATIO ON MIXING ENHANCEMENT NEAR THE TUBE EXIT IN A TONE EXCITED RICH FLAME

An experimental investigation has been made with the objective of studying the limit of equivalent ratio (ϕ) on mixing enhancement in a tone excited jet rich flame. The jet is pulsed by means of a loudspeaker-driven cavity and experiments are limited to very rich flames (ϕ>1⋅5). The excitation frequency is chosen for the resonant frequency identified as a pipe resonance due to acoustic excitation. Methane, propane and butane are used to examine the effect of mixture property on the limit of equivalence ratio. Mixing is always enhanced in a methane/air flame as the excitation intensity increases. In the case of propane/air and butane/air flames, mixing enhancement can be obtained only when the equivalence ratio lies in the range from a certain value (the equivalence ratio limit) to infinity (non-premixed flame), irrespective of mean mixture velocities. It is also found that the equivalence ratio limit is related to flame instability; the lower the Lewis number, the higher the equivalence ratio limit. As the excitation intensity increases, flame separation occurs below the equivalence ratio limit; an inner (premixed) flame is transformed into a cellular flame which then moves upstream, but the height of an outer (non-premixed) flame is not decreased. Acoustic pressure measurements using a microphone are made to quantify the oscillating velocity. The oscillating velocity amplitude at the cellular flame position is proportional only to mean mixture velocity regardless of fuel type. © 1997 by John Wiley & Sons, Ltd.     

Experimental and numerical study of the role of NCN in prompt-NO formation in low-pressure CH4-O2-N2 and C2H2-O2-N2 flames

We report an experimental and modeling study on prompt-NO formation in low-pressure (5.3 kPa) premixed flames. Special emphasis is given to the quantitative detection (and prediction) of NCN, whose role in prompt-NO formation has recently been confirmed in alkane flames. Here a rich (Φ = 1.25) CH₄-O₂-N₂ flame and rich (Φ = 1.25) and stoichiometric C₂H₂-O₂-N₂ flames have been investigated. Absolute concentration profiles of CH and NCN radicals and NO species are obtained by combining laser-induced fluorescence (LIF) and cavity ring-down spectroscopy (CRDS). Temperature profile is determined in each flame using OH and NO-LIF thermometry. Flame modeling is performed to determine the role of NCN in prompt-NO formation and to test the capacity of the present chemical mechanisms to predict some intermediate species involved in prompt-NO formation. The methane flame is modeled using GDFkin®3.0_NCN mechanism [El Bakali et al., Fuel 85 (2006), 896-909]. The acetylene flames are modeled using the Lindstedt and Skevis C/H/O mechanism [Lindstedt and Skevis, Proc. Combust. Inst. 28 (2000), 1801-1807], completed by the submechanism issued from GDFkin®3.0_NCN for nitrogen chemistry. This submechanism includes the initiation reaction CH + N₂ = NCN + H. Rate constants of NO-sensitive reactions of the submechanism are modified by taking into account the recent literature. In particular, the C₂O route could be explored thanks to a significant presence of C₂O in acetylene flames. Globally, the modified submechanism of nitrogen chemistry coupled with the two hydrocarbon mechanisms leads to a satisfying prediction of NCN and NO mole fraction profiles, even though refinements of rate constant determination is still required. The role of NCN in prompt-NO formation in acetylene flames is demonstrated.     

Comparison of species profiles between O2 and NO2 oxidizers in premixed methane flames

The profiles of the species H, OH, CH, NH, CN, NCO, NO₂, and CH₃O are compared in a series of five premixed stoichiometric 15-torr CH₄/O₂/NO₂/N₂ flames with NO₂ comprising between 0% and 40% of the oxidizer. Relative species concentrations were measured by laser-induced fluorescence (LIF) and these results are compared with calculations using measured temperature profiles. The reaction mechanism of Miller and Bowman incorrectly predicts the standoff from the burner in flames containing more than 20% NO₂; addition of several reactions involving NO₂ and HONO produces excellent agreement with experiment for most species. The reaction CH₃ + NO₂ → CH₃O + NO is found to be particularly important in the reaction mechanism. LIF profiles of CH₃O show this species to be present in far larger quantities in the NO₂ supported flames than in the CH₄/O₂ system. The nitrogen-containing intermediates CN, NCO, and NH are all overpredicted by a factor of two in the 40% NO₂ flame relative to the 10% NO₂ flame. This indicates an inaccuracy in either the reburn reactions or the fuel nitrogen chemistry when large amounts of NO are present. The kinetic modeling shows that in the 40% NO₂ flame, the dominant pathway to N₂ formation is through N₂O, which is produced primarily by the reaction of NCO with NO. Comparison of emission profiles of NO₂* for the various flames indicates that the appearance of an orange-yellow luminous zone at the base of NO₂ supported flames is caused by thermal excitation of NO₂, not by a chemiluminescence mechanism.