掺混H_(2)对CH_(4)-空气对冲扩散火焰温度和NO_(x)生成影响的数值研究北大核心CSCD

文章基于CHEMKIN软件对CH_(4)-空气对冲扩散火焰燃烧过程中掺混H_(2)对火焰温度以及NO_(x)生产量的影响进行了数值研究,分析了不同H_(2)摩尔分数和火焰拉伸率下火焰温度的变化特性以及NO_(x)的生成特性。研究结果表明:受到燃料气体传质能力和燃烧产热能力的综合影响,随着H_(2)摩尔分数的增加,混合燃料主燃烧区的峰值火焰温度点更靠近空气区;随着火焰拉伸率的增大,主燃烧区的范围变窄,反应物在燃烧区的滞留时间缩短,NO的生成受到抑制;NO_(2)和NO的摩尔分数表现出正相关的关系;随着混合燃料中H_(2)摩尔分数的增大,NO和NO_(2)的峰值摩尔分数显著增大。     

柴油机燃用不同含氧掺混燃料燃烧火焰自发光研究

通过一台改造的光学单缸柴油机,研究了柴油与3种不同含氧燃料掺混后对柴油机缸内燃烧火焰发光的影响.3种含氧燃料分别为丁酸甲酯(MB)、正丁醇(B)和2,5-二甲基呋喃(DMF),掺混体积分数为20%,分别用MB,20、B,20和DMF,20表示.光学发动机转速为1,200,r/min、循环喷油量为20,mg,喷油压力为60,MPa.结果表明:柴油的发光滞燃期与放热滞燃期的差距最大,3种含氧掺混燃料在燃烧过程初期出现明显的、持续时间更长的"蓝焰"化学发光;3种含氧掺混燃料对燃烧碳烟的降低能力依次为DMF 20MB 20B 20,DMF 20降低碳烟的能力受滞燃期主导,后两者主要由含氧量决定;含氧燃料的加入减小了燃烧过程中的火焰温度和火焰面积,降低了燃烧过程中的碳烟.     

新配方乙二醇单乙醚含氧燃料对柴油机燃烧和排放的影响

将乙二醇单乙醚(EGME)作为柴油机含氧燃料,选用乙二醇二甲醚(EGDE)作为助溶剂和十六烷值改进剂,按两者体积比2:1配制了新配方乙二醇单乙醚含氧燃料.对新配方含氧燃料的燃烧与排放特性进行研究.结果表明,在柴油中加入体积分数为15%～25%的新配方乙二醇单乙醚含氧燃料,柴油机热效率明显提高,燃烧速度加快,燃烧持续期缩短,着火略有延迟.碳烟和CO排放大幅度降低,碳烟排放降低40.7%～75.0%,CO在高负荷下降低40.0%～60.9%,NOx排放稍有下降,HC排放基本不变.     

生物柴油同分异构替代燃料丁酸甲酯和丙酸乙酯预混燃烧对比研究

在CO2/O2/Ar气氛下对生物柴油两种同分异构替代燃料丁酸甲酯和丙酸乙酯的预混燃烧（当量比为0.8）进行了对比研究,重点分析了生物柴油替代燃料的同分异构化对燃烧主要产物、稳定中间产物以及自由基的影响,同时揭示CO2对两种同分异构替代燃料燃烧的化学作用,给出了潜在典型污染物的生成趋势和规律。结果表明,CO2的加入对两种燃料中重要的烟黑前驱物C2H2和C3H3具有抑制作用。CO2的稀释和热作用对C2H2生成的抑制作用在丙酸乙酯火焰中更加显著,而对C3H3的抑制作用在丁酸甲酯火焰中更加明显,并且CO2的化学作用可进一步加强对两种火焰中C2H2和C3H3生成的抑制。同时,CO2的存在可有效降低两种燃料非常规污染物醛酮类产物的浓度,其中CH2O和CH3CHO的浓度在丙酸乙酯火焰中的减小更为显著。两种火焰中抑制CH2O生成的主要作用是CO2的稀释和热作用,而CO2的化学作用则是抑制CH3CHO生成的主导作用。由产物消耗速率分析得知,对丁酸甲酯消耗影响最大的化学反应是脱氢反应MB+H=H2+MB2J,而对丙酸乙酯消耗影响最大的则是分解反应EP=C2H5COOH+C2H4。     

航空发动机燃烧室环境中非预混旋流火焰的标量特征

参考航空发动机燃烧室典型工况设计了微型模型非预混旋流燃烧室并进行了直接数值模拟,基于火焰因子对火焰标量特征进行了分析．研究发现,在非预混燃烧中,预混燃烧模态广泛存在且是放热的主要贡献者．不同工况中火焰面的分布位置和预混火焰的产生机制均存在显著差异．在贫燃工况中,火焰分布在内剪切层中,氧气优先向燃料侧输运从而在富燃料侧产生预混火焰;而在富燃工况中,火焰主要分布在外剪切层中,由于燃料中间产物优先向空气侧输运,预混火焰主要产生于富氧气侧．对标量通量的研究发现：非守恒标量(如YCO2)通量在各燃烧模态中均基本符合梯度假设;守恒标量(如混合分数Z)通量仅在预混燃烧模态中符合假设,在扩散火焰面附近不遵循这一假设．     

二甲醚/聚甲氧基二甲醚-3简化机理的构建及验证

采用基于误差的直接关系图法(DRGEP)、敏感性分析法以及同分异构聚合法对二甲醚/聚甲氧基二甲醚-3(DME/DMM₃)的联合详细化学反应机理进行简化，最终构建了一个包括65个组分和308个反应方程式的DME/DMM₃简化化学动力学机理．为了验证其可靠性，分别用二甲醚(DME)和聚甲氧基二甲醚-3(DMM₃)详细机理及试验数据与DME/DMM₃简化机理计算得到的着火延迟、层流火焰燃烧速度和组分摩尔分数等进行了比较，并分析了DME/DMM₃反应路径．最后验证了柴油机转速为1 600 r/min，当量比为0.18和0.34，燃料DME与DMM₃体积配比为1∶9时的仿真与试验的缸内压力和放热率以及CO、CO₂、NO_x和HC排放物．结果表明：该DME/DMM₃简化机理的着火延迟时间、层流火焰燃烧速度及射流搅拌反应器(JSR)中组分摩尔分数、缸内压力、放热率以及CO、CO₂、NO...     

DME与LPG预混平面火焰的甲醛及NO_x排放特性

对二甲醚与液化石油气预混平面火焰中不同掺混比例下的甲醛生成、NOx的排放特性进行了实验研究.实验结果表明,固定燃料质量流量和当量比条件下,火焰中甲醛质量浓度随着二甲醚掺混比例的增加而增加,其峰值质量浓度为相同工况纯LPG燃烧时的2～5倍,表明混合燃料中的二甲醚仍然是甲醛产生的主要来源;尾气中的NOx质量浓度随二甲醚比例的增加而降低,但均不高于16.08,mg/m3.控制二甲醚的完全氧化是二甲醚与液化石油气掺混燃烧中减少甲醛排放的关键途径.     

基于详细模型汽油替代燃料碳烟生成特性分析

基于矩方法算法和预混火焰程序,实现一维预混火焰中汽油替代燃料燃烧生成碳烟的详细数值模拟.其中气相详细模型为包含多环芳烃(PAHs)生成机理的多组分汽油替代燃料反应动力学模型,详细碳烟颗粒相模型包含成核、凝并、PAHs表面沉积、表面生长和表面氧化过程.结果表明:燃烧过程中苯(C6H6)分子摩尔分数是影响PAHs生成的直接原因,决定了汽油替代燃料生成PAHs摩尔分数的大小;芘(C16H10)分子摩尔分数直接影响碳烟的生成过程,决定汽油替代燃料碳烟排放水平;可依据汽油替代燃料的组成评价燃烧生成PAHs和碳烟的水平,汽油替代燃料中甲苯/二异丁烯总含量越高,其生成PAHs和碳烟量越大,反之亦然.     

地沟油生物柴油与正丁醇/乙醇混合燃料燃烧火焰特性

为研究不同配比下生物柴油混合燃料燃烧特性,设计了一套生物质液体燃料雾化蒸发燃烧系统,该系统可产生生物柴油及其混合燃料层流预混火焰,结合OH-PLIF平面激光诱导荧光技术测定并分析燃烧火焰的高度和锋面面积以及层流预混火焰的传播速度和OH-PLIF总信号强度等燃烧特性.结果表明随着正丁醇或乙醇添加比例的增大,两种混合燃料燃烧火焰高度、火焰锋面面积呈下降趋势;火焰传播速度呈上升趋势.在混合燃料中,正丁醇的体积分数越大,燃烧火焰OH-PLIF总信号强度越大,而乙醇的体积分数越大,混合燃料燃烧火焰OH-PLIF总信号强度越小.     

二甲醚燃烧中甲醛生成的分光光度法测量

含氧替代燃料二甲醚在燃烧过程中会产生一定浓度的甲醛等有害物质,利用国标乙酰丙酮分光光度法来测量二甲醚燃烧的甲醛生成特性,确立了一种数据可靠、简便实用、成本低廉的甲醛分析方法.由于吸收液加热和冷却时间对测量结果影响较大,为了保证测试结果准确,对这些环节的分析步骤必须严格控制.对二甲醚和甲烷燃烧的试验结果表明,二甲醚燃烧后尾气中的甲醛排放浓度比甲烷燃烧高10倍以上.     

An experimental and kinetic investigation of premixed furan/oxygen/argon flames

The detailed chemical structures of three low-pressure (35 Torr) premixed laminar furan/oxygen/argon flames with equivalence ratios of 1.4, 1.8 and 2.2 have been investigated by using tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry. About 40 combustion species including hydrocarbons and oxygenated intermediates have been identified by measurements of photoionization efficiency spectra. Mole fraction profiles of the flame species including reactants, intermediates and products have been determined by scanning burner position with some selected photon energies near ionization thresholds. Flame temperatures have been measured by a Pt–6%Rh/Pt–30%Rh thermocouple. A new mechanism involving 206 species and 1368 reactions has been proposed whose predictions are in reasonable agreement with measured species profiles for the three investigated flames. Rate-of-production and sensitivity analyses have been performed to track the key reaction paths governing furan consumption for different equivalence ratios. Both experimental and modeling results indicate that few aromatics could be formed in these flames. Furthermore, the current model has been validated against previous pyrolysis results of the literature obtained behind shock waves and the agreement is reasonable as well.     

A comprehensive experimental study of low-pressure premixed C3-oxygenated hydrocarbon flames with tunable synchrotron photoionization

Lean and rich premixed flames of three different C₃-oxygenated hydrocarbons (acetone, n-propanol, and i-propanol) at low pressure have been investigated with tunable synchrotron photoionization and molecular-beam mass spectrometry. Flame species, including isomeric intermediates, are unambiguously identified with measurements of photoionization efficiency spectra by scanning the photon energy. Mole fraction profiles of most observed species are measured by scanning the burner position at selected photon energies near ionization thresholds, and the flame temperature profiles are recorded using a Pt/Pt-13%Rh thermocouple. Compared with previous studies, some new flame species, e.g., vinyl, propargyl, allyl, ethenol, methyl ketene, propenols, ethyl ketene, methyl ethyl ketone, butenols, and 1,3,5-hexatriyne, are detected in this work, which will extend our knowledge of the intermediate pools of oxygenated hydrocarbon combustion. On the other hand, comparisons among chemical structures of these flames have been performed, based on the comprehensive experimental data. It is concluded that different structural features of fuel molecules will cause a lot of variation in intermediate pools, isomeric compositions, and formation channels of flame species, especially for the oxygenated intermediates. Combined with previous research on hydrocarbon flames, analyses of pollutant emissions and soot formation are presented. It is consistent with previous studies that the oxygenated fuels have reduced sooting tendencies and potential emissions of toxic oxygenated by-products.     

Demonstration of a burner for the investigation of partially premixed low-temperature flames

A burner, which stabilizes near-one-dimensional low-temperature flames at atmospheric pressure, was designed to access the combustion regime near 1500 K for quantitative species diagnostics. Combustion temperatures between 1300 and 1800 K in argon-diluted methane-oxygen flames were achieved by preheating the burner and adapting the inert gas flow. Mass spectrometry with electron ionization was used to determine mole fractions profiles of reactants, products, and intermediates. Combustion parameters were varied including stoichiometry, diluent mole fraction and preheat temperature. Mole fraction profiles resemble those taken in regular premixed flat flames. A number of C₁- and C₂-intermediates as well as some oxygenated species were identified. Higher-mass species (m/z > 42) were not detected in the low-temperature methane-oxygen flames which contain 90% argon in the cold gases.     

Measurements and modeling study of intermediates in ethanol and dimethy ether low-pressure premixed flames using synchrotron photoionization

A molecular-beam flame-sampling photoionization mass spectrometer, employing synchrotron radiation, was applied to detect new intermediates and to measure mole fractions in the low-pressure laminar premixed stoichiometric ratio dimethyl ether (DME)/O₂/Ar and ethanol/O₂/Ar flames respectively. With the help of the means mentioned above, flame species of the two fuels, including isomeric intermediates, were unambiguously identified and the temperature profiles and mole fraction profiles of intermediates were also compared and analyzed. Additionally, five detailed oxidation mechanisms were applied to the modeling study to verify current mechanism for the two fuels. Based on the comparative experimental and modeling study of the two flames at low pressure, the oxidation mechanism for DME and ethanol was revised and a new one was suggested in the present work. The results calculated by the revised mechanism showed that the modeling prediction agreed with those experimental results measured in DME and ethanol flames as well as with the ones in five flames from the experiments done by the other people. In the measurement all species with mole fraction higher than 10⁻⁴ were considered, including those which have not been included in the present five mechanisms. Meanwhile, the mole fractions of the species which had not been considered by current mechanism such as ethenol, acetone and ethyl methyl ether (EME) were also included in the modeling study. Besides, the reaction paths and species conversion ratio analysis were also conducted to show the difference before and after the mechanism revised.     

Identification of combustion intermediates in isomeric fuel-rich premixed butanol-oxygen flames at low pressure

Laminar premixed low-pressure flames fueled by either one of the four isomers of butanol were investigated by a molecular-beam photoionization mass spectrometer using vacuum ultraviolet (VUV) synchrotron radiation as the ionization source. The photoionization efficiency (PIE) spectra of most flame intermediates were measured between 7.75 and 11.00 eV. By comparing the resulting PIE spectra to known ionization energies (IEs) or known PIE spectra of pure substances, most hydrocarbon and oxygenated combustion intermediates, including some radicals, in the mass range from m/z=15 to 106 were assigned and identified in the four butanol flames. The results show that the higher-mass oxygenated species in butanol flames are strongly affected by the fuel structure, while many hydrocarbon isomers appear almost independent of the fuel structure. The respective dissociation mechanisms of the fuels, including complex fission, simple fission, and H-atom abstraction, are in good agreement with previous results from nonpremixed butanol flames.     

Combustion chemistry of the propanol isomers — investigated by electron ionization and VUV-photoionization molecular-beam mass spectrometry

The combustion of 1-propanol and 2-propanol was studied in low-pressure, premixed flat flames using two independent molecular-beam mass spectrometry (MBMS) techniques. For each alcohol, a set of three flames with different stoichiometries was measured, providing an extensive data base with in total twelve conditions. Profiles of stable and intermediate species, including several radicals, were measured as a function of height above the burner. The major-species mole fraction profiles in the 1-propanol flames and the 2-propanol flames of corresponding stoichiometry are nearly identical, and only small quantitative variations in the intermediate species pool could be detected. Differences between flames of the isomeric fuels are most pronounced for oxygenated intermediates that can be formed directly from the fuel during the oxidation process. The analysis of the species pool in the set of flames was greatly facilitated by using two complementary MBMS techniques. One apparatus employs electron ionization (EI) and the other uses VUV light for single-photon ionization (VUV-PI). The photoionization technique offers a much higher energy resolution than electron ionization and as a consequence, near-threshold photoionization-efficiency measurements provide selective detection of individual isomers. The EI data are recorded with a higher mass resolution than the PI spectra, thus enabling separation of mass overlaps of species with similar ionization energies that may be difficult to distinguish in the photoionization data. The quantitative agreement between the EI- and PI-datasets is good. In addition, the information in the EI- and PI-datasets is complementary, aiding in the assessment of the quality of individual burner profiles. The species profiles are supplemented by flame temperature profiles. The considerable experimental efforts to unambiguously assign intermediate species and to provide reliable quantitative concentrations are thought to be valuable for improving the mechanisms for higher alcohol combustion.     

The effect of strain rate on polycyclic aromatic hydrocarbon (PAH) formation in acetylene diffusion flames

Acetylene is a ubiquitous combustion intermediate that is also believed to be the major precursor for aromatic, polycyclic aromatic hydrocarbon (PAH), and soot formation in both hydrocarbon and halogenated hydrocarbon flames. However, in spite of its important role as a flame intermediate, the detailed chemical structures of acetylene diffusion flames have not been studied in the past. Here the detailed chemical structures of counterflow diffusion flames of acetylene at strain rates of 37.7 and 50.3 s⁻¹ are presented. Both flames possessed the same carbon density of 0.37 g/L corresponding to an acetylene mole fraction of 0.375 in argon on the fuel side, and an oxygen mole fraction of 0.22 in argon on the oxidizer side. Concentration profiles of a large number of major, minor, and trace species, including a wide spectrum of aromatics and PAH, have been determined by direct sampling from flames using a heated quartz microprobe coupled to an online gas chromatograph/mass selective detector (GC/MSD). Temperature profiles were made using a thermocouple and the rapid insertion technique. Although the major species concentrations were nearly the same in the two flames, the mole fraction profiles of trace combustion by-products were significantly lower in the higher-strain-rate flame, by nearly two orders of magnitude for PAH. These comparative results provide new information on the trace chemistries of acetylene flames and should be useful for the development and validation of detailed chemical kinetic mechanisms describing the formation of toxic by-products in the combustion of hydrocarbons and halogenated hydrocarbons.     

An experimental and kinetic modeling study of premixed NH3/CH4/O2/Ar flames at low pressure

An experimental and modeling study of 11 premixed NH₃/CH₄/O₂/Ar flames at low pressure (4.0 kPa) with the same equivalence ratio of 1.0 is reported. Combustion intermediates and products are identified using tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry. Mole fraction profiles of the flame species including reactants, intermediates and products are determined by scanning burner position at some selected photon energies near ionization thresholds. Temperature profiles are measured by a Pt/Pt-13%Rh thermocouple. A comprehensive kinetic mechanism has been proposed. On the basis of the new observations, some intermediates are introduced. The flames with different mole ratios (R) of NH₃/CH₄ (R0.0, R0.1, R0.5, R0.9 and R1.0) are modeled using an updated detailed reaction mechanism for oxidation of CH₄/NH₃ mixtures. With R increasing, the reaction zone is widened, and the mole fractions of H₂O, NO and N₂ increase while those of H₂, CO, CO₂ and NO₂ have reverse tendencies. The structural features by the modeling results are in good agreement with experimental measurements. Sensitivity and flow rate analyses have been performed to determine the main reaction pathways of CH₄ and NH₃ oxidation and their mutual interaction.     

Combustion of butanol isomers – A detailed molecular beam mass spectrometry investigation of their flame chemistry

The combustion chemistry of the four butanol isomers, 1-, 2-, iso- and tert-butanol was studied in flat, premixed, laminar low-pressure (40 mbar) flames of the respective alcohols. Fuel-rich (? = 1.7) butanol–oxygen–(25%)argon flames were investigated using different molecular beam mass spectrometry (MBMS) techniques. Quantitative mole fraction profiles are reported as a function of burner distance. In total, 57 chemical compounds, including radical and isomeric species, have been unambiguously assigned and detected quantitatively in each flame using a combination of vacuum ultraviolet (VUV) photoionization (PI) and electron ionization (EI) MBMS.Synchrotron-based PI-MBMS allowed to separate isomeric combustion intermediates according to their different ionization thresholds. Complementary measurements in the same flames with a high mass-resolution EI-MBMS system provided the exact elementary composition of the involved species. Resulting mole fraction profiles from both instruments are generally in good quantitative agreement.In these flames of the four butanol isomers, temperature, measured by laser-induced fluorescence (LIF) of seeded nitric oxide, and major species profiles are strikingly similar, indicating seemingly analog global combustion behavior. However, significant variations in the intermediate species pool are observed between the fuels and discussed with respect to fuel-specific destruction pathways. As a consequence, different, fuel-specific pollutant emissions may be expected, by both their chemical nature and concentrations.The results reported here are the first of their kind from premixed isomeric butanol flames and are thought to be valuable for improving existing kinetic combustion models.     

Methyl butanoate inhibition of n-heptane diffusion flames through an evaluation of transport and chemical kinetics

The extinction limits of methyl butanoate, n-heptane, and methyl butanoate/n-heptane diffusion flames have been measured as a function of fuel mole fraction with nitrogen dilution in counterflow with air. On a mole fraction basis, methyl butanoate diffusion flames are observed to have a much lower extinction strain rate than n-heptane diffusion flames and the extinction strain rate of n-heptane/methyl butanoate diffusion flames is observed to increase significantly as the n-heptane fraction is increased.Based on previous works, detailed chemical kinetic models to describe the high temperature oxidation of these fuel mixtures are assembled, tested and reduced. When the transport properties of ester species are re-evaluated by means of a thorough literature review, numerical computations of extinction generally reproduce experimental results for the pure fuels as well as for mixtures. An in-depth analysis of the kinetic model computations reveals that the extinction behaviour of both fuels is due to (1) fuel energy content affects and (2) the chemical kinetic potential of each fuel to produce the hydroperoxy radical. Comparatively, in n-heptane flames reactive ethyl radicals and ethylene are the major intermediates formed, but in methyl butanoate flames the major intermediates are formyl radicals and formaldehyde. In all flames studied, increased strain rates affect an increased interaction of formyl and/or vinyl radicals with molecular oxygen leading to a transition from hydrogen atom production at low strain rates, to the production of large quantities of the hydroperoxy radical at higher strain rates. The formation of the hydroperoxy radical induces extinction in each flame by directly interfering with the important radical chain branching and exothermic elementary reactions of H atoms and OH radicals that are dominant in weakly strained flames.It is postulated that the similar inhibitive effect of methyl butanoate fuelled flames will also be observed for more biodiesel like, larger n-alkyl esters when compared to equivalent n-alkanes. The diffusive extinction limits of methyl decanoate diffusion flames are also measured and show reactivity comparable to n-heptane diffusion flames by a molar comparison.