扩散过滤燃烧的数值模拟研究

通过二维双温模型,使用甲烷详细的化学反应机理,设计了甲烷与氧气扩散燃烧器模型,并与实验值进行对比验证了其有效性。分析了小球直径,燃料质量分数等对火焰高度的影响。结果表明:火焰高度随着入口流速和甲烷质量分数的增大而增大;随着小球直径、固体导热系数、分子扩散系数和质量弥散系数的增大而减小。与预混气体过滤燃烧相比,扩散过滤燃烧的反应区域的气体和固体的温度分布是不同的,二者之间的最大温差达到188 K。     

富氧空气/甲烷扩散燃烧的NO抑制机理的数值研究

为了开发适用于富氧燃烧的NO抑制技术,以对向流扩散火焰这一扩散燃烧的典型形态为对象,利用所建立的基元反应动力学模型研究了燃料稀释（CO2为稀释剂）以及速度梯度的改变对富氧空气/甲烷扩散火焰中NO生成的影响.用CO2稀释燃料甲烷得到的计算结果表明,随着燃料中CO2浓度的增大,火焰结构和NO生成的机理发生了显著变化,NO排放指数EINO（Emission index of NO）单调减少.改变速度梯度发现,随着速度梯度的增加,热力型NO质量生成速率以及EINO快速下降.这些研究表明,用CO2稀释燃料以及增加速度梯度可以减少富氧火焰中NO的生成.     

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。     

CO2稀释及合成气构成对预混燃烧特性的影响

用CO2代替N2作氧化剂的稀释气体不但可以减少NOx的排放,还能有效回收利用CO2.CO2具有与N2不同的物理、化学性质,通过对合成气与不同CO2稀释程度氧化剂的预混燃烧计算发现,CO2体积分数的增加会降低火焰温度进而降低燃烧速度,增加化学反应滞留时间.通过对贫燃拉伸火焰的研究发现,在CO2稀释氧化剂环境下,合成气中H...     

CO_2稀释对合成气扩散火焰中氮氧化物生成排放特性的影响

为揭示合成气燃烧过程中氮氧化物的生成机理和抑制措施,利用详细化学反应机理动力学模型研究了CO2稀释对合成气对冲扩散火焰中氮氧化物生成的影响,结果表明:随着合成气成分的变化及稀释剂CO2的添加,扩散火焰结构及不同NO生成机理对总NOx排放的贡献发生显著变化;低火焰拉伸率下主要表现为热力型NO,但在高火焰拉伸率下,因CH4存在,使总NO生成高于不含CH4的合成气;随CO2稀释剂的添加,NOx的排放指数EI<,NOx>呈单调下降趋势,并且稀释空气的效果优于稀释燃料的效果.     

不同空气稀释剂对合成气扩散火焰NO_x生成特性的影响

针对合成气燃烧中NOx的生成机理,以结构简单的对冲火焰作为研究对象,利用化学反应动力学模型研究了不同稀释剂对火焰特性、自由基浓度及NOx生成的影响.结果表明:3种稀释剂降低NO排放效果的顺序为:CO2＞H2O＞N2,少量的CO2或H2O稀释空气时能有效地降低NOx排放;稀释剂量的增加对合成气中是否存在CH4时的影响趋势基本一致;合成气中CH4的存在降低了火焰温度和热力型NO生成,促进了快速型NO的生成;火焰拉伸率的提高使火焰温度和NO的生成降低.说明采用CO2和H2O稀释空气能有效抑制NOx的生成.     

组分H2/CO的比例对合成气非预混射流火焰燃烧特性的影响

采用一种14组分37步简化机理模型、RNG k-ε湍流模型以及稳态层流小火焰(SLF)燃烧模型,研究了N₂稀释条件下组分H₂/CO的比例对合成气燃烧特性的影响。数值模拟结果表明：当组分H₂/CO的体积比从3:7变化至7:3时,合成气燃烧过程中生成的OH自由基浓度上升,燃烧位置向入口靠近;火焰燃烧峰值温度随H₂/CO体积比的增大而下降,火焰峰值温度所在位置向燃料入口靠近;火焰传播速度随H₂/CO体积比的增大而加快,燃烧反应在更短的距离和时间内完成。     

N2和CO2稀释对氢气-空气湍流扩散燃烧及NO生成特性的影响

采用18组分47步H2-N2-CO2反应机理模型、可实现k-ε模型及涡流耗散概念(EDC)模型研究了N2和CO2稀释作用对氢气-空气同轴射流湍流扩散燃烧过程的影响.结果 表明:2种稀释剂均能有效降低氢气燃烧温度,降低NO质量分数,且NO峰值质量分数随着火焰峰值温度的升高而上升;与稀释剂N2相比,CO2对降低氢气燃烧温度和NO质量分数的效果较好;2种稀释剂对火焰峰值温度及NO峰值质量分数的影响是非线性的,随着稀释率的增大,稀释剂降低火焰峰值温度的效果明显增强,而抑制NO生成的效果逐渐减弱;当稀释剂为N2、稀释率为0.5或稀释剂为CO2、稀释率为0.3时,能使火焰峰值温度处于中等水平情况下NO峰值质量分数依然较低,有利于实现氢气的高效低污染燃烧.     

电场作用下的液体乙醇微尺度层流扩散燃烧

采用实验和数值模拟方法研究了电场作用下液体乙醇层流扩散微燃烧的火焰结构、火焰无量纲高度和宽度以及Re之间的关系.结果表明,实验拍摄火焰结构图像与数值模拟高温区结构相似,正向电场火焰结构尺寸较反向电场高;H/d与Re呈线性关系,正向电场的H/d随Re增加幅度较反向电场快;W/d随Re先是线性增加,后期变化幅度较小;火焰中...     

Dilution effects analysis on NOx emissions of opposed-jet H2/CO syngas diffusion flames

Syngas is a promising alternative fuel for stationary power generation due to cleaner combustion than convectional fossil fuels. During the gasification processes, the by-products of CO₂, H₂O, or N₂ may be present in the syngas mixture to control the flame temperature and emissions. Several studies indicated that syngas with dilutions is capable of reducing pollutant emissions such as NO_x emissions. This work applied a numerical model of opposed-jet diffusion fames to explore the dilution effects on NO_x formation and differentiate the inert effect, thermal/diffusion effect, chemical effect, and radiation effect from CO₂, H₂O, or N₂ dilutions. The numerical study was performed by a revised OPPDIF program coupling with narrowband radiation model and detail chemical mechanism. The dilution effects on NO_x formation were analyzed by comparing the realistic and hypothetical cases. Regardless the diluent types, the inert effect is the main cause to reduce NO production, followed by chemical effect and radiation effect. The thermal/diffusion effect may promote NO formation because the preferential diffusion due to different diffusivities between diluents and syngas magnifies the reaction rate locally. CO₂ dilution reduces NO by radiation effect at low strain rate, and contributes NO reduction by chemical effect at high strain rate. At the same dilution percentage, CO₂ dilution reduces NO production the most, followed by H₂O and N₂. Besides the thermal/diffusion effect, the chemical effect of H₂O enhances NO production through thermal route and reburn route.     

Effect of CO2/N2/CH4 dilution on NO formation in laminar coflow syngas diffusion flames

The effect of CO₂/N_2/CH₄ dilution on NO formation in laminar coflow H₂/CO syngas diffusion flames was experimentally and numerically investigated. The results reveal that the NO emission index increases with H₂/CO mole ratio. In all cases, CO₂/N₂/CH₄ dilution can reduce the peak temperature of syngas flame and have the ability to reduce peak flame temperature is decreased in the following order: CO₂>N₂>CH₄. CO₂/N₂ dilution reduces the NO formation in syngas flame while CH₄ dilution promotes the NO formation. Besides, the dilution of CO₂/N₂/CH₄ can reduce the peak mole fraction of OH and its variations with H₂/CO mole ratio and dilution ratio show the same trend as the peak flame temperature variations. The height of the flame with CO₂ and N₂ dilution increases with dilution ratio. The flame with CH₄ dilution becomes higher and wider with the increase of dilution ratio.     

Effect of oxygen-rich combustion on soot formation in laminar co-flow propane diffusion flames

Oxygen-rich combustion is a new type of clean combustion technology with important application prospects. In this work, the effects of oxygen-rich combustion on soot formation in the propane/(O₂+N₂) laminar flow coaxial jets diffusion flame were numerically investigated by using the detailed gas-phase chemical reaction model with the mechanism of tetracyclic aromatic hydrocarbons and the complex thermodynamic properties and transport characteristics parameters. Soot surface growth follows the hydrogen-abstraction-carbon-addition (HACA) model. A hybrid gas-phase mechanism was adopted, which contains a DLR-based polycyclic aromatic hydrocarbons (PAHs) formation, growth model and a gas-phase model. Results show that the oxygen-rich combustion has a great influence on the flame temperature, especially the high temperature region. With the increase of oxygen concentration, the soot formation region of flame broadens and the maximum of soot volume fraction increase from 3.95 ppm to 10.87 ppm. The extra oxygen makes PAHs increased around the nozzle, leading to larger rate in early soot nucleation and surface growth, eventually more soot yield.     

Effect of syngas fuel compositions on the occurrence of instability of laminar diffusion flame

The paper presents a numerical investigation of the critical roles played by the chemical compositions of syngas on laminar diffusion flame instabilities. Three different flame phenomena – stable, flickering and tip-cutting – are formulated by varying the syngas fuel rate from 0.2 to 1.4 SLPM. Following the satisfactory validation of numerical results with Darabkhani et al. [1], the study explored the consequence of each species (H₂, CO, CH₄, CO₂, N₂) in the syngas composition. It is found that low H₂:CO has a higher level of instability, which however does not rise any further when the ratio is less than 1. Interestingly, CO encourages the heat generation with less fluctuation while H₂ plays another significant role in the increase of flame temperature and its fluctuation. Diluting CH₄ into syngas further increases the instability level as well as the fluctuation of heat generation significantly. However, an opposite effect is found from the same action with either CO₂ or N₂. Finally, considering the heat generation and flame stability, the highest performance is obtained from 25%H₂+75%CO (81 W), followed by EQ+20%CO₂, and EQ+20%N₂ (78 W).     

Effect of radiation emission and reabsorption on flame temperature and NO formation in H2/CO/air counterflow diffusion flames

The radiation effect on flame temperature and NO emission of H₂-lean (0.2H₂ + 0.8CO) and H₂-rich (0.8H₂ + 0.2CO) syngas/air counterflow diffusion flames was numerically investigated using OPPDIF code incorporated with the optical thin model, statistical narrow band model and adiabatic condition. Firstly, the coupled effect of strain rate and radiation was studied. Disparate tendencies of NO emission with an increasing strain rate between H₂-lean and H₂-rich syngas flames were found at very small strain rate, and the effect of radiation reabsorption on NO formation can be neglected when the strain rate was greater than 100 s^?1 for both H₂-lean and H₂-rich syngas flames. Because the radiation effect is vital to flames with small strain rate, its impact on flame temperature and NO emission was investigated in detail at a strain rate of 10 s^?1. The results indicated that NO formation is more sensitive to radiation reabsorption than flame temperature, especially for the H₂-rich syngas flame. The underlying mechanism was discovered by using reaction pathway analysis. Furthermore, the radiation effect under CO₂ dilution of the syngas fuel was examined. It was demonstrated that the radiation effect on flame temperature became more prominent with the increase of CO₂ concentration for both H₂-lean and H₂-rich syngas. The radiation effect on NO emission increased first and then decreased with an increasing CO₂ content for H₂-lean syngas, whereas for H₂-rich syngas the radiation effect is monotonic.     

NO mechanisms of syngas MILD combustion diluted with N2, CO2, and H2O

The NO mechanism under the moderate or intense low-oxygen dilution (MILD) combustion of syngas has not been systematically examined. This paper investigates the NO mechanism in the syngas MILD regime under the dilution of N₂, CO₂, and H₂O through counterflow combustion simulation. The syngas reaction mechanism and the counterflow combustion simulation are comprehensively validated under different CO/H₂ ratios and strain rates. The effects of oxygen volume fraction, CO/H₂ ratio, pressure, strain rate, and dilution atmosphere are systematically investigated. For all the MILD cases, the contribution of the prompt and NO-reburning routes to the overall NO emission is less than 0.1% due to the lack of CH₄ in fuel. At atmospheric pressure, the thermal route only accounts for less than 20% of the total NO emission because of the low reaction temperature. Moreover, at atmospheric pressure, the contribution of the NNH route to NO emission is always larger than 55% in the N₂ atmosphere. The N₂O-intermediate route is enhanced in CO₂ and H₂O atmospheres due to the increased third-body effects of CO₂ and H₂O through the reaction N₂ + O (+M) ? N₂O (+M). Especially in the H₂O atmosphere, the N₂O-intermediate route contributes to 60% NO at most. NO production is reduced with increasing CO/H₂ ratio or pressure, mainly due to decreased NO formation from the NNH route. Importantly, a high reaction temperature and low NO emission are simultaneously achieved at high pressure. To minimize NO emission, the reactions should be operated at high values of CO/H₂ ratios (i.e., >4) and pressures (e.g., P > 10 atm), low oxygen volume fractions (e.g., X_O2 < 15%), and using H₂O as a diluent. This study provides a new fundamental understanding of the NO mechanism of syngas MILD combustion in N₂, CO₂, and H₂O atmospheres.     

NO formation of opposed-jet syngas diffusion flames: Strain rate and dilution effects

The NO formation characteristics and reaction pathways of opposed-jet H₂/CO syngas diffusion flames were analyzed with a revised OPPDIF program which coupled a narrowband radiation model with detailed chemical kinetics in this work. The effects of strain rates ranging from 0.1 to 1000 s^?1 and diluents including CO₂, H₂O and N₂ on NO production rates were investigated for three typical syngas compositions. The numerical results demonstrated that NO is produced primary through NNH-intermediate route and thermal route at high strain rates, where the reaction of NH + O = NO + H (R51) also become more active. Near the strain rate of 10 s^?1, the flame temperature is the highest and thermal route is the dominant NO formation route, but NO would be consumed by reburn route where NO is converted to NH through HNO, especially for H₂-rich syngas. At low strain rates, radiative heat loss results in a lower flame temperature and further reduce NO formation, while the reaction of N + CO₂ = NO + CO (R140) become more important, especially for CO-rich syngas. With the diluents, NO production rates decreased with increasing dilution percentages. When the flame temperature is very high as the thermal route is dominant near strain rate of 10 s^?1, CO₂ dilution makes flame temperature and NO production rate the lowest. Toward both lower and higher strain rates, adding H₂O is more effective in reducing NO because R140 and NNH-intermediate route are suppressed the most by H₂O dilution respectively.     

NOx formation in post-flame gases from syngas/air combustion at atmospheric pressure

Species concentration measurements specifically those associated with nitrogen oxides (NO_x) can act as important validation targets for developing kinetic models to predict NO_x emissions under syngas combustion accurately. In the present study, premixed combustion of syngas/air mixtures, with equivalence ratio (Φ) from 0.5 to 1.0 and H₂/CO ratio from 0.25 to 1.0 was conducted in a McKenna burner operating at atmospheric pressure. Temperature and NO_x concentrations were measured in the post-combustion zone. For a given H₂/CO ratio, increasing the equivalence ratio from lean to stoichiometric resulted in an increase in NO and decrease in NO₂ concentration near the flame. Increasing the H₂/CO ratio led to a decrease in the temperature as well as the NO concentration near the flame. Based on the axial profiles above the burner, NO concentration increases right above the flame while NO₂ concentration decreases through NO₂-NO conversion reactions according to the path flux analysis. In addition, the present experiments were operated in the laminar region where multidimensional transport effects play significant roles. In order to account for the radial and axial diffusive and convective coupling to chemical kinetics in laminar flow, a multidimensional model was developed to simulate the post-combustion species and temperature distribution. The measurements were compared against both multidimensional computational fluid dynamics (CFD) simulations and one-dimensional burner-stabilized flame simulations. The multidimensional model predictions resulted in a better agreement with the measurements, clearly highlighting the effect of multidimensional transport.     

Numerical modeling for structure and NOx formation characteristics of oxygen-enriched syngas turbulent non-premixed jet flames

The present study numerically investigated the effect of oxygen enrichment on the precise structure and NO_x formation characteristics of turbulent syngas non-premixed flames. The turbulence-chemistry interactions were represented by a Lagrangian flamelet model. In context with the Lagrangian flamelet model, the NO concentration was obtained directly from the flamelet calculation based on full NO_x chemistry, with radiative heat loss being accounted for through the flamelet energy equation. Computations were performed for three different syngas compositions with a designated nitrogen dilution level. Numerical results indicated that, for the CO-rich composition with the lowest LHV yielding the highest scalar dissipation rate and shortest flight time, the flame structure was dominantly influenced by turbulence-chemistry interactions. On the other hand, with regard to the H₂-rich composition with the highest LHV yielding the lowest injection velocity and longest flight time, the flame structure was strongly influenced by radiative cooling. The peak NO level was remarkably elevated by increased oxygen level due to the elevated temperature of the oxygen-enriched flame. In the enhanced oxygen level (30%), the H₂-rich case produced the highest NO level due to a higher temperature and longer residence time within the hot flame zone, while the CO-rich case yielded the lowest NO level due to a lower temperature and shorter residence time. It was also found that, by enhancing the oxygen level, contributions of NNH and N₂O to total NO emission rapidly decreased while the contributions of the thermal NO path were progressively dominant for all cases.     

Numerical investigation of the effects of H2/CO/syngas additions on laminar premixed combustion characteristics of NH3/air flame

Co-firing NH₃ with H₂/CO/syngas (SYN) is a promising method to overcome the low reactivity of NH₃/air flame. Hence, this study aims to systematically investigate the laminar premixed combustion characteristics of NH₃/air flame with various H₂/CO/SYN addition loadings (0–40%) using chemical kinetics simulation. The numerical results were obtained based on the Han mechanism which can provide accurate predictions of laminar burning velocities. Results showed that H₂ has the greatest effects on increasing laminar burning velocities and net heat release rates of NH₃/air flame, followed by SYN and CO. CO has the most significant effects on improving NH₃/air adiabatic flame temperatures. The H₂/CO/SYN additions can accelerate NH₃ decomposition rates and promote the generation of H and NH₂ radicals. Furthermore, there is an evident positive linear correlation between the laminar burning velocities and the peak mole fraction of H + NH₂ radicals. The reaction NH₂ + NH <=> N₂H₂ + H and NH₂ + NO <=> NNH + OH have remarkable positive effects on NH₃ combustion. The mole fraction of OH × NH₂ radicals positively affects the net heat release rates. Finally, it was discovered that H radicals play an important role in the generation of NO. The H₂/CO/SYN additions can reduce the hydrodynamic and diffusional-thermal instabilities of NH₃/air flame. The NH₃ reaction pathways for NH₃–H₂/CO/SYN-air flames can be categorized mainly into NH₃–NH₂–NH–N–N₂, NH₃–NH₂–HNO–NO(?N₂O)–N₂ and NH₃–NH₂(?N₂H₂)–NNH–N₂. CO has the greatest influence on the proportions of three NH₃ reaction routes.