生物燃气及生物燃气掺氢燃烧特性的实验研究

为了获得生物燃气及生物燃气掺氢的燃烧特性,文章通过高速纹影系统在定容燃烧弹中研究了CH_4/CO_2在不同CO_2比例、不同当量比、不同初始压力以及不同掺氢比下的层流燃烧特性。研究结果表明:生物燃气的层流燃烧速度随着CO_2掺混比例的增大而降低,随着初始压力的升高而降低;生物燃气掺氢后的层流燃烧速度随着掺氢比的增大而增大;对于生物燃气及生物燃气掺氢而言,火焰的稳定性随着初始压力及掺氢比的增大而降低,随着当量比增大而增大,生物燃气中CO_2的比例对火焰稳定性的影响不大。     

一维平面火焰OH基绝对浓度值的测量与分析

针对甲烷/空气预混气体层流平面火焰,利用LIF技术,采用双向光路法对燃烧过程中所产生的OH基进行了测量,得到其一维绝对浓度值及其空间分布,并分析其产生的动力学因素.结果表明,当量比为0.8~1.0范围内,OH基绝对浓度差别不大;当量比在1.1~1.3的范围内,OH基的绝对浓度呈下降趋势,一方面由于OH基消耗反应OH+CH_4→CH_3+H2O,随着CH_4量的增大,OH基的消耗速度增加,另一方面OH基生成反应CH_3+O_2→OH+CH2O,虽然CH_3不断生成,但由于O_2的流量逐渐减小,所以OH基的生成率逐渐下降,二者共同作用导致OH基绝对浓度随着当量比的增大而减小.实验所测OH基绝对浓度值与Versluis等测得的OH基绝对浓度的数量级相差不大.     

甲烷-正庚烷混合物的预混层流火焰研究

利用球形传播火焰试验和一维平面火焰数值模拟对0.1 MPa、393 K条件下甲烷-正庚烷混合物的预混层流燃烧特性进行了研究,其中当量比从0.7到1.5、甲烷含量从0到100%,.结果表明:随着甲烷含量的提高,甲烷-正庚烷混合物的层流燃烧速度和Markstein长度首先基本保持不变,呈现出与正庚烷类似的特性,当甲烷含量达到75%,时,其层流燃烧速度和Markstein长度开始不同于正庚烷,逐渐发生变化.结合层流燃烧速度的敏感性分析,对混合物一维平面火焰中主要自由基的浓度和主要基元反应的反应速率随甲烷含量的变化进行了研究,发现当甲烷含量增至75%,时,主要自由基(·H、·OH和·O)的摩尔分数峰值和基元反应(R1 H+O_2=O+OH、R24 CO+OH=CO_2+H)的反应速率峰值都开始发生变化.     

等离子体辅助甲烷燃烧的主要反应路径/通道

通过实验和模拟的方法,对等离子体辅助甲烷燃烧主要基元反应进行了研究.实验测量了等离子体辅助甲烷燃烧火焰不同位置的发射光谱图,得到了参与该过程的重要组分;模拟过程中,基于密度泛函理论,研究了甲烷燃烧反应中O_2+H→OH+O、CH_4+OH/O/H→CH_3+H_2O/OH/H_2、CH_3+O→CH_2O+H、CH_2O+OH/O/H→CO+H_2O/(H+H_2O)/(H+H_2)等几个重要基元反应,在B3LYP/6-311++G**水平找到了各反应可能的反应路径/通道,并进行分析.在此基础上,对比分析了放电等离子体作用下各反应路径/通道的变化,结果表明,在研究等离子体辅助甲烷燃烧机理时,应基于甲烷燃烧机理进行完善,通过加入*2HO(v)、O(1D)、CH_4(v)等激发态组分,并对一些基元反应进行拆分或合并,重新计算各反应活化能;生成的激发态组分会提升反应势能面,降低活化能,加速反应进程;在CH_2O与O/OH/H的反应中,放电等离子体可将生成的HCO直接分解,降低了HCO对H原子的消耗,同时还会释放一个H原子,有利于燃烧反应的进行.     

汽油压燃发动机中燃烧化学动力学特性的模拟

利用PRF70掺混燃料作为汽油表征燃料进行仿真研究,将三维计算软件CONVERGE和Chemkin结合,研究了汽油压燃燃烧反应过程的主要放热反应并对其进行了相关的化学反应路径分析。结果表明:在汽油压燃燃烧反应过程中,不同反应对总放热率的贡献不同,由HCO+O_2CO+HO_2、CH2O+OHHCO+H2O、CH2CCH2OH+O_2CH2OH+CO+CH2O是燃料燃烧过程中对放热贡献最大的3个反应,其放出的热量远大于其他反应;汽油压燃燃烧反应过程中参与夺氢反应的自由基主要有4种,分别是HO_2、OH、H、O,在不同曲轴转角处上述4种自由基参与夺氢反应的重要性不同,HO_2夺氢反应所占比例始终领先其他3种,H、O参与的夺氢反应所占比例随曲轴转角的增加而增加,相应的OH参与夺氢反应所占比例减少;汽油压燃燃烧反应过程中,随着曲轴转角的变化,缸内温度升高,异辛烷发生高温裂解的比例增加。     

Oxy-Steam燃烧方式下的NO生成特性

以甲烷掺混氨气为燃料对oxy-steam气氛下NO生成特性进行实验和模拟研究.在常压柱塞流反应器中开展了一系列O_2/N_2和O_2/H_2O气氛的对比实验,当量比从富燃至贫燃(1.6、1.0、0.2),温度范围为973~1,773,K.实验结果表明,O2/H_2O气氛下高浓度的水蒸气在当量和贫燃工况下抑制NO的生成,而在富燃工况下促进NO生成.更新并构建的详细化学机理能够很好地重现并解释实验中NO的生成特性.当量和贫燃工况下,极少的O基团抑制了反应NH_2+O H+HNO从而抑制了NH_2→HNO→NO反应路径,最终降低了NO的生成.富燃工况下,充足的OH基团显著促进了反应NH_2+OH=NH+H_2O从而促进了NH_2→NH→HNO→NO反应路径,最终增加了NO的生成.     

分层充量压燃简化动力学模型及燃烧机理

在前期工作的基础上,加入了与浓混合气相关的反应,构建了适合分层压燃的简化动力学模型,包括39种物质和49个基元反应.新构建的分层压燃简化动力学模型,零维模拟的结果与详细机理吻合很好;与三维CFD耦合模拟的结果与预混/直喷条件下的实验结果吻合也很好.对于预混/直喷分层燃烧来说,低温反应最先发生在压缩余隙和凹坑内活塞表面附近的燃料均质分布的区域,而高温反应最先发生在喷雾导致的浓混合气区域.到低温反应结束为止,缸内生成的H2O2和CH2O浓度分布不均匀,混合气较浓的区域生成的H2O2和CH2O浓度较大.高温反应开始后,浓混合气区域温度比燃料均质分布的区域高,因此在较浓的混合气区域,H2O2、CH2O、HCO和CO等重要中间产物消耗的速度都比燃料均质分布的区域快.     

掺氢天然气充量均质压燃发动机燃烧和排放特性的数值模拟研究

为研究掺氢对天然气充量均质压燃（homogeneous charge compression ignition, HCCI）发动机的燃烧及排放特性的影响,基于化学反应动力学软件包CHEMKIN中的零维模型模拟天然气HCCI发动机在掺氢比为0%~40%时的燃烧过程及排放,并对其进行数值分析。结果表明：掺入氢气后缸内温度和放热率先略下降后上升而缸内压力变化不明显,着火时刻随掺氢比增加不断提前,CO和CO₂的生成浓度降低而NO的生成浓度上升。掺入氢气为体系提供大量H,使链分支和链传递反应加快,HO₂和OH生成速度加快,造成着火时刻提前。NO的总生成速率随掺氢比增加而加快。     

边界条件对正庚烷均质压燃燃烧过程的影响

应用零维详细化学反应动力学模型，对不同边界条件下正庚烷（n—heptane）均质压燃燃烧反应的化学反应动力学过程进行了数值模拟研究，得出了以初始温度和燃料当量空燃比这两类边界条件为函数，压缩比为17，转速为1400r／min的HCCI全工况解。结果表明：HCCI燃烧分为完全燃烧区域、低温反应和蓝焰反应区域、仅发生低温反应区域和失火区域；不发生热焰反应的关键是反应H＋O2=O＋OH进行程度浅，不能生成足够的OH自由基使CO氧化成CO2；蓝焰反应也不发生而仅发生低温反应的关键是H2O2分解反应的进行程度浅，H2O2只有在缸内温度达到1000K时才能快速分解，这就不能生成足够的OH自由基使甲醛转化成CO2低温反应和蓝焰反应区域是高CO排放区，仅发生低温反应的区域是高甲醛排放区。     

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。     

天然气/氢气燃烧特性研究

在定容燃烧弹中研究了不同氢气掺混比例、燃空当量比和初始压力下的大然气／氢气混合气的燃烧特性，建立了适合用于容弹计算的准维双区模型。研究结果表明：在各种当量比和初始压力下，随着掺氢比例的增加，混合气的质量燃烧速率明显增加，燃烧持续期和火焰发展期娃著缩短。随着掺氢比例的增加，短的燃烧持续期所对应的当量比范围变宽，稀混合气和浓混合气条件下天然气掺氢对火焰发展期缩短的效果更明显。化学计量比附近（1．0—1．1）掺氢燃烧对燃烧最大压力值影响不大，浓混合气（燃空当量比大于1．1）和稀混合气燃烧时，随着掺氢比例的增加，最大燃烧压力值增加。     

Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures

Effect of hydrogen addition on early flame growth of lean burn natural gas–air mixtures was investigated experimentally and numerically. The flame propagating photos of premixed combustion and direct-injection combustion was obtained by using a constant volume vessel and schlieren photographic technique. The pressure derived initial combustion durations were also obtained at different hydrogen fractions (from 0% to 40% in volumetric fraction) at overall equivalence ratio of 0.6 and 0.8, respectively. The laminar premixed methane–hydrogen–air flames were calculated with PREMIX code of CHEMKIN II program with GRI 3.0 mechanism. The results showed that the initial combustion process of lean burn natural gas–air mixtures was enhanced as hydrogen is added to natural gas in the case of both premixed combustion and direct-injection combustion. This phenomenon is more obvious at leaner mixture condition near the lean limit of natural gas. The mole fractions of OH and O are increased with the increase of hydrogen fraction and the position of maximum OH and O mole fractions move closing to the unburned mixture side. A monotonic correlation between initial combustion duration with the reciprocal maximum OH mole fraction in the flames is observed. The enhancement of the spark ignition of natural gas with hydrogen addition can be ascribed to the increase of OH and O mole fractions in the flames.     

Effect of gas-phase reaction on catalytic reaction for H2/O2 mixture in micro combustor

The catalytic reaction characteristics of the hydrogen and oxygen mixture in the catalytic micro-combustor were detected by thermocouple, infrared thermal imager and OH-PLIF. The equivalence ratio of transition stage from coupling reaction (coupled homogeneous-heterogeneous combustion) to pure catalytic reaction (heterogeneous combustion) was obtained. The three-dimensional pure catalytic reaction model with detailed catalytic reaction mechanism was established in a rectangular catalytic micro-combustor. The simulation model was validated with experimental data. The effects of the intermediate and final products from gas-phase reactions on the pure catalytic reaction were discussed as well as the effects of gas-phase reactions on the catalytic reaction in the process of coupling reactions were studied. The intermediate product OH radical can improve the hydrogen conversion of the surface reaction, and the effect of O radical is not obvious. The final product H₂O has an inhibitory effect on the surface reaction. Since the mass fraction of H₂O is much higher than other gas-phase reaction products, the dominant effect of the gas-phase reaction on the catalytic reaction is suppression. In the coupling reactions, the fuel consumed by the gas-phase reaction weakened the catalytic reaction.     

Numerical study on laminar burning velocity and NO formation of premixed methane–hydrogen–air flames

Numerical study on laminar burning velocity and NO formation of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity, adiabatic flame temperature, and radical mole fractions of H, OH and NO are obtained at various equivalence ratios and hydrogen fractions. The results show that the unstretched laminar burning velocity is increased with the increase of hydrogen fraction. Methane-dominated combustion is presented when hydrogen fraction is less than 40%, where laminar burning velocity is slightly increased with the increase of hydrogen addition. When hydrogen fraction is larger than 40%, laminar burning velocity is exponentially increased with the increase of hydrogen fraction. A strong correlation exists between burning velocity and maximum radical concentration of H + OH radicals in the reaction zone of premixed flames. High burning velocity corresponds to high radical concentration in the reaction zone. With the increase of hydrogen fraction, the overall activation energy of methane–hydrogen mixture is decreased, and the inner layer temperature and Zeldovich number are also decreased. All these factors contribute to the enhancement of combustion as hydrogen is added. The curve of NO versus equivalence ratio shows two peaks, where they occur at the stoichiometric mixture due to Zeldovich thermal-NO mechanism and at the rich mixture with equivalence ratio of 1.3 due to the Fenimore prompt-NO mechanism. In the stoichiometric flames, hydrogen addition has little influence on NO formation, while in rich flames, NO concentration is significantly decreased. Different NO formation responses to stretched and unstretched flames by hydrogen addition are discussed.     

Experimental and numerical study on laminar burning characteristics of premixed methane–hydrogen–air flames

An experimental and numerical study on laminar burning characteristics of the premixed methane–hydrogen–air flames was conducted at room temperature and atmospheric pressure. The unstretched laminar burning velocity and the Markstein length were obtained over a wide range of equivalence ratios and hydrogen fractions. Moreover, for further understanding of the effect of hydrogen addition on the laminar burning velocity, the sensitivity analysis and flame structure were performed. The results show that the unstretched laminar burning velocity is increased, and the peak value of the unstretched laminar burning velocity shifts to the richer mixture side with the increase of hydrogen fraction. Three regimes are identified depending on the hydrogen fraction in the fuel blend. They are: the methane-dominated combustion regime where hydrogen fraction is less than 60%; the transition regime where hydrogen fraction is between 60% and 80%; and the methane-inhibited hydrogen combustion regime where hydrogen fraction is larger than 80%. In both the methane-dominated combustion regime and the methane-inhibited hydrogen combustion regime, the laminar burning velocity increases linearly with the increase of hydrogen fraction. However, in the transition regime, the laminar burning velocity increases exponentially with the increase of hydrogen fraction in the fuel blends. The Markstein length is increased with the increase of equivalence ratio and is decreased with the increase of hydrogen fraction. Enhancement of chemical reaction with hydrogen addition is regarded as the increase of H, O and OH radical mole fractions in the flame. Strong correlation is found between the burning velocity and the maximum radical concentrations of H and OH in the reaction zone of the premixed flames.     

二甲基醚/天然气双燃料均质压燃过程详细化学反应动力学数值模拟研究(Ⅰ)--二甲基醚反应机理研究

应用零维详细化学反应动力学模型对二甲基醚均质压燃燃烧反应机理进行了数值模拟研究。结果表明二甲基醚放热反应为典型的双阶段放热反应，经历低温反应、负温度系数区域和高温反应三个过程．高温反应又分为蓝焰和热焰两个阶段。二甲基醚自燃着火由过氧化氢(H2O2)分解所控制，甲醛(CH2O)是过氧化氢的主要来源。基于化学敏感性分析．得到了均质压燃二甲基醚反应的主要途径：首先是二甲基醚脱氢，经过两次加氧后得到甲醛基；然后生成甲酸基(HCO)；最后生成一氧化碳(CO)。在二甲基醚的氧化反应过程中，氢氧根(OH)发挥着重要的作用，它是二甲基醚脱氢反应和CO氧化过程中的主要自由基。     

Effect of partially premixed and hydrogen addition on natural gas direct-injection lean combustion

Effect of partially premixed mixture and hydrogen addition on natural gas direct-injection lean combustion was studied experimentally using a constant volume vessel. Flame propagating photos and pressure derived combustion parameters were analysed at different premixed ratios (from 0% to 80%) and hydrogen fractions (from 0% to 40%) at overall equivalence ratio of 0.6, 0.8 and 1.0, respectively. The results show that the flame kernel is concentrated to the spark position with the increase of premixed ratio and/or hydrogen fraction. Flame propagating speed is decreased with the increase of premixed ratio while it increases as hydrogen is added to natural gas. Hydrogen addition has little effect on the partially direct-injection natural gas combustion at the stoichiometric fuel-air mixture condition and all premixed ratios. However, hydrogen addition significantly enhances the combustion rate of natural gas direct-injection combustion at lean mixture condition. Both the initial and main combustion durations are increased with the increase of premixed ratio, while they show the decreasing trend as hydrogen is added to natural gas at the lean mixture condition. Partially premixed direct-injection combustion combining with hydrogen addition can achieve the stable spark ignition and fast combustion at the lean mixture condition.     

氢气预混合微尺度催化燃烧的数值模拟

通过耦合计算流体力学软件FLUENT和化学反应动力学软件CHEMKIN并采用空间气相和表面催化详细化学反应机理,对氢气和空气的预混合气体在微型管道内的催化燃烧过程进行了数值模拟,讨论了不同反应模型的燃烧特性以及预混合气体入口速度、当量比Φ和管径对催化燃烧反应的影响。计算结果表明：表面催化反应对空间气相反应有抑制作用;随着入口速度的增大,燃烧过程同时存在着表面催化反应和空间气相反应两种控制因素;当量比Φ和管径对氢气的催化燃烧过程有重要的影响。     

A computational study to analyze the effect of equivalence ratio and hydrogen volume fraction on the ultra-lean burning of the syngas-fueled HCCI engine

This computational study investigates the equivalence ratio and hydrogen volume fraction effect on the ultra-lean burning of the syngas-fueled homogeneous charge compression ignition (HCCI) engine. In this research, low calorific syngas, composed of different compositions of H2, CO, and CO2, is used as a fuel in the HCCI engine that is operated under an overly lean air-fuel mixture. ANSYS Forte CFD package with Gri-Mech 3.0 chemical kinetics was used to analyze the in-cylinder combustion phenomena, and the simulation results were validated with experimental tests in the form of in-cylinder pressure and heat release rate at different equivalence ratios.The results indicate that changing the equivalence ratio produces a negligible change in combustion phasing, while it positively impacts the combustion and thermal efficiency of this syngas-fueled HCCI engine under lean conditions due to the high burning rate in the squish region. Moreover, an increased equivalence ratio increases MPRR due to the rich mixture combustion. The results also represent that the high-volume fraction of H2 in syngas fuel causes an advanced burning phase, improves the combustion performance of the HCCI engine at all equivalence ratio conditions, and causes slightly high NOx emissions.