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。     

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。     

生物柴油燃烧化学动力学数值模拟

应用零维单区模型对生物柴油在发动机燃烧中的化学动力学过程进行了数值模拟,通过分析生物柴油在内燃机边界条件下燃烧氧化过程的关键基元反应、关键中间产物的生成速率(ROP)以及重要自由基,总结出了生物柴油燃烧氧化的主要反应路径.生物柴油的燃烧过程经历低温放热和高温放热两个阶段;在低温放热阶段生物柴油燃料主要通过脱氢反应进行消耗,在高温阶段则通过直接裂解和脱氢反应进行消耗,OH自由基无论在低温阶段还是在高温阶段对燃料的脱氢都起到了主导作用.     

正庚烷预混火焰中PAHs的生成机理

应用CHEMKIN软件对正庚烷预混火焰中碳黑的先驱物PAHs的生成机理进行研究,得到了包含49种组分、94个基元反应的简化模型.该饥理包含正庚烷的燃烧和PAHs的生成两部分,正庚烷的燃烧模型构建在Patel等人模型的基础上,增加了3个低温区关键反应;PAHs生成机理主要根据脱氢加乙炔(HACA)反应机理添加.新模型能够模拟正庚烷预混燃烧的冷焰和热焰反应以及预测PAHs的生成过程,与详细模型计算结果吻合较好.为CFD多维模型与化学反应动力学模型相耦合的燃烧计算提供了可行的途径.     

一个新的用于HCCI发动机燃烧研究的正庚烷化学反应动力学简化模型

提出了一个新的适用于HCCI发动机燃烧研究的正庚烷化学反应动力学简化模型，包含44种组分和72个反应。由四个子模型组成：低温反应子模型是在Li等人模型的基础上，定义具体的醛类(RcH0)产物和小分子碳氢产物(Rs)而构建；增加了用于链接低温反应向高温反应过渡的大分子直接裂解成小分子反应子模型；高温反应子模型是在Griffiths等人模型的基础上，去除了无关的基元反应，增加两个关于CO和CH3O的氧化反应而构建；此外，还采用了Golovitchev简化模型中NOx生成子模型。新模型能够模拟正庚烷HCCI燃烧的冷焰和热焰反应以及NOx生成的整个过程，与详细模型计算结果吻合较好。CPU计算时间是详细模型的1／1000，为CFD多维模型与化学反应动力学模型相耦合的燃烧计算提供了可行的途径。     

二甲基醚/天然气双燃料均质压燃化学动力学数值模拟

使用零维详细化学反应动力学模型，研究了二甲基醚和天然气双燃料均质压燃燃烧的化学反应动力学过程，缸内压力计算值和实测结果相当一致，计算结果表明，双燃料燃烧过程分为低温反应和高温反应两个阶段，低温反应主要是二甲基醚燃烧氧化，而高温反应主要是天然气的氧化，低温反应二甲基醚生成了大量自由基加速了天然气的燃烧反应．混合气初始温度升高，放热率增大，燃烧持续期缩短；二甲基醚浓度主要影响低温燃烧过程，天然气浓度则主要影响高温燃烧过程；惰性气体(CO2)使燃烧反应推迟，燃烧反应速率降低．通过控制二甲基醚、天然气和惰性气体浓度可以有效控制均质压燃燃烧过程，拓宽运行范围。     

乙醇/正庚烷燃烧过程PAH形成的数值模拟

应用CHEMKIN化学动力学软件,构建乙醇/正庚烷的芳香烃(PAH)形成机理(包含241种组分,1,703个基元反应).在激波管和预混火焰条件下,研究乙醇/正庚烷燃烧过程中前驱体单环芳香烃(苯)和多环芳香烃(萘、菲、芘)的形成,探讨乙醇掺混比对PAH形成的影响.对生成速率的分析结果表明,丙炔基的聚合与环化、苯基的加氢反应是形成苯的主要反应;多个苯环的形成主要通过脱氢加乙炔反应;具有氧化性的H和OH自由基减少了PAH的生成.敏感性分析结果表明,乙醇/正庚烷燃烧过程中,促进苯生成的主要反应是戊基、戊烯、烯丙基和丙基的分解.随着乙醇掺混比的增加,燃烧过程中分解形成的活性羟基(—OH)摩尔分数增加,PAH的生成速率下降,表明乙醇具有抑制PAH形成的作用.     

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

应用零维详细化学反应动力学模型研究了二甲基醚／天然气双燃料均质压燃燃烧反应机理。结果表明由于两种燃料相互作用，DME低温反应进行程度很小，没有第二次加氧过程，β-scission起主导作用，大部分甲醛由CH3O生成，而不是DME的低温反应；H2O2主要由DME控制，H2O2浓度升高促进了天然气的低温反应进行；另一方面，天然气低温反应放热也促进了DME的氧化反应，OH浓度升高，使CO能够全部氧化。计算结果表明，在压缩比较高的条件下，天然气浓度变化对DME稀燃极限几乎没有影响，但压缩比较低时，随着天然气浓度升高，DME稀燃极限浓度升高。     

甲烷氛围下正庚烷低温氧化特性

利用射流搅拌反应器研究了不同摩尔分数甲烷氛围中正庚烷在0.1 MPa、500～750 K条件下的低温氧化特性,当量比和滞留时间分别为0.5和1.4 s;分析了重要组分摩尔分数随反应温度的变化规律,研究了甲烷对正庚烷低温氧化的物理和化学作用及正庚烷低温氧化过程的化学动力学特性.结果表明:甲烷摩尔分数较低时甲烷表现出惰性,其对正庚烷低温氧化的物理作用与氮气相当;当甲烷摩尔分数达到70%时,甲烷从化学反应角度抑制了正庚烷的低温氧化.正庚烷低温氧化主要与含氧组分相关的反应路径有关,后续氧化以CO、烯烃、醛与醚等含氧物质的反应为主.甲烷主要通过与正庚烷竞争OH而抑制其氧化,甲烷摩尔分数的提高促进了CH_3生成,从而使甲醇摩尔分数上升.     

正庚烷HCCI燃烧过程的数值模拟及试验研究

结合详细化学动力学机理,运用最新的CHEMKIN—Ⅳ化学动力学软件包中的IC enginc模块模拟了正庚烷的HCCI燃烧过程。并在一台高速四缸柴油机上进行了单缸正庚烷HCCI燃烧试验。分别从理论和试验上研究了正庚烷的HCCI燃烧过程以及各种参数变化对燃烧过程的影响,研究表明：正庚炕燃烧过程由低温反应和高温反应两阶段组成,NC7KET裂解生成OH是低温阶段最重要的反应,高温阶段着火是由H2O2裂解所触发的,低温阶段着火对高温阶段着火起着关键作用;各种参数的变化会导致燃烧过程的显著变化。     

缸内活化热氛围对柴油机燃烧与排放特性的影响

采用气口喷射正庚烷形成稀薄均匀混合气,利用其低温和高温化学反应释放的热量和生成的活性基团在柴油喷入气缸前形成活化热氛围,研究了活化热氛围强度对直喷柴油燃烧和排放的影响.研究发现:随着正庚烷比例的增加,活化热氛围强度增大,柴油扩散燃烧着火时刻略有提前,最大放热率明显降低,高负荷下的NOx和烟度排放显著降低;活化热氛围强度超过某一临界点后,NOx抑制效果开始减弱并单调升高,而烟度排放开始升高到极大值后再次降低.综合各个负荷下的排放,热氛围强度不适宜过强,在试验条件下20 %～30 %最佳.     

柴油机燃烧多环芳香烃前驱体等物质的化学动力学研究

为了揭示混合气浓度对柴油机排放的影响规律,采用正庚烷氧化详细反应机理及化学动力学分析软件对不同燃空当量比下柴油机燃烧初级碳烟粒子前驱体等重要反应中间产物或自由基的形成及发展历程进行了数值模拟．模拟结果表明,降低混合气浓度可以实现低温燃烧,使燃烧温度远离“碳烟形成温度窗”,大幅度降低柴油机碳烟排放．混合气浓度对反应中间产物或自由基有重要影响,通过改变混合气浓度可以控制燃烧过程中多环芳香烃(PAH)前体物乙炔(C2H2)、炔丙基(C3H3)及其他重要物质羟基(OH)、过氧羟自由基(HO2)、过氧化氢(H2O2)、甲醛(CH2O)和一氧化碳(CO)等的生成量,从而实现控制柴油机排放．     

Oxidation of 1-butanol and a mixture of n-heptane/1-butanol in a motored engine

The oxidation of neat 1-butanol and a mixture of n-heptane and 1-butanol was studied in a modified CFR engine at an equivalence ratio of 0.25 and an intake temperature of 120 °C. The engine compression ratio was gradually increased from the lowest point to the point where significant high temperature heat release was observed. Heat release analyses showed that no noticeable low temperature heat release behavior was observed from the oxidation of neat 1-butanol while the n-heptane/1-butanol mixture exhibited pronounced cool flame behavior. Species concentration profiles were obtained via GC-MS and GC-FID/TCD. Quantitative analyses of the reaction products from the oxidation of neat 1-butanol indicate that 1-butanol is consumed mainly through H-atom abstraction. Among the H-atom abstraction reactions, it is observed that the H-atom abstraction from the α-carbon of 1-butanol is particularly favored. The investigation on the oxidation of the mixture of n-heptane/1-butanol showed that the oxidation of 1-butanol is facilitated at low temperatures through the radical pool generated from the oxidation of n-heptane.     

Inhibition effect of doping methyl tert-butyl ether compounds to n-heptane on homogenous charge compression ignition combustion

This article reports an experimental study on the combustion characteristics and emissions of homogenous charge compression ignition (HCCI) combustion using n-heptane doped with methyl tert-butyl ether (MTBE). The experiments were conducted on a single cylinder HCCI engine using neat n-heptane and 10%, 20%, 30%, 40% and 50% (by volume) MTBE/n-heptane blends at constant engine speed. The experimental results reveal that the ignition timing of the low temperature reaction (LTR) gets retarded, the peak values of heat release during the LTR decrease and the negative temperature coefficient (NTC) duration gets prolonged with the increase of MTBE in the blends. Consequently, the ignition timing of the high temperature reaction (HTR) gets delayed and both the attainable maximum indicated mean effective pressure (IMEP) and the lowest stable IMEP increase. Parametric studies on CO and HC emissions reveal that the maximum combustion temperature, pressure rise rate, IMEP, ignition timing of the HTR, combustion duration and fuel components have important impacts on HC emission, while the main parameters that show an important influence on CO emissions are the maximum combustion temperature, pressure rise rate, IMEP and combustion duration. Moreover, in order to suppress the CO and HC emissions to a low level, the maximum combustion temperature should be higher than 1500 K, the maximum pressure rise rate larger than 0.5 MPa/°CA, the IMEP above 0.3 MPa and the combustion duration shorter than 9 °CA.     

Numerical study of ignition mechanism of n-heptane direct injection compression-ignition engine

A detailed chemical dynamical mechanism of oxidation of n-heptane was implemented into kiva-3 code to study the ignition mechanism of a high-temperature, high-pressure, three-dimensional-space, transient turbulent, non-homogeneous, mono-component fuel in the engine. By testing the quantity of the heat released by the chemical reaction within the cylinder cell, the elementary reaction showing an obvious increase in the cell temperature was defined as ignition reaction and the corresponding cell as ignition position. The main pathway of the ignition reaction was studied by using the reverse deducing method. The result shows that the ignition in the engine can be divided into low-temperature ignition and high-temperature ignition, both of which follow the same rule in releasing heat, called the impulse heat releasing feature. Low-temperature ignition reaction, whose ignition reaction is c5h9o1-4=ch3cho+c3h5-a, follows the oxidation mechanism, while high-temperature ignition reaction, whose ignition reaction is c2h3o1-2=ch3co, follows the decomposition mechanism. No matter which ignition it is in, the chemical reaction that restrains the ignition reaction from lasting is the deoxidization reaction of alkylperoxy radicals.     

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.     

Effects of CO content on laminar burning velocity of typical syngas by heat flux method and kinetic modeling

Variations in syngas composition could bring a challenge for its combustion with both high efficiency and low emission. In this study, the effect of CO content on the laminar burning velocity of typical syngas was determined by the heat flux method and by kinetic simulations. For the 0% H₂ in syngas, the laminar burning velocity increased monotonically with CO content until its maximum value and then dropped rapidly with further increase of CO content, while for the 25% H₂ case, the laminar burning velocity increased almost linearly with CO content. Based on the kinetic simulations, consumption rate changes of CO and OH and the discrepancy of the heat release rate in the preheat zone contribute to these trends. At sufficient OH, the increase in the reaction rate between OH and CO corresponds to a faster heat release in the preheat zone, whereas at insufficient OH, oxidation of CO by OH is inhibited and the heat release process is delayed, decelerating the release rate and decreasing the laminar burning velocity.