基于改进的详细碳烟模型的柴油燃烧碳烟颗粒物的生成特性

采用改进的详细碳烟模型,耦合了由正庚烷和甲苯组成的混合燃料的简化反应机理.在碳烟模型中,考虑了反应中生成的乙炔类异构体和多环芳香烃前驱物对碳烟颗粒成长过程的影响作用,分别以可视化发动机和单缸柴油机台架试验的结果,对详细碳烟模型预测柴油机碳炯生成和氧化过程的准确性和有效性进行了验证.结果表明,模拟得到的缸内压力、放热率曲线以及着火时刻和试验结果吻合较好,得到的碳烟浓度瞬态二维分布与采用双色法得到的试验结果较为接近,碳烟排放量的模拟值与试验值的变化趋势基本一致,因而本文的详细碳烟模型可较好地预测不同工况条件下柴油机碳烟颗粒排放的变化趋势.同时,采用提出的详细碳烟模型对柴油机内碳烟颗粒数密度和平均尺寸进行数值模拟,研究其在柴油机缸内的变化情况,结果表明,碳烟颗粒的直径在预混燃烧期和急燃期急剧增加,在缓燃期以及燃烧后期收敛于某一稳定值.     

柴油机燃烧生成芳香烃、碳烟及NO_x的化学动力学研究

在LLNL详细正庚烷氧化反应模型基础上,将苯(A1)和多环芳香烃(A2、A3、A4)的详细反应机理和NOx的生成机理加入其中形成了一个包含芳香烃和NOx的正庚烷氧化反应组合模型,并通过激波管试验验证了该组合模型。在CHEMKIN软件上运用该组合模型对柴油机燃烧反应产物中的芳香烃、碳烟和NOx进行了研究,结果表明:芳香烃的生成在Ф-T图中都呈钟形分布,随着环数的增加它们所对应的生成温度大致呈逐渐升高的趋势。利用芳香烃-Ф-T图可以较准确地预测碳烟-Ф-T图,再结合NOx-Ф-T图可以绘制出芳香烃-碳烟-NOx-Ф-T图,结果表明:芳香烃和碳烟在1 150～2 200K、当量比大于2的范围内大量生成,NOx在温度高于2 350K、当量比小于1.2的范围内大量生成。     

乙烯-正庚烷混合燃料在碳烟生成中的协同效应

在对冲扩散火焰中对乙烯-正庚烷混合燃料的燃烧进行了模拟研究.耦合的反应机理包含正庚烷的裂解、C0-C4核心小分子反应、初始苯环生成、拓展到六苯并苯(A7)的PAH化学和包含36步成核反应的碳烟表面机理.在预混火焰中对正庚烷的燃烧进行了模拟,得到的关键组分的浓度变化与文献中的试验数据吻合较好.在对冲扩散火焰中对乙烯-正庚烷掺混燃料的燃烧进行模拟,结果显示:随着正庚烷掺混比的增加,碳烟体积分数(SVF)、苯及PAH的峰值均呈现先增加后降低的趋势,当掺混比为5%,时,SVF的峰值达到最大.对苯的生成进行详细分析,发现丙基和甲基在这种协同效应中起着关键作用.此外,随着掺混比的增加,苯与PAH的反应生成区逐渐往氧化剂侧移动,越过滞止平面后苯的生成率和消耗率均大大降低.     

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.     

基于CFD耦合现象学碳烟模型研究温度对碳烟生成的影响

基于9步法现象学碳烟模型,根据4种情况考虑了碳烟表面氧化对碳烟数密度的影响,并将改进后的模型耦合到KIVA-3V Release 2程序中.应用该模型,对柴油在定容弹中不同初始温度(800、900和1,000,K)下的燃烧和碳烟生成进行了多维数值模拟,并通过对应的试验结果进行校准.结果表明:各工况下预测的碳烟生成过程与试验值能定性地吻合.随着初始温度降低,点火时刻推迟,燃烧持续期缩短,燃烧模式由扩散燃烧向预混燃烧转变,碳烟生成降低.低初始温度下,碳烟的生成和氧化机理均受到抑制,然而局部燃烧温度峰值的降低和高温区域的缩减并不明显,碳烟的降低主要由于高燃油当量比区域的缩减.     

柴油机燃用正庚烷及柴油时多环芳香烃与碳烟的演化规律

基于电控燃油喷射柴油机的全气缸取样系统,使用气相色谱-质谱联用仪及程序升温大体积进样方法,对柴油机燃用正庚烷和柴油过程中缸内多环芳香烃(Polycyclic aromatic hydrocarbons,PAHs)和碳烟的演化规律进行了试验研究.结果表明:在燃烧过程中,芘、苯并[a]芘质量随曲轴转角呈单峰变化,萘、芴质量随曲轴转角呈"S"型变化趋势(先降低后升高再降低);碳烟的生成趋势与缸内温度呈较好的一致性,且碳烟与芘随曲轴转角具有相似的单峰状质量变化趋势,只是碳烟峰值出现的时刻稍有滞后.此外,正庚烷与柴油在柴油机中燃烧时生成的PAHs与碳烟具有相似的演化规律.     

混合气的氧燃比和温度对富氧燃烧柴油机PAH、碳烟和NO_x排放和燃烧路径的影响

构建了包含PAH和NOx反应的正庚烷氧化反应动力学详细组合模型,并对其进行了验证.以该组合模型为基础,开展了混合气的氧燃比(R)和温度(T)对富氧燃烧柴油机排放和燃烧路径影响的理论计算工作.结果表明,所构建的组合模型是可信的,可以用来对柴油机的燃烧和排放特性进行预测.富氧燃烧柴油机有害排放物PAH、碳烟和NOx受混合气的氧燃比和温度的影响显著,其生成规律在R-T图中呈"半岛"型分布.在R-T图中,富氧柴油机的燃烧路径有别于传统柴油机,导致其PAH和碳烟排放降低,而NOx排放增加.最后,为了降低富氧燃烧柴油机的NOx排放,提出了两种富氧燃烧路径的控制策略.     

进气压力对柴油机缸内碳烟颗粒微观形貌的影响

采用全气缸取样系统获取不同进气条件下柴油机燃烧过程中的缸内碳烟颗粒,运用透射电子显微镜(TEM)测量碳烟样品在微观尺度下的二维形貌,并计算碳烟颗粒的分形维数和碳烟基本粒子直径,进而研究不同进气压力对柴油机缸内碳烟颗粒物微观形貌的影响.结果表明:在碳烟生成主导阶段,进气压力的升高会增大生成的碳烟基本粒子的平均直径;而在碳烟氧化主导阶段,进气压力的升高会导致碳烟氧化过程加快,从而使碳烟基本粒子的平均直径快速减小.另外,进气压力的增加对尾气碳烟中基本粒子的平均直径影响不显著.在碳烟氧化主导阶段,碳烟破碎现象会导致碳烟分形维数下降,增加进气压力会使下降起始点提前.     

柴油机缸内碳烟颗粒形成过程与尺寸分布特性

采用了唯象的半经验碳烟排放模型,考虑了碳粒成核、表面成长、凝结和氧化的基本过程,模拟计算了柴油机缸内燃烧条件下缸内碳烟形成过程中的活性基核、碳粒核的生成规律及碳粒尺寸分布规律,并对碳烟排放进行了分析。计算结果表明:燃烧温度和混合物当量比对初始碳核的生成有很重要的影响,碳核粒子首先在活塞凹坑底部的浓混合区域生成,随燃烧的扩散过程,燃烧室中心位置和凹坑唇边挤流区相继出现较高浓度的基核,但存在浓度相位差,缸内不同位置生成碳粒数量和尺寸分布是不同的,不同尺寸范围的碳粒数量和质量之间存在对应关系。     

掺混比例和喷油压力对柴油/正丁醇低温燃烧的影响

将正丁醇、正庚烷分别与柴油以20∶80和40∶60体积比掺混,在100、130,MPa两个喷油压力下进行柴油机直喷燃烧试验,研究喷油压力和掺混比例对柴油/正丁醇低温燃烧特性和碳烟排放的影响,结果表明:提高喷油压力能够改善柴油/正丁醇掺混燃料高比例EGR燃烧相位滞后问题,在更大EGR率范围实现高效燃烧.随喷油压力提高,十六烷值和含氧对降低碳烟的作用增强,沸点等其他特性作用变小.提高掺混比例,正丁醇对燃烧的影响更明显,燃烧放热始点明显推迟、燃烧反应速度加快,在一定的EGR率范围内热效率提高.掺混比例从20%提高到40%,各燃料特性对碳烟排放的降低作用都显著增大,沸点等其他特性作用成为降低碳烟排放的重要因素,十六烷值的作用更加显著.     

Modeling study of soot formation and oxidation in DI diesel engine using an improved soot model

Particulate emission is one of the most deleterious pollutants generated by Diesel fuel combustion. The ability to predict soot formation is one of the key elements needed to optimize the engine performance and minimize soot emissions. This paper reports work on developing, a phenomenological soot model to better model the physical and chemical processes of soot formation in Diesel fuel combustion. This hybrid model features that the effect of turbulence on the chemical reaction rate was considered in soot oxidation. Soot formation and oxidation processes were modeled with the application of a hybrid method involving particle turbulent transport controlled rate and soot oxidation rate. Compared with the original soot model, the in-cylinder pressures, heat release rate and soot emissions predicted by this hybrid model agreed better with the experimental results. The verified hybrid model was used to investigate the effect of injection timing on engine performance. The results show that the new soot model predicted reasonable soot spatial profiles within the combustion chamber. The high temperature gas zone in cylinder for hybrid model case is distributed broadly soot and NOx emission dependence on the start-of-injection (SOI) timing. Retarded SOI timing increased the portion of diffusion combustion and the soot concentration increased significantly with retarding of the fuel injection timing. The predicted distributions of soot concentration and particle mass provide some new insights on the soot formation and oxidation processes in direct injection (DI) engines. The hybrid phenomenological soot model shows greater potential for enhancing understanding of combustion and soot formation processes in DI diesel engines.     

直喷式柴油机缸内热辐射多区(多维)模型的研究

以准维现象学多区喷雾燃烧模型和碳粒生成预测了模型为基础,建立了缸内空间辐射多区（多维）模型,并以Ｇ４１３５直喷式柴油机为研究对象,用蒙特卡洛（Ｍｏｎｔｅ－Ｃａｒｌｏ）法计算和分析了燃烧室壁面辐射热量的分布,结果表明,热流量分布规律和数值与柴油机缸内燃烧过程,有关试验结果相符。     

A phenomenological model for the prediction of soot formation in diesel spray combustion

A phenomenological soot model coupled with complex chemistry mechanism for the prediction of soot formation in diesel spray combustion is presented. The prototype of the model is one proposed by Leung and Lindstedt in which soot formation is treated by four global stages: particle nucleation, surface growth, surface oxidation, and particle coagulation, each of which is represented by only a few reaction steps. In the present study, the model is modified according to recent literature data. The formation of soot particles is linked with gas-phase chemistry via diacetylene and naphthalene, which are presumed to be indicative species of particle inception/nucleation. The soot surface growth is described using Frenklach et al.'s active site model, and the oxidation mechanism includes both Nagle and Strickland-Constable's O₂ oxidation and Neoh et al.'s OH oxidation models. The soot model integrated with the gas-phase kinetics is then applied in multidimensional spray simulations. The KIVA3 code that is widely used in diesel combustion studies is modified and employed for the simulations. The turbulent flow is predicted using the compressible k-ε model, and the turbulence-chemistry interaction handled by a partially stirred reactor model. The IDEA experimental data for n-heptane sprays in diesel-like conditions (800 K and 50 bar) are used for evaluation of the model. Some reaction rate constants are adjusted to achieve better agreement with the measurements. Further, sensitivity studies have been carried out and the effects of some parameters that affect the predictions are discussed. The results indicate that the model, if applied together with other models that properly describe sprays and turbulent flow, can be used for qualitative and even quantitative prediction of soot formation in diesel combustion.     

A reaction mechanism for gasoline surrogate fuels for large polycyclic aromatic hydrocarbons

This work aims to develop a reaction mechanism for gasoline surrogate fuels (n-heptane, iso-octane and toluene) with an emphasis on the formation of large polycyclic aromatic hydrocarbons (PAHs). Starting from an existing base mechanism for gasoline surrogate fuels with the largest chemical species being pyrene (C₁₆H₁₀), this new mechanism is generated by adding PAH sub-mechanisms to account for the formation and growth of PAHs up to coronene (C₂₄H₁₂). The density functional theory (DFT) and the transition state theory (TST) have been adopted to evaluate the rate constants for several PAH reactions. The mechanism is validated in the premixed laminar flames of n-heptane, iso-octane, benzene and ethylene. The characteristics of PAH formation in the counterflow diffusion flames of iso-octane/toluene and n-heptane/toluene mixtures have also been tested for both the soot formation and soot formation/oxidation flame conditions. The predictions of the concentrations of large PAHs in the premixed flames having available experimental data are significantly improved with the new mechanism as compared to the base mechanism. The major pathways for the formation of large PAHs are identified. The test of the counterflow diffusion flames successfully predicts the PAH behavior exhibiting a synergistic effect observed experimentally for the mixture fuels, irrespective of the type of flame (soot formation flame or soot formation/oxidation flame). The reactions that lead to this synergistic effect in PAH formation are identified through the rate-of-production analysis.     

Measurements of sooting tendency in laminar diffusion flames of n-heptane at elevated pressure

This research focuses on the effects of an increasing pressure on the soot formation during combustion of vaporized liquid fuel. Therefore soot formation is measured in a laminar diffusion flame, with n-heptane as fuel, over a range of pressures from 1.0 to 3.0 bar. The soot volume fraction in the diffusion flames has been measured using Laser-Induced Incandescence (LII) calibrated by means of the Line Of Sight Attenuation (LOSA) technique. The values of the calibration factors between LII intensities and soot volume fraction from LOSA are slightly varied for different pressure. The integral soot volume fractions show power law dependence on pressures, being proportional to pⁿ, with n being 3.4 ± 0.3 in the pressure range of 1.0–3.0 bar.     

喷油策略对汽油压燃燃烧及碳烟生成过程影响的数值模拟研究

基于三维计算流体动力学(CFD)软件CONVERGE,耦合甲苯掺比燃料(toluene reference fuel,TRF)简化动力学机理及多步现象学碳烟模型,建立汽油压燃(GCI)的数值模拟模型。通过改变气道喷射比例、主喷时刻和预主喷间隔研究了高负荷条件下气道喷射结合缸内直喷的喷油策略对GCI燃烧及碳烟生成过程的影响。研究结果表明,增加气道喷射比例、提前主喷时刻和增大预主喷间隔都能够缩短燃烧持续期,使放热更为集中,从而降低碳烟排放;改变气道喷射比例对碳烟成核及表面生长有较大的影响,主喷时刻提前能够提高氧化速率。当气道喷射比例为40%,主喷时刻为-8°,预主喷间隔为15°时,碳烟排放为0.015 1g/(kW·h),相比试验基准工况降低了33.8%,而最大压升率也控制在可接受的范围内。     

正庚烷甲苯混合物燃烧简化机理分析

提出了一个新的包括多环芳香烃(PAH)生成的正庚烷/甲苯混合物燃烧化学动力学简化机理.该机理包括64种物质,120个反应,与激波管内滞燃期实验结果吻合较好.在不同进气氧体积分数下,使用该机理对柴油机缸内燃烧过程进行了计算,其结果与缸内的实验结果吻合良好.通过机理的敏感性分析发现,PAH的重要前驱物乙炔主要是由甲苯反应路径中的C6H5及C6H4O2生成,说明在正庚烷中加入甲苯会对模拟柴油的燃烧特别是碳烟的生成有很大的影响;过氧化氢自由基HO2和羟自由基OH在甲苯、正庚烷的分解反应及小分子烃的裂解和氧化反应中都起着非常重要的作用.