【作者】 王浒；

【导师】 尧命发；

【作者基本信息】 天津大学 ， 动力机械及工程， 2012， 博士

【摘要】 柴油机颗粒物排放是柴油机排放控制的重点和难点，研究降低柴油机碳烟排放的控制策略、发展详细且通用的柴油机碳烟生成机理和模型具有很重要的理论意义和实际应用价值。本文采用试验研究和CFD与化学动力学耦合数值模拟手段相结合的方法，对柴油机碳烟生成机理及控制策略进行了系统研究。本文首先通过台架试验研究的方法，研究了降低柴油机碳烟排放的控制策略。综合采用进气增压、EGR、后喷射和正丁醇含氧燃料，可以明显改善柴油机的经济性和排放；多次喷射是改善柴油机NO_x和碳烟折衷关系的有效措施；在将NO_x排放控制在2.0g/kW-h情况下，采用多次喷射耦合EGR结合正丁醇含氧燃料，是降低柴油机排放的有效技术途径，可以显著改善柴油机碳烟排放。随后本文构建了正庚烷-正丁醇-PAH简化动力学模型，并利用滞燃期、组分浓度和HCCI燃烧等多种试验数据对其进行了广泛验证。该简化机理耦合CFD计算程序计算得到的油束贯穿距离、火焰浮起长度、碳烟体积浓度和分布及其生成区域都与试验结果吻合较好。多维数值模拟结果表明，进气压力主要通过降低缸内当量比和增强碳烟氧化对碳烟排放产生影响；高EGR率条件下碳烟生成量增加，而氧化速率明显降低，造成碳烟排放随EGR率上升而明显上升；通过后喷射提高燃烧后期温度、增强碳烟氧化和增强燃烧后期缸内充量运动，是其降低碳烟排放的主要原因；正丁醇降低碳烟排放主要是通过降低碳烟生成前驱物PAH浓度和增强油气混合过程，降低当量比实现的。计算结果表明，采用后喷射结合正丁醇含氧燃料，可以在NO_x保持不变的前提下大幅度降低碳烟排放；将所发展的正庚烷-PAH机理拓展至正庚烷-甲苯-PAH简化动力学机理，并对其进行了验证。采用CFD计算程序耦合简化机理进行了柴油和正庚烷/甲苯混合燃料缸内直喷燃烧和碳烟排放的多维数值模拟计算。计算结果表明简化机理可以预测由EGR率和燃料组分变化引起的缸内压力和燃烧放热率的变化以及不同燃料碳烟排放随EGR率的变化趋势；简化机理也可以在与典型柴油机运行工况类似的环境下准确预测正庚烷和甲苯混合燃料的燃烧和碳烟生成过程。对燃烧过程中的重要参数，如当量比、火焰温度、OH和PAH浓度等的分析结果表明，甲苯分子结构和化学反应特性对燃烧过程和碳烟生成的影响要远大于其物理特性的影响；本文所发展的简化机理可以很容易扩展至正庚烷-异辛烷-正丁醇-甲苯-PAH简化动力学模型；所发展的简化机理中与芳烃和PAH生成相关的反应路径是合理的，这对于发展更为完善的柴油多组分替代物反应机理，并实现对PAH和碳烟排放的准确预测有非常重要的理论意义。更多还原

【Abstract】 The particle matter is one of the major pollutant emissions of diesel engine.Exploring the control strategies to reduce the soot emissions, developing moredetailed and generalized soot formation mechanisms, and models have very importanttheoretical and practical significance. In the current investigation, experimental andnumerical studies have been conducted to explore the soot emission reduction controlstrategies and to develop more sophisticated soot formation mechanisms and models.Experiments were conducted to investigate the soot emission reduction controlstrategies. It is found that the fuel economy and emissions of diesel engine can begreatly improved by applying boosting, EGR, post injection and n-butanol additive.By applying multi-injection coupling EGR, combined with n-butanol oxygenatedadditive, it is possible to reduce the NO_x and soot emissions simultaneously whilestill maintaining good fuel economy, thus it is one of the most competitive controlstrategies to realize high efficiency and clean diesel combustion.A reduced n-heptane/n-butanol/PAH chemical kinetics mechanism has beendeveloped, and experimental data from shock tube ignition delay, premixed flamespecies concentrations and HCCI combustion were taken to validate the proposedmechanism. The simulation results with CFD coupled the reduced mechanism showthat the predicted spray liquid and vapor penetrations, the lift-off length, theconcentration and distribution of soot volume fraction, as well as soot formationregions, agree quite well with the experimental results in constant volume n-heptanespray combution cases. Simulations were also conducted to study the effects ofboosting, EGR, post injection and n-butanol additive on combustion and sootemissions. Results show that increasing the intake pressure can reduce the overallin-cylinder equivalence ratio and enhance the soot oxidation, thus effectively reducethe soot emission; the soot formation rate increases while the soot oxidation ratedecreases as the EGR rate increases, which results in much more soot emissions,especially under high EGR conditions; post injection is an effective measure to reducethe soot emission, and this can be attributed to the higher temperature caused by thepost injected fuel to accelerate the soot oxidation process, and also the enhancedin-cylinder air motion which can improve the combustion during the late combustion phase, the in-cylinder air utilization can also be improved by splitting single injectioninto double injections; soot emission can be greatly reduced by addition of n-butanol.By blending n-butanol into a non-oxygenated hydrocarbon fuel, air entrainment isenhanced by prolonging the lift-off length and the overall equivalence ratio is reducedby introducing extra available oxygen through the n-butanol molecule. The oxygenatom in the n-butanol molecule can also effectively reduce the carbon remaining inthe form of soot precursors by forming the stable C-O bond. The simulation resultsconfirm that the combination of post injection and n-butanol additive has the potentialto greatly reduce the soot emissions.The proposed n-heptane/PAH mechanism has been further extended to formulatean n-heptan/toluene/PAH mechanism, and the mechanism has also been extensivelyvalidated with experimental data from shock tube ignition delay, premixed flamespecies concentrations and HCCI combustion. Simulations were conducted bycouping the reduced mechanism with the KIVA CFD code to investigate thecombustion process and soot emissions of diesel and n-heptane/toluene blended fuelsin direct injection diesel engine. Results show that the proposed mechanism has theability to predict the in-cylinder pressure and heat release of various fuels underdifferent EGR rate conditions, the soot emissions of various fuels were also wellcaptured by the reduced mechanism. The effects of toluene on combustion and sootformation processes in the condition similar to that of a typical diesel engineoperating condtions have also be studied. The effects of toluene in the blended fuelson the distributions of soot volume fraction, equivalence ratio, flame temperature, OHand A4species concentrations show that the toluene molecule structure and chemicalkinetics characteristic play dominat role in the soot formation process compared to thephysical properties.It should be pointed out that the proposed mechanisms can be easily extended to ann-heptane/iso-octane/n-butanol/toluene/PAH mechanism; the reaction pathways ofbenzene and PAH formation have been well validated. These indicate that it will bevery helpful for the development of multicomponent diesel surrogate mechanism andfor the accurate predictions of PAH formation and soot emissions with such asophisticated surrogate mechanism.更多还原