碳氢燃料在高温空气燃烧过程中的裂解和烟炱生成

碳氢燃料在空气中燃烧时会生成烟炱,并影响火焰的结构。燃烧稳定性,燃烧效率和火焰的辐射特性,过量的烟炱排放到大气中还会对环境造成不良的影响,研究表明,燃烧过程中空气中可吸入悬浮颗粒物的主要来源之一。悬浮颗粒物的存在与肺癌等疾病有密切的关系。特别是直径在2.5μm以下的粒子对人体的危害性更大,文章总结了关于烟炱研究的最新进展,论述了碳氢燃料在空气中燃烧时烟炱生成的化学机理和生成模型。烟炱对燃烧的辐射特性的影响,总结了实验和计算结果,并介绍了燃料在燃烧过程中裂解中间成分的测定。通过对烟炱生成机理的研究为高温空气燃烧技术中燃料在高温下的裂解和烟炱的生成与排放以及它的辐射特性提供研究的基础。     

小缸径直喷式柴油机喷雾,燃烧和碳粒生成过程试验研究

本介绍了作采用同步高速摄影技术和图象分析技术研究柴油机缸内碳粒生成和氧化过程的研究结果。研究结果表明：在柴油机燃烧过程中碳粒最早出现在燃烧室内有空气渗混的富油区；速燃期内高温富碳燃烧区的形状与着火前一时刻喷雾场的形状相吻合；富油区大直径油滴的燃烧裂解和活塞顶隙处外溢燃气的低温燃烧是柴油机产生碳烟微粒排放的主要原因。     

高温低氧燃烧条件下氮氧化物的生成特性

高温低氧燃烧原理是高温空气燃烧技术赖以发展的基础，使得高温燃烧条件下的氮氧化物的生成与排放受到大大抑制。为了掌握这种非常规燃烧现象及污染物生成的基本规律，采用扩散燃烧模型、热力NO生成模拟与湍流N－S方程，数值研究了燃烧空间中空气氧浓度对燃烧特性和氮氧化物排放浓度的影响，再现了高温与低氧两种条件相结合，形成的稳定的低氮氧化物排放的燃烧特性。计算结果与实验数据吻合，为发展高温空气燃烧技术提供了理论基础。     

燃烧参数影响高温空气燃烧特性的数值分析

高温空气燃烧技术具有高效节能和低NOx排放等多重优越性，是一种新型燃烧技术。为了深入研究高温空气燃烧机理和低氮氧化物排放特性，将湍流N—S方程与扩散燃烧模型和热力型NO生成模型相结合，研究了低氧浓度条件下，燃烧参数，如燃气供应量，过量空气系数，进口空气预热温度以及进口空气氧含量对燃烧的影响，为发展高温空气燃烧技术提供了理论依据。     

中小型燃气锅炉NO_x源头控制及低氮燃烧技术研究进展

介绍了燃气燃烧过程中热力型NO_x和快速型NO_x的生成机理,从对空气的预处理、对燃气和空气的混合分配两个方面综述了燃烧源头控制降低NO_x排放的技术,主要包括高温空气燃烧、柔和燃烧、浓淡燃烧、分级分段燃烧、脉动燃烧、烟气再循环等单一技术和旋流与富氧燃烧、烟气再循环与高温空气燃烧等组合技术;结合低NO_x原理,综述了低NO_x技术的实现形式,指出低NO_x燃烧技术的发展前景。     

双层分流燃烧室结构及喷射参数智能优化

通过采用NSGA-Ⅱ遗传算法耦合KIVA-3V程序,以双层分流燃烧室为研究对象,对其燃烧系统结构参数与燃油喷射参数进行了多目标智能优化研究,过程中优化了柴油喷射正时、喷孔夹角、涡流比、压缩比及燃烧室结构参数．研究表明：NO_x和碳烟排放呈现出明显的trade-off关系,NO_x排放和指示燃油消耗率之间存在一定的trade-off关系．对于带碰撞凸台的双层分流燃烧室,射流油束撞击燃烧室喉口碰撞凸台后分为上、下两束,其中下部油束向燃烧室内部卷吸,上部油束进入活塞压缩余隙区域．此时气缸内部空气利用率较高,燃料燃烧较为充分,高温区域分布较广;因而NO_x生成区域分布较广、生成量较高．同时因受到双层燃烧室燃料喷射分流的影响,在燃烧室内部以及压缩余隙区域均有明显的碳烟生成,使得碳烟生成区域较为分散,大部分碳烟能够在燃烧后期与新鲜空气混合从而被氧化．     

进气加湿对船用柴油机燃烧和排放的影响

运用CFD数值模拟软件AVL Fire建立了船用柴油机燃烧过程模型,研究不同相对湿度的进气成分对船用柴油机燃烧和排放特性的影响,并着重研究对NO_x和碳烟排放的影响规律和作用机理.结果表明:随着进气加湿率的增加,缸内压力和燃烧温度均有降低;当加湿率达到100%,时,峰值压力相比进气为干空气时下降0.67,MPa,最高燃烧温度下降220,K;滞燃期和燃烧持续期延长,预混燃烧比例增多;燃烧重心CA50向后偏移,燃烧定容度和热效率下降.NO_x排放不断减少,燃烧火焰温度的降低和水蒸气对富氧区的稀释是NO_x排放下降的主要原因;碳烟排放随着加湿率的增加不断恶化,进气加湿虽然可以促进油气混合、抑制碳烟生成,OH基团加速碳烟前驱物的氧化,但加湿后氧气质量分数的减少和燃烧温度的降低导致碳烟后期氧化能力严重减弱.     

喷油定时和燃烧室形状对柴油机燃烧及排放的影响

在保持压缩比不变的条件下,设计了3种不同径深比的燃烧室,并采用CFD数值模拟方法研究了喷油定时和燃烧室形状对柴油机燃烧过程、碳烟和NOx排放的影响.结果表明,随着喷油的推迟,各燃烧室NOx排放均呈下降趋势,碳烟排放则有所增加,尤其是缩口较小的燃烧室,其碳烟生成较多而氧化速率却很低,因此碳烟排放较高.此外,缸内温度为1500～2400K的区域是碳烟生成的主要区域,温度过低不利于碳烟的生成,而较高的温度会促进已生成碳烟的氧化.     

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.     

汽油/柴油双燃料不同HPCC燃烧模式的数值模拟

针对汽油/柴油双燃料高比例预混燃烧(HPCC)模式,在总喷油量、主放热时刻(CA50)基本相当的条件下,对柴油早喷(E-HPCC)与柴油晚喷(L-HPCC)情况下的燃烧特性和排放特性进行了数值模拟研究.结果表明,HPCC着火和燃烧过程主要受缸内活性自由基控制,较晚喷射会导致缸内局部正庚烷浓度高、活性较大,造成高温放热提前;较早喷射正庚烷会抑制低温放热,从而增加着火时刻缸内活性自由基的数量,是放热率较高、最大压升率较大的主要原因.L-HPCC模式下,高温放热时刻缸内局部正庚烷浓度较大造成了缸内局部温度较高,促进了NOx生成,相对E-HPCC模式NOx排放较高;虽然L-HPCC局部浓度较高会促进碳烟的生成,但同时较高的燃烧温度加强了碳烟的氧化,因此L-HPCC模式的碳烟排放低于E-HPCC模式.     

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.     

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。     

Modeling of combustion characteristics and NOx emission in highly preheated and diluted air combustion

The gas diffusion combustion in a regenerative furnace with highly preheated and diluted air has been numerically investigated in this paper. The highly preheated air combustion possesses high combustion intensity and high level temperature, but the NO_x emission also has an unwanted high level. Decreasing the oxygen concentration in the highly preheated air could decrease the NO_x emission and improve the uniformity of temperature distribution in the furnace. The combustion characteristics of highly preheated and diluted air combustion have been studied, including temperature distribution, soot formation, OH radical distribution, as well as NO_x emission. The influence of the preheated air temperature, the oxygen concentration, and the air diluent has also been investigated. The optimal combinations of the preheated air temperature and the oxygen concentration have been predicted in the case of flue gas recirculation, which could provide the highest possible temperature in the furnace while keeping the NO_x emission lower than the permitted value. © 1998 by John Wiley & Sons, Ltd.     

废塑料流化床焚烧及排放特性的试验研究

在一内径40mm、高500mm小型电加热流化床焚烧炉上对废塑料的燃烧特性，包括燃烧效率、挥发份析出特性及烟黑的生成进行了试验，分析了主要气体污染物（SO2、NO、HCl)的排放特性及运行条件（过剩空气率、床温及水份等参数）对废塑料燃烧和排放的影响。     

Study of the characteristic of diesel spray combustion and soot formation using laser-induced incandescence (LII)

In this paper, the planar images of diesel spray combustion flame and soot formation were measured and analyzed by using LII, in a constant volume combustion vessel. The effects of combustion flame and fuel–air mixing characteristics on soot formation and distribution of soot concentration were studied at different conditions. The result indicates that, with increase in ambient temperature and pressure, the ignition delay of diesel fuel is shorter. The increase of ambient temperature and pressure and the reduction of injection pressure shorten the diesel flame lift-off length. The lower the ambient temperature and pressure, the weaker LII signal intensity. At the same ambient temperature and pressure condition, the higher the diesel injection pressure, the smaller the soot production in diesel jet spray, and soot particles are primarily produced in the relative fuel-rich region, which is encompassed by the flame surface front at the downstream of the diesel jet.     

适用于柴油HCCI燃烧的碳烟半经验模型研究

将改进的碳烟半经验模型和简化正庚烷的化学反应机理纳入KIVA-3V程序中,以描述柴油燃烧过程中碳烟的生成和氧化历程。通过以正庚烷为燃料的激波管试验验证发现,在较宽的温度和压力范围内,该碳烟半经验模型可以相对准确地预测碳烟的生成率、颗粒直径和数密度。在定容燃烧器中典型的传统柴油机的扩散燃烧和接近于均质压燃(HCCI)发动机的预混燃烧状况下,应用此碳烟模型进一步研究了喷孔直径和喷射压力对碳烟排放的影响。结果发现模型预测得到的碳烟体积分数分布与试验吻合得较好,同时显示当控制局部当量比小于2.0时可以避免碳烟的生成。     

Soot processes in compression ignition engines

While diesel engines are arguably superior to any other power-production device for the transportation sector in terms of efficiency, torque, and overall driveability, they suffer from inferior performance in terms of noise, NO_x and particulate emissions. The majority of particulate originates with soot particles which are formed in fuel-rich regions of burning diesel jets. Over the past two decades, our understanding of the formation process of soot in diesel combustion has transformed from inferences based on exhaust measurements and laboratory flames to direct in-cylinder observations that have led to a transformation in diesel engine combustion. In-cylinder measurements show the diesel spray to produce a jet which forms a lifted, partially premixed, turbulent diffusion flame. Soot formation has been found to be strongly dependent on air entrainment in the lifted portion of the jet as well as by oxygen in the fuel and to a lesser extent the composition and structure of hydrocarbons in the fuel. Soot surviving the combustion process and exiting in the exhaust is dominated by soot from fuel-rich pockets which do not have time to mix and burn prior to exhaust valve opening. Higher temperatures at the end of combustion enhance the burnout of soot, while high temperatures at the time of injection reduce air entrainment and increase soot formation. Using a conceptual model based on in-cylinder soot and combustion measurements, trends seen in exhaust particulate can be explained. The current trend in diesel engine emissions control involves multi-injection combustion strategies which are transforming the picture of diesel combustion rapidly into a series of low temperature, stratified charge, premixed combustion events where NO_x formation is avoided because of low temperature and soot formation is avoided by leaning the mixture or increasing air entrainment prior to ignition.