基于消光辐射法和燃烧成像测速技术的柴油火焰中碳烟生成机理研究

Understanding the mechanism of soot formation and oxidation in Diesel combustion is critical for creating accurate soot numerical model and developing low emission diesel engine. As a fluorescence-free diagnostics fuel with known chemistry and properties, n-dodecane is widely used as an alternative fuel of diesel in Engine Combustion Network (ECN). Consequently, n-dodecane, as well as two injectors equipped with a single-hole nozzle and a three-hole nozzle respectively, are applied in this project. A new combined extinction and radiation methodology is applied to obtain the soot concentration and temperature simultaneously to avoid the uncertainties caused by signal self-absorption of Two-color method. Furthermore, it can be used to validate and develop chemical reaction mechanism of soot. Besides, Combustion Image Velocimetry is applied to obtain the soot particle velocity field under both quiescent (combustion vessel) and non-quiescent (optical engine) conditions. In addition, coupling with the spray velocity field from Particle Image Velocimetry and soot amount distribution from Diffused-background illumination extinction imaging, the interaction mechanism of air entrainment with soot formation and oxidation process under both quiescent and non-quiescent conditions will be obtained. The study of this project will provide a detailed database and theoretical basis for the development of more accurate and more reliable soot numerical model. Additionally, it can also promote the development of optical techniques on soot diagnostics.

明晰柴油燃烧火焰中碳烟生成氧化机理是准确建立碳烟模型，进而研发超低颗粒物排放柴油机的关键之一。正十二烷由于其跟柴油相似的着火特性和较为熟知的物理化学性质，在发动机燃烧合作组织（ECN）中作为柴油的替代燃料被广为研究。本项目以一个单孔喷油器和一个三孔喷油器以及正十二烷为研究对象。提出一种将碳烟消光特性和辐射特性相结合的测试方法，实现对稳态火焰中碳烟浓度和温度的同步测量，解决了双色法自吸光带来的不确定性，进一步用来准确验证和发展碳烟的化学反应机理。另外，采用燃烧图像测速技术获得碳烟颗粒在静态环境（容弹）和非静态环境下（光学发动机）的速度场变化过程。结合粒子成像测速技术获得的燃烧喷雾的速度场和扩散背景光消光技术测得的瞬时碳烟生成量，揭示空气卷吸在静态与非静态环境中对碳烟生成氧化过程的影响机理。本项目的研究将为准确、可靠燃烧碳烟模型的建立提供数据和理论基础，也必将极大推动碳烟光学测试技术的发展。

本项目基于多重光学诊断技术在高温高压燃烧弹中的多种工况条件下，对压燃式发动机喷雾火焰中的碳烟生成过程进行了全面系统的测量，并结合数值计算，揭示了喷雾火焰中的碳烟生成氧化机理。本项目首先应用消光辐射结合法同步获得了不同环境温度、喷油压力和环境氧含量下稳态火焰中碳烟浓度分布和温度分布，得出局部火焰温度对碳烟生成并没有产生显著影响，而火焰浮起长度处的油气混合对后续碳烟生成起到了更为决定性的作用；进一步，在分段喷油策略对碳烟影响的研究中发现，喷油间隔和初次喷油时间对二次喷油中的碳烟生成并没有产生显著影响，这主要是由于一次喷油发生了显著的“回火”现象，而二次喷油相似的着火延迟、火焰浮起长度以及喷油结束后卷吸波发展最后导致了相似的碳烟发展；最后，对甲醇/加氢催化生物柴油混合燃油的喷雾燃烧特性研究发现，甲醇的增加提高了火焰浮起长度并增加了燃料的氧含量，显著的降低了火焰中的碳烟生成。