基于低温冷焰特性的船用LNG-柴油双燃料燃烧化学动力学机理及其在发动机数值模拟中的敏感性研究

The marine engine employed in the LNG-fueled ship is operating mainly with the LNG-diesel dual-fuel mode. Specifically, LNG is ignited by the pilot diesel during the engine running processes. For this type of engine, the chemistry of the fuel plays a significant role in the in-cylinder combustion processes. However, the data of the combustion characteristics for the LNG-diesel dual fuel under low temperature conditions is less available, and the corresponding combustion kinetic mechanism is not of enough accuracy. In addition, the insight into the sensitivity of the output of the engine simulation to the scale of the combustion kinetic mechanism for the fuel is not sufficient. As a result, it is somewhat difficult to model the in-cylinder combustion processes effectively and accurately. In the present project, the cool premixed laminar flames of the LNG-diesel dual fuel will be studied using an experimental setup for spherical flames which mainly consists of a constant-volume combustion vessel and a schlieren optical system through adding a certain amount of ozone into the initial combustible mixtures. According to the experimental result, the low-temperature combustion characteristics for the dual fuel under various initial conditions will be obtained. On this basis, the combustion kinetic mechanism for the dual fuel will be validated and improved with the emphasis on the accurate prediction of the low-temperature combustion processes. Then based on the model for the in-cylinder combustion processes of the dual-fuel engine and the mechanism reduction methods, the sensitivity of the output of the engine simulation to the level of detail of the reaction mechanism will be studied and revealed, and the roles played by the chemistry of the fuel during the various stages of the in-cylinder combustion processes will be elucidated. The present investigation will provide a solid support for the effective and accurate simulation of the in-cylinder combustion processes of the dual-fuel engine, then accelerating the design and development of the engine.

对于LNG燃料动力船舶，其发动机主要采用柴油引燃LNG的双燃料燃烧模式，化学反应对该类型发动机的缸内燃烧过程具有显著影响，但低温条件下LNG-柴油双燃料的燃烧特性数据较为缺乏，低温燃烧化学动力学机理不完善，关于发动机数值模拟对双燃料动力学机理详细程度的敏感性规律认识也尚不充分，从而制约了双燃料发动机缸内燃烧过程的高效准确数值计算。本项目将基于定容弹高速纹影摄像试验装置，通过引入臭氧来开展双燃料的预混层流低温冷焰试验，获得双燃料的低温燃烧特性随初始条件的变化规律；据此验证和完善双燃料的化学动力学机理，重点提高低温条件下机理的预测准确性；在此基础上，结合双燃料发动机缸内燃烧数值计算模型和机理简化方法，研究和阐明发动机数值模拟对双燃料化学动力学机理详细程度的敏感性，揭示缸内不同燃烧阶段中化学动力学的影响。该研究可为双燃料发动机缸内燃烧过程的高效准确计算以及发动机设计开发效率的提高提供重要支撑。

以LNG燃料作为船用发动机的替代燃料，对于降低船舶碳排放、硫氧化物和颗粒物排放等具有重要意义。LNG燃料动力船舶的发动机主要采用柴油引燃LNG的双燃料燃烧模式，缸内燃烧经历了从低温到高温的全过程，温度跨度范围较大。化学反应对该类型发动机的缸内燃烧过程具有显著影响，但低温条件下LNG-柴油双燃料的燃烧特性数据较为缺乏，低温燃烧的机制不明确，关于发动机数值模拟对双燃料动力学机理详细程度的敏感性规律认识也尚不充分，从而制约了双燃料发动机缸内燃烧过程的高效准确数值计算。为此，项目组搭建了球形传播火焰与对冲火焰试验装置，获得了柴油模型燃料组分高碳直链烷烃的熄灭极限等数据，并研究了其对氧气摩尔分数的依赖性；以甲烷-正十二烷混合物作为双燃料的模型燃料，对其熄灭极限等冷焰特性进行了分析；同时，也研究阐明了双燃料的低温氧化与着火特性，以全面认识双燃料的低温燃烧特性。在船用LNG-柴油双燃料燃烧化学动力学机理方面，比较分析了不同机理在高碳直链烷烃低温冷焰预测准确性上的差异，通过引入密度加权焓，分析了低温化学性质在高碳直链烷烃冷焰熄灭极限中的作用；对于甲烷-正十二烷混合物的低温冷焰，从火焰结构角度分析了添加甲烷的作用，并分离和研究了甲烷的物理和化学作用，同时结合敏感性和反应路径分析探讨了甲烷和正十二烷的化学耦合作用；此外，对双燃料的低温氧化和着火过程进行了化学动力学分析，以全面揭示双燃料低温燃烧时的化学动力学特征。在此基础上，对双燃料模型燃料的化学动力学机理进行了简化和验证，获得了不同规模的简化机理；据此构建了化学动力学和流体力学耦合的双燃料发动机燃烧数值计算模型，结合模型比较分析了不同简化机理的发动机性能和排放数值模拟结果，并结合组分浓度和温度等参数的云图研究了不同简化机理的缸内燃烧过程模拟结果差异。本项目取得的研究成果可为船用LNG-柴油双燃料发动机缸内燃烧过程的高效准确数值模拟提供理论支撑。