吸热型碳氢燃料裂解引发剂的分子设计及作用机制

Using hydrocarbon fuels as a coolant presents a new application scope for hypersonic fuels, and its performance is mainly evaluated by heat sink capability from endothermic cracking. To increase the rate of thermal cracking reactions and to inhibit coke formation, this work will carry out the oriented molecular design investigation of initiators in the controllable cracking of different hydrocarbons by combining theoretical calculation, experimental means, and molecular simulation technique based on molecular fragment method. Firstly, the cracking mechanisms of different hydrocarbons are investigated to explore the correlation of free radical transformation process and endothermic cracking capacity, which will provide foundation for the controllable molecular design of initiators. Secondly, interaction of functional groups with different hydrocarbons is investigated by theoretical calculations to elucidate the structure-property relationship, which help to establish the functional fragment library of initiators. Then the multi-mode hybrid optimization algorithm is developed for identifying and assembly of functional fragments to obtain the molecular structure of initiator. Lastly, experimental platform, which simulates the operation condition of cooling channel, is built to evaluate the physicochemical properties and initiated cracking activity of initiators. Meanwhile, theoretical calculations are performed to elucidate their promoting effect on the cracking of different hydrocarbons and derive the mechanistic kinetics model. With the development of pyolysis initiators, the research project will help achieve technological innovation for the heat sink enhancements of hydrocarbon fuels by an effective control of the thermal cracking process, which is anticipated to provide practical foundation for the thermal management design of hypersonic vehicles.

吸热型碳氢燃料是高超音速推进燃料应用的新概念，决定其工作成效的关键是化学裂解吸热能力。为了提高裂解反应速率，抑制结焦生炭行为，本项目以“不同烃类裂解反应历程可控”为研究主线，结合理论计算、实验手段与分子模拟技术，基于分子碎片法进行裂解引发剂的分子设计与性能调控研究。从深入认识不同烃类裂解反应机理入手，构建自由基转化历程与化学吸热效应关联机制，为裂解引发剂的分子设计提供依据；模拟计算特征基团与烃类分子间的定向作用，探索结构与性质间的关系，构建裂解引发剂的官能团碎片库，通过发展多模式混合优化算法实现特征基团的识别与组装；模拟冷却通道实际工况进行引发剂理化性质与引发裂解活性的验证评价，并解析引发剂对不同烃类裂解的促进机制，建立机理型反应动力学模型。本项目的研究有助于实现吸热型碳氢燃料裂解反应历程有效调控，吸热能力显著增强的技术创新，从而为高超音速热防护系统设计提供一定的实践基础。

针对吸热型碳氢燃料（用于高超音速飞行器冷却）裂解反应要求温度高、易结焦生炭的问题，本项目以不同烃类裂解反应历程可控为研究主线进行裂解引发剂的分子设计与性能调控研究。通过理论计算结合实验手段进行了正庚烷和甲基环己烷裂解反应机理的研究，深入认识不同烃类裂解的转化历程和反应特性，确定出自由基生成是裂解链反应中的速率决定步骤，烷基自由基C–C键断裂生成烯烃是整个裂解链反应中的主要吸热反应，燃料化学热沉的主要源泉。与碳原子数相同的正烷烃相比，环烷烃裂解生成的烯烃分子数少，对燃料的化学热沉贡献较小。环烷烃裂解产物中的二烯烃与烯烃间的Diels-Alder反应活化能较低，在裂解反应体系中容易进行，然而Diels-Alder反应是个放热量较大的反应，会使燃料化学热沉显著减少。相比正烷烃和环烷烃，异构烷烃起裂温度低，裂解反应速率常数大。通过理论计算模拟特征基团与烃类分子间的定向作用，发现相比硝基，烷基和甲氧基等自由基易于与烃类发生夺氢反应，促进链反应进行，结合连接基团构建出裂解引发剂的官能团碎片库，并通过邻接矩阵、键合参数、原子特征向量和键级进行结构表征，并采用向量矩阵进行数学描述使其功能化。开发出的多模式混合优化算法涉及遗传算法和模拟退火思想，采用自适应多重退火交叉和变异策略，增加种群多样性，避免了种群早熟和局部最优陷阱，以键解离能为约束条件实现了特征基团的识别与组装。通过引发裂解活性的验证评价，发现硝基烷烃类引发剂能够促进碳氢燃料裂解，建立的机理型反应动力学模型有助于实现吸热型碳氢燃料裂解反应历程有效调控，吸热能力显著增强的技术创新，但距离工程应用还有较长的距离，尚需加深认识多场耦合对燃料热沉的影响。