高气压下微波等离子体辅助点火改善燃烧效能的机理研究

Microwave plasma assisted ignition and combustion is one of the potential reasonable methods in practical use in future. This work is based on the fundamental theories of plasma and combustion and will focus on two of the key scientific issues, including the mechanism of interaction between microwave plasma and combustible mixture and the relationship between combustion performance and microwave plasma ignition process. The characteristics and combustion performance of methane-air mixture by microwave plasma ignition will be investigated experimentally in a constant volume chamber under 6-10 atm. Based on the optimized configuration of microwave plasma system parameters by real-time impedance measurement and microwave frequency adjustment, a methodology that can excite microwave plasma by moderate power under elevated pressure will be proposed. The propagation mechanism of microwave plasma ignition will be investigated through the active particle density measurement during the ignition process by optical emission spectroscopy(OES), laser-induced fluorescence spectroscopy(LIFS) and cavity ringdown spectroscopy(CRDS). The combustion performance of microwave plasma ignition will be systematically evaluated by ignition delay time, pressure history and emission components density, and will be compared with that of spark plug ignition. Attempt will be made to improve the current methane combustion mechanism and to establish the relationship between plasma process and combustion performance. This work is hopeful to promote the practical applications of microwave assisted ignition and combustion.

微波等离子体辅助点火可能成为未来等离子体辅助点火及燃烧实用化的主要手段之一。本课题拟围绕高气压下微波等离子体对燃料气体混合物的作用机制和等离子体点火过程与燃烧效能之间的关系两大关键科学问题，结合等离子体放电和点火燃烧领域的基础理论开展研究工作。在6-10个大气压下对定容燃烧室中甲烷、空气混合物的微波等离子体点火特性及燃烧效能进行实验研究，通过实时阻抗测量及微波频率动态调节，实现优化微波系统参数的优化配置，提出一套高气压下稳定激发微波等离子体的方法；通过发射光谱、激光诱导荧光光谱和光腔衰荡光谱测量微波等离子体点火过程中的活性粒子浓度，分析等离子体点火过程的发展机制；通过测量点火延迟时间、压力曲线和燃烧尾气成分，并与火花塞点火进行对比，对微波等离子体的点火效能进行系统化评估，并尝试改进甲烷的燃烧机理，以建立等离子体过程与燃烧效能之间的联系。研究结果将指导微波等离子体辅助点火和燃烧的实用化。

微波等离子体点火是一种有望应用于汽车的新型点火方式，与传统的火花塞点火相比，微波等离子体点火具有提高点火效率、节能减排的潜力。而汽车点火的典型工作环境是6个大气压以上，目前对该气压下微波等离子体点火的基础研究还不充分，制约了该技术的实用化。.本项目首先研究了高气压下微波等离子体的稳定激发方法，依据微波谐振腔理论，设计了与汽车发动机气缸尺寸相近的实验腔体，研制了微波等离子体点火实验系统。采用甲烷-空气混合物作为燃烧气体，通过微波频率的反馈调节以及微波点火装置的结构设计，实现了高大气压下的微波等离子体稳定点火。.在实现微波等离子体的稳定激发后，通过测量压力曲线、高速摄影等手段对高气压下的微波等离子体点火和燃烧特性进行了研究，并研究了这些特性与微波功率、脉宽、重复频率和气压之间的关系，为优化微波等离子体点火参数提供了依据。.最后，对微波等离子体点火的燃烧效能进行了实验研究，将稀燃极限、压力特性和尾气成分等特性与同样实验条件下的火花塞点火进行了对比。结果表明，微波等离子体点火将火花塞点火的稀燃极限从0.7拓展到0.6；当量比较低时，微波等离子体的动力性能较好；在同样的实验条件下，微波等离子体点火生成的氮氧化物大约比火花塞点火减少40%左右。总体而言，在本文的研究条件下，微波等离子体点火的燃烧效能优于火花塞点火，该点火方式具有替代火花塞点火的应用前景。