临近空间对于国家安全具有重要的战略意义。超燃冲压发动机具有结构简单，比冲高等优势，是临近空间一种重要的推进器。超燃冲压发动机燃烧室内部气流滞留时间在ms量级，这为发动机的启动以及稳定工作提供了极大挑战，诸多助燃手段便应运而生。超燃冲压发动机实现成功点火和稳定燃烧有两种途径：降低点火延迟和提高火焰速度。

研究表明，微波可以显著的提高层流火焰速度，但微波参数与火焰参数的耦合关系仍然不明确。微波助燃实验涉及到复杂的微波系统、微波诊断系统和火焰诊断系统，因此微波助燃系统的搭建是研究微波助燃的关键问题之一。本文基于矩形波导设计了一套独立的微波助燃谐振系统并对该系统进行了详细的测试，获得了微波与流场谐振耦合的定量化参数。本文的实验研究内容主要分为两部分：基于微波谐振腔系统的微波辅助本生灯预混火焰实验和基于直连式超声速燃烧室的微波辅助煤油超声速燃烧实验。

第一部分通过搭建可调精密微波谐振腔系统，研究微波参数与火焰速度的定量化关系。采用数值仿真软件对谐振腔结构设计进行了详细的计算并对窗口参数进行了优化，自制探针对谐振腔的驻波电场分布进行了测试。开展了不同功率的微波对甲烷/空气预混火焰的影响研究。采用火焰图像处理的方式来提取火焰速度并建立了微波功率和火焰速度的定量化关系。研究表明，对于 ϕ=0.7，出口速度为1m/s和直径4mm的甲烷/空气预混火焰，加入15W的连续微波时层流火焰速度的增幅为6.98%，加入45W的连续微波时层流火焰速度的增幅为14.0%，层流火焰速度的提高与电场强度成正比。

第二部分通过微波与直连式超声速燃烧室的耦合，研究了微波对超声速燃烧过程中火焰位置、火焰稳定性、CH*转振温度以及发动机性能的影响。实验结果显示，当加入频率为2.45GHz，功率为700W的连续微波时，通过CH*图像可以计算出火焰稳定位置向上游移动了7.4mm。对比有无微波下的火焰传播速度，发现微波对超声速下的火焰速度提高比较明显。在火焰进入凹腔之前，以燃烧室为坐标，火焰向上游传播的瞬时速度均值提高了约64%，火焰离开凹腔继续向上游传播时，火焰的瞬时速度均值提高了约61%。微波的加入对发动机性能产生正反馈，实验结果显示，当加入700W连续微波时，发动机的比冲提高了约4%。

煤油燃烧火焰发射光谱的结果显示，由于点火过程中存在大量的氢，微波对于点火过程具有很强的增益效果。激发态的NO和N₂的数量均得到了明显的提高，中间产物OH、CH和C₂等基团的数量明显提高。对于煤油单独燃烧过程，微波虽然使CH等基团的数量有所提高，但是活性基的增益远不如有大量氢存在的情况，说明2.45GHz的微波作用机理可能与氢元素有较大关系，其机理还需要进一步明晰。

Near space has important strategic significance for national security. scramjet is an important propeller for its simple structure and high efficiency. The time of air flow stays in the scramjet chamber in the order of millisecond, which makes it difficult for the start and work stably of the scramjet. Therefore, many means of techniques to assist combustion came into being. There are two ways for scramjet to achieve successful ignition and stable combustion: reducing ignition delay and increasing flame speed.

Research shows that microwave can increase the flame speed obviously, but the relationship between microwave factor and flame factor is still waiting to be defined. The microwave assisted combustion experiment involves complex microwave system, microwave diagnosis system and flame diagnose system, so the construction of microwave combustion system is one of the key issues for microwave assisted combustion experiment. An independent microwave assisted-combustion resonance system is designed based on the rectangular waveguide and detailed debugging work is carried out on this system. Experiments were conducted to obtain quantitative microwave and flow field resonance coupling parameters. The experimental research content of this article is mainly divided into two parts: the microwave-assisted premixed Bunsen flame experiment based on the microwave resonator system and the microwave-assisted kerosene supersonic combustion experiment based on the combustion chamber of direct-connected supersonic combustion test facility.

The first part established the quantitative relationship between microwave parameters and flame speed by building an adjustable precise microwave-assisted combustion system. The resonator structure and the windows factor was optimized in detail by numerical simulation software. Electric field probe designed by the author was used to detect the electric field distribution of standing wave in the resonator. The effect of microwave with different powers on the methane / air premixed flame was studied. Flame image processing is used to extract flame speed and establish a quantitative relationship between microwave power and flame speed. Research shows that for methane/air premixed flame with an equivalence ratio of 0.7 and velocity of 1 m/s and the flow diameter of 0.4 mm, when 15W continuous microwave is added, the increase of laminar flame speed is 6.98%, and when 45W continuous microwave is added, the increase of laminar flame speed is 14.0%. The increase of laminar flame speed is proportional to the electric field strength.

The second part studies the influence of microwave on the flame position, flame stability, CH* rotational temperature and scramjet performance during supersonic combustion through the coupling of directly connected supersonic combustion chamber and microwave. The experimental results show that when a continuous microwave with a frequency of 2.45 GHz and a power of 700 W is added, the flame stable position has moved upstream by 7.4 mm calculated by the CH* images. The microwave has more obvious effect on the flame speed under supersonic speed by comparing the flame propagation speed with or without microwave in different flow conditions. Before the flame enters the cavity, with the combustion chamber as the coordinate, the average instantaneous velocity of flame propagation upstream increases by about 64%. When the flame leaves the cavity and propagate upstream continuously, the average speed of the flame increases by about 61%, which shows that microwave also has a strong gain effect on supersonic turbulent flame. The addition of microwaves produces positive feedback on scramjet performance. The experimental results show that when 700W continuous microwave was added, the specific impulse of the scramjet is increased by about 4%.

The results of kerosene combustion flame emission spectrum show that due to the large amount of hydrogen in the ignition process, microwave can enhance ignition process effectively. The number of excited NO and N₂ have been significantly increased, and the number of intermediate products OH, CH and C₂ and other groups have increased significantly. However, for the kerosene alone combustion process, although microwaves have increased the number of CH and other radicals, the gain of the radicals is far less than the process when there are large amounts of hydrogen, which shows that the microwave-assisted combustion mechanism of 2.45GHz may closely related to hydrogen element, and its mechanism needs to be further clarified.